@node Date and Time, Non-Local Exits, Arithmetic, Top @c %MENU% Functions for getting the date and time and formatting them nicely @chapter Date and Time This chapter describes functions for manipulating dates and times, including functions for determining what the current time is and conversion between different time representations. The time functions fall into three main categories: @itemize @bullet @item Functions for measuring elapsed CPU time are discussed in @ref{Processor Time}. @item Functions for measuring absolute clock or calendar time are discussed in @ref{Calendar Time}. @item Functions for setting alarms and timers are discussed in @ref{Setting an Alarm}. @end itemize @menu * Processor Time:: Measures processor time used by a program. * Calendar Time:: Manipulation of ``real'' dates and times. * Precision Time:: Manipulation and monitoring of high accuracy time. * Setting an Alarm:: Sending a signal after a specified time. * Sleeping:: Waiting for a period of time. * Resource Usage:: Measuring various resources used. * Limits on Resources:: Specifying limits on resource usage. * Priority:: Reading or setting process run priority. @end menu @node Processor Time @section Processor Time If you're trying to optimize your program or measure its efficiency, it's very useful to be able to know how much @dfn{processor time} or @dfn{CPU time} it has used at any given point. Processor time is different from actual wall clock time because it doesn't include any time spent waiting for I/O or when some other process is running. Processor time is represented by the data type @code{clock_t}, and is given as a number of @dfn{clock ticks} relative to an arbitrary base time marking the beginning of a single program invocation. @cindex CPU time @cindex processor time @cindex clock ticks @cindex ticks, clock @cindex time, elapsed CPU @menu * Basic CPU Time:: The @code{clock} function. * Detailed CPU Time:: The @code{times} function. @end menu @node Basic CPU Time @subsection Basic CPU Time Inquiry To get the elapsed CPU time used by a process, you can use the @code{clock} function. This facility is declared in the header file @file{time.h}. @pindex time.h In typical usage, you call the @code{clock} function at the beginning and end of the interval you want to time, subtract the values, and then divide by @code{CLOCKS_PER_SEC} (the number of clock ticks per second), like this: @smallexample @group #include <time.h> clock_t start, end; double elapsed; start = clock(); @dots{} /* @r{Do the work.} */ end = clock(); elapsed = ((double) (end - start)) / CLOCKS_PER_SEC; @end group @end smallexample Different computers and operating systems vary wildly in how they keep track of processor time. It's common for the internal processor clock to have a resolution somewhere between hundredth and millionth of a second. In the GNU system, @code{clock_t} is equivalent to @code{long int} and @code{CLOCKS_PER_SEC} is an integer value. But in other systems, both @code{clock_t} and the type of the macro @code{CLOCKS_PER_SEC} can be either integer or floating-point types. Casting processor time values to @code{double}, as in the example above, makes sure that operations such as arithmetic and printing work properly and consistently no matter what the underlying representation is. @comment time.h @comment ISO @deftypevr Macro int CLOCKS_PER_SEC The value of this macro is the number of clock ticks per second measured by the @code{clock} function. @end deftypevr @comment time.h @comment POSIX.1 @deftypevr Macro int CLK_TCK This is an obsolete name for @code{CLOCKS_PER_SEC}. @end deftypevr @comment time.h @comment ISO @deftp {Data Type} clock_t This is the type of the value returned by the @code{clock} function. Values of type @code{clock_t} are in units of clock ticks. @end deftp @comment time.h @comment ISO @deftypefun clock_t clock (void) This function returns the elapsed processor time. The base time is arbitrary but doesn't change within a single process. If the processor time is not available or cannot be represented, @code{clock} returns the value @code{(clock_t)(-1)}. @end deftypefun @node Detailed CPU Time @subsection Detailed Elapsed CPU Time Inquiry The @code{times} function returns more detailed information about elapsed processor time in a @w{@code{struct tms}} object. You should include the header file @file{sys/times.h} to use this facility. @pindex sys/times.h @comment sys/times.h @comment POSIX.1 @deftp {Data Type} {struct tms} The @code{tms} structure is used to return information about process times. It contains at least the following members: @table @code @item clock_t tms_utime This is the CPU time used in executing the instructions of the calling process. @item clock_t tms_stime This is the CPU time used by the system on behalf of the calling process. @item clock_t tms_cutime This is the sum of the @code{tms_utime} values and the @code{tms_cutime} values of all terminated child processes of the calling process, whose status has been reported to the parent process by @code{wait} or @code{waitpid}; see @ref{Process Completion}. In other words, it represents the total CPU time used in executing the instructions of all the terminated child processes of the calling process, excluding child processes which have not yet been reported by @code{wait} or @code{waitpid}. @item clock_t tms_cstime This is similar to @code{tms_cutime}, but represents the total CPU time used by the system on behalf of all the terminated child processes of the calling process. @end table All of the times are given in clock ticks. These are absolute values; in a newly created process, they are all zero. @xref{Creating a Process}. @end deftp @comment sys/times.h @comment POSIX.1 @deftypefun clock_t times (struct tms *@var{buffer}) The @code{times} function stores the processor time information for the calling process in @var{buffer}. The return value is the same as the value of @code{clock()}: the elapsed real time relative to an arbitrary base. The base is a constant within a particular process, and typically represents the time since system start-up. A value of @code{(clock_t)(-1)} is returned to indicate failure. @end deftypefun @strong{Portability Note:} The @code{clock} function described in @ref{Basic CPU Time}, is specified by the @w{ISO C} standard. The @code{times} function is a feature of POSIX.1. In the GNU system, the value returned by the @code{clock} function is equivalent to the sum of the @code{tms_utime} and @code{tms_stime} fields returned by @code{times}. @node Calendar Time @section Calendar Time This section describes facilities for keeping track of dates and times according to the Gregorian calendar. @cindex Gregorian calendar @cindex time, calendar @cindex date and time There are three representations for date and time information: @itemize @bullet @item @dfn{Calendar time} (the @code{time_t} data type) is a compact representation, typically giving the number of seconds elapsed since some implementation-specific base time. @cindex calendar time @item There is also a @dfn{high-resolution time} representation (the @code{struct timeval} data type) that includes fractions of a second. Use this time representation instead of ordinary calendar time when you need greater precision. @cindex high-resolution time @item @dfn{Local time} or @dfn{broken-down time} (the @code{struct tm} data type) represents the date and time as a set of components specifying the year, month, and so on, for a specific time zone. This time representation is usually used in conjunction with formatting date and time values. @cindex local time @cindex broken-down time @end itemize @menu * Simple Calendar Time:: Facilities for manipulating calendar time. * High-Resolution Calendar:: A time representation with greater precision. * Broken-down Time:: Facilities for manipulating local time. * Formatting Date and Time:: Converting times to strings. * Parsing Date and Time:: Convert textual time and date information back into broken-down time values. * TZ Variable:: How users specify the time zone. * Time Zone Functions:: Functions to examine or specify the time zone. * Time Functions Example:: An example program showing use of some of the time functions. @end menu @node Simple Calendar Time @subsection Simple Calendar Time This section describes the @code{time_t} data type for representing calendar time, and the functions which operate on calendar time objects. These facilities are declared in the header file @file{time.h}. @pindex time.h @cindex epoch @comment time.h @comment ISO @deftp {Data Type} time_t This is the data type used to represent calendar time. When interpreted as an absolute time value, it represents the number of seconds elapsed since 00:00:00 on January 1, 1970, Coordinated Universal Time. (This date is sometimes referred to as the @dfn{epoch}.) POSIX requires that this count ignore leap seconds, but on some hosts this count includes leap seconds if you set @code{TZ} to certain values (@pxref{TZ Variable}). In the GNU C library, @code{time_t} is equivalent to @code{long int}. In other systems, @code{time_t} might be either an integer or floating-point type. @end deftp @comment time.h @comment ISO @deftypefun double difftime (time_t @var{time1}, time_t @var{time0}) The @code{difftime} function returns the number of seconds elapsed between time @var{time1} and time @var{time0}, as a value of type @code{double}. The difference ignores leap seconds unless leap second support is enabled. In the GNU system, you can simply subtract @code{time_t} values. But on other systems, the @code{time_t} data type might use some other encoding where subtraction doesn't work directly. @end deftypefun @comment time.h @comment ISO @deftypefun time_t time (time_t *@var{result}) The @code{time} function returns the current time as a value of type @code{time_t}. If the argument @var{result} is not a null pointer, the time value is also stored in @code{*@var{result}}. If the calendar time is not available, the value @w{@code{(time_t)(-1)}} is returned. @end deftypefun @node High-Resolution Calendar @subsection High-Resolution Calendar The @code{time_t} data type used to represent calendar times has a resolution of only one second. Some applications need more precision. So, the GNU C library also contains functions which are capable of representing calendar times to a higher resolution than one second. The functions and the associated data types described in this section are declared in @file{sys/time.h}. @pindex sys/time.h @comment sys/time.h @comment BSD @deftp {Data Type} {struct timeval} The @code{struct timeval} structure represents a calendar time. It has the following members: @table @code @item long int tv_sec This represents the number of seconds since the epoch. It is equivalent to a normal @code{time_t} value. @item long int tv_usec This is the fractional second value, represented as the number of microseconds. Some times struct timeval values are used for time intervals. Then the @code{tv_sec} member is the number of seconds in the interval, and @code{tv_usec} is the number of additional microseconds. @end table @end deftp @comment sys/time.h @comment BSD @deftp {Data Type} {struct timezone} The @code{struct timezone} structure is used to hold minimal information about the local time zone. It has the following members: @table @code @item int tz_minuteswest This is the number of minutes west of UTC. @item int tz_dsttime If nonzero, daylight saving time applies during some part of the year. @end table The @code{struct timezone} type is obsolete and should never be used. Instead, use the facilities described in @ref{Time Zone Functions}. @end deftp It is often necessary to subtract two values of type @w{@code{struct timeval}}. Here is the best way to do this. It works even on some peculiar operating systems where the @code{tv_sec} member has an unsigned type. @smallexample /* @r{Subtract the `struct timeval' values X and Y,} @r{storing the result in RESULT.} @r{Return 1 if the difference is negative, otherwise 0.} */ int timeval_subtract (result, x, y) struct timeval *result, *x, *y; @{ /* @r{Perform the carry for the later subtraction by updating @var{y}.} */ if (x->tv_usec < y->tv_usec) @{ int nsec = (y->tv_usec - x->tv_usec) / 1000000 + 1; y->tv_usec -= 1000000 * nsec; y->tv_sec += nsec; @} if (x->tv_usec - y->tv_usec > 1000000) @{ int nsec = (y->tv_usec - x->tv_usec) / 1000000; y->tv_usec += 1000000 * nsec; y->tv_sec -= nsec; @} /* @r{Compute the time remaining to wait.} @r{@code{tv_usec} is certainly positive.} */ result->tv_sec = x->tv_sec - y->tv_sec; result->tv_usec = x->tv_usec - y->tv_usec; /* @r{Return 1 if result is negative.} */ return x->tv_sec < y->tv_sec; @} @end smallexample @comment sys/time.h @comment BSD @deftypefun int gettimeofday (struct timeval *@var{tp}, struct timezone *@var{tzp}) The @code{gettimeofday} function returns the current date and time in the @code{struct timeval} structure indicated by @var{tp}. Information about the time zone is returned in the structure pointed at @var{tzp}. If the @var{tzp} argument is a null pointer, time zone information is ignored. The return value is @code{0} on success and @code{-1} on failure. The following @code{errno} error condition is defined for this function: @table @code @item ENOSYS The operating system does not support getting time zone information, and @var{tzp} is not a null pointer. The GNU operating system does not support using @w{@code{struct timezone}} to represent time zone information; that is an obsolete feature of 4.3 BSD. Instead, use the facilities described in @ref{Time Zone Functions}. @end table @end deftypefun @comment sys/time.h @comment BSD @deftypefun int settimeofday (const struct timeval *@var{tp}, const struct timezone *@var{tzp}) The @code{settimeofday} function sets the current date and time according to the arguments. As for @code{gettimeofday}, time zone information is ignored if @var{tzp} is a null pointer. You must be a privileged user in order to use @code{settimeofday}. The return value is @code{0} on success and @code{-1} on failure. The following @code{errno} error conditions are defined for this function: @table @code @item EPERM This process cannot set the time because it is not privileged. @item ENOSYS The operating system does not support setting time zone information, and @var{tzp} is not a null pointer. @end table @end deftypefun @comment sys/time.h @comment BSD @deftypefun int adjtime (const struct timeval *@var{delta}, struct timeval *@var{olddelta}) This function speeds up or slows down the system clock in order to make gradual adjustments in the current time. This ensures that the time reported by the system clock is always monotonically increasing, which might not happen if you simply set the current time. The @var{delta} argument specifies a relative adjustment to be made to the current time. If negative, the system clock is slowed down for a while until it has lost this much time. If positive, the system clock is speeded up for a while. If the @var{olddelta} argument is not a null pointer, the @code{adjtime} function returns information about any previous time adjustment that has not yet completed. This function is typically used to synchronize the clocks of computers in a local network. You must be a privileged user to use it. The return value is @code{0} on success and @code{-1} on failure. The following @code{errno} error condition is defined for this function: @table @code @item EPERM You do not have privilege to set the time. @end table @end deftypefun @strong{Portability Note:} The @code{gettimeofday}, @code{settimeofday}, and @code{adjtime} functions are derived from BSD. @node Broken-down Time @subsection Broken-down Time @cindex broken-down time @cindex calendar time and broken-down time Calendar time is represented as a number of seconds. This is convenient for calculation, but has no resemblance to the way people normally represent dates and times. By contrast, @dfn{broken-down time} is a binary representation separated into year, month, day, and so on. Broken down time values are not useful for calculations, but they are useful for printing human readable time. A broken-down time value is always relative to a choice of local time zone, and it also indicates which time zone was used. The symbols in this section are declared in the header file @file{time.h}. @comment time.h @comment ISO @deftp {Data Type} {struct tm} This is the data type used to represent a broken-down time. The structure contains at least the following members, which can appear in any order: @table @code @item int tm_sec This is the number of seconds after the minute, normally in the range @code{0} through @code{59}. (The actual upper limit is @code{60}, to allow for leap seconds if leap second support is available.) @cindex leap second @item int tm_min This is the number of minutes after the hour, in the range @code{0} through @code{59}. @item int tm_hour This is the number of hours past midnight, in the range @code{0} through @code{23}. @item int tm_mday This is the day of the month, in the range @code{1} through @code{31}. @item int tm_mon This is the number of months since January, in the range @code{0} through @code{11}. @item int tm_year This is the number of years since @code{1900}. @item int tm_wday This is the number of days since Sunday, in the range @code{0} through @code{6}. @item int tm_yday This is the number of days since January 1, in the range @code{0} through @code{365}. @item int tm_isdst @cindex Daylight Saving Time @cindex summer time This is a flag that indicates whether Daylight Saving Time is (or was, or will be) in effect at the time described. The value is positive if Daylight Saving Time is in effect, zero if it is not, and negative if the information is not available. @item long int tm_gmtoff This field describes the time zone that was used to compute this broken-down time value, including any adjustment for daylight saving; it is the number of seconds that you must add to UTC to get local time. You can also think of this as the number of seconds east of UTC. For example, for U.S. Eastern Standard Time, the value is @code{-5*60*60}. The @code{tm_gmtoff} field is derived from BSD and is a GNU library extension; it is not visible in a strict @w{ISO C} environment. @item const char *tm_zone This field is the name for the time zone that was used to compute this broken-down time value. Like @code{tm_gmtoff}, this field is a BSD and GNU extension, and is not visible in a strict @w{ISO C} environment. @end table @end deftp @comment time.h @comment ISO @deftypefun {struct tm *} localtime (const time_t *@var{time}) The @code{localtime} function converts the calendar time pointed to by @var{time} to broken-down time representation, expressed relative to the user's specified time zone. The return value is a pointer to a static broken-down time structure, which might be overwritten by subsequent calls to @code{ctime}, @code{gmtime}, or @code{localtime}. (But no other library function overwrites the contents of this object.) The return value is the null pointer if @var{time} cannot be represented as a broken-down time; typically this is because the year cannot fit into an @code{int}. Calling @code{localtime} has one other effect: it sets the variable @code{tzname} with information about the current time zone. @xref{Time Zone Functions}. @end deftypefun Using the @code{localtime} function is a big problem in multi-threaded programs. The result is returned in a static buffer and this is used in all threads. POSIX.1c introduced a varient of this function. @comment time.h @comment POSIX.1c @deftypefun {struct tm *} localtime_r (const time_t *@var{time}, struct tm *@var{resultp}) The @code{localtime_r} function works just like the @code{localtime} function. It takes a pointer to a variable containing the calendar time and converts it to the broken-down time format. But the result is not placed in a static buffer. Instead it is placed in the object of type @code{struct tm} to which the parameter @var{resultp} points. If the conversion is successful the function returns a pointer to the object the result was written into, i.e., it returns @var{resultp}. @end deftypefun @comment time.h @comment ISO @deftypefun {struct tm *} gmtime (const time_t *@var{time}) This function is similar to @code{localtime}, except that the broken-down time is expressed as Coordinated Universal Time (UTC)---that is, as Greenwich Mean Time (GMT)---rather than relative to the local time zone. Recall that calendar times are @emph{always} expressed in coordinated universal time. @end deftypefun As for the @code{localtime} function we have the problem that the result is placed in a static variable. POSIX.1c also provides a replacement for @code{gmtime}. @comment time.h @comment POSIX.1c @deftypefun {struct tm *} gmtime_r (const time_t *@var{time}, struct tm *@var{resultp}) This function is similar to @code{localtime_r}, except that it converts just like @code{gmtime} the given time as Coordinated Universal Time. If the conversion is successful the function returns a pointer to the object the result was written into, i.e., it returns @var{resultp}. @end deftypefun @comment time.h @comment ISO @deftypefun time_t mktime (struct tm *@var{brokentime}) The @code{mktime} function is used to convert a broken-down time structure to a calendar time representation. It also ``normalizes'' the contents of the broken-down time structure, by filling in the day of week and day of year based on the other date and time components. The @code{mktime} function ignores the specified contents of the @code{tm_wday} and @code{tm_yday} members of the broken-down time structure. It uses the values of the other components to compute the calendar time; it's permissible for these components to have unnormalized values outside of their normal ranges. The last thing that @code{mktime} does is adjust the components of the @var{brokentime} structure (including the @code{tm_wday} and @code{tm_yday}). If the specified broken-down time cannot be represented as a calendar time, @code{mktime} returns a value of @code{(time_t)(-1)} and does not modify the contents of @var{brokentime}. Calling @code{mktime} also sets the variable @code{tzname} with information about the current time zone. @xref{Time Zone Functions}. @end deftypefun @node Formatting Date and Time @subsection Formatting Date and Time The functions described in this section format time values as strings. These functions are declared in the header file @file{time.h}. @pindex time.h @comment time.h @comment ISO @deftypefun {char *} asctime (const struct tm *@var{brokentime}) The @code{asctime} function converts the broken-down time value that @var{brokentime} points to into a string in a standard format: @smallexample "Tue May 21 13:46:22 1991\n" @end smallexample The abbreviations for the days of week are: @samp{Sun}, @samp{Mon}, @samp{Tue}, @samp{Wed}, @samp{Thu}, @samp{Fri}, and @samp{Sat}. The abbreviations for the months are: @samp{Jan}, @samp{Feb}, @samp{Mar}, @samp{Apr}, @samp{May}, @samp{Jun}, @samp{Jul}, @samp{Aug}, @samp{Sep}, @samp{Oct}, @samp{Nov}, and @samp{Dec}. The return value points to a statically allocated string, which might be overwritten by subsequent calls to @code{asctime} or @code{ctime}. (But no other library function overwrites the contents of this string.) @end deftypefun @comment time.h @comment POSIX.1c @deftypefun {char *} asctime_r (const struct tm *@var{brokentime}, char *@var{buffer}) This function is similar to @code{asctime} but instead of placing the result in a static buffer it writes the string in the buffer pointed to by the parameter @var{buffer}. This buffer should have at least room for 16 bytes. If no error occurred the function returns a pointer to the string the result was written into, i.e., it returns @var{buffer}. Otherwise return @code{NULL}. @end deftypefun @comment time.h @comment ISO @deftypefun {char *} ctime (const time_t *@var{time}) The @code{ctime} function is similar to @code{asctime}, except that the time value is specified as a @code{time_t} calendar time value rather than in broken-down local time format. It is equivalent to @smallexample asctime (localtime (@var{time})) @end smallexample @code{ctime} sets the variable @code{tzname}, because @code{localtime} does so. @xref{Time Zone Functions}. @end deftypefun @comment time.h @comment POSIX.1c @deftypefun {char *} ctime_r (const time_t *@var{time}, char *@var{buffer}) This function is similar to @code{ctime}, only that it places the result in the string pointed to by @var{buffer}. It is equivalent to (written using gcc extensions, @pxref{Statement Exprs,,,gcc,Porting and Using gcc}): @smallexample (@{ struct tm tm; asctime_r (localtime_r (time, &tm), buf); @}) @end smallexample If no error occurred the function returns a pointer to the string the result was written into, i.e., it returns @var{buffer}. Otherwise return @code{NULL}. @end deftypefun @comment time.h @comment ISO @deftypefun size_t strftime (char *@var{s}, size_t @var{size}, const char *@var{template}, const struct tm *@var{brokentime}) This function is similar to the @code{sprintf} function (@pxref{Formatted Input}), but the conversion specifications that can appear in the format template @var{template} are specialized for printing components of the date and time @var{brokentime} according to the locale currently specified for time conversion (@pxref{Locales}). Ordinary characters appearing in the @var{template} are copied to the output string @var{s}; this can include multibyte character sequences. Conversion specifiers are introduced by a @samp{%} character, followed by an optional flag which can be one of the following. These flags are all GNU extensions. The first three affect only the output of numbers: @table @code @item _ The number is padded with spaces. @item - The number is not padded at all. @item 0 The number is padded with zeros even if the format specifies padding with spaces. @item ^ The output uses uppercase characters, but only if this is possible (@pxref{Case Conversion}). @end table The default action is to pad the number with zeros to keep it a constant width. Numbers that do not have a range indicated below are never padded, since there is no natural width for them. Following the flag an optional specification of the width is possible. This is specified in decimal notation. If the natural size of the output is of the field has less than the specified number of characters, the result is written right adjusted and space padded to the given size. An optional modifier can follow the optional flag and width specification. The modifiers, which are POSIX.2 extensions, are: @table @code @item E Use the locale's alternate representation for date and time. This modifier applies to the @code{%c}, @code{%C}, @code{%x}, @code{%X}, @code{%y} and @code{%Y} format specifiers. In a Japanese locale, for example, @code{%Ex} might yield a date format based on the Japanese Emperors' reigns. @item O Use the locale's alternate numeric symbols for numbers. This modifier applies only to numeric format specifiers. @end table If the format supports the modifier but no alternate representation is available, it is ignored. The conversion specifier ends with a format specifier taken from the following list. The whole @samp{%} sequence is replaced in the output string as follows: @table @code @item %a The abbreviated weekday name according to the current locale. @item %A The full weekday name according to the current locale. @item %b The abbreviated month name according to the current locale. @item %B The full month name according to the current locale. @item %c The preferred date and time representation for the current locale. @item %C The century of the year. This is equivalent to the greatest integer not greater than the year divided by 100. This format is a POSIX.2 extension. @item %d The day of the month as a decimal number (range @code{01} through @code{31}). @item %D The date using the format @code{%m/%d/%y}. This format is a POSIX.2 extension. @item %e The day of the month like with @code{%d}, but padded with blank (range @code{ 1} through @code{31}). This format is a POSIX.2 extension. @item %F The date using the format @code{%Y-%m-%d}. This is the form specified in the @w{ISO 8601} standard and is the preferred form for all uses. This format is a @w{ISO C 9X} extension. @item %g The year corresponding to the ISO week number, but without the century (range @code{00} through @code{99}). This has the same format and value as @code{%y}, except that if the ISO week number (see @code{%V}) belongs to the previous or next year, that year is used instead. This format is a GNU extension. @item %G The year corresponding to the ISO week number. This has the same format and value as @code{%Y}, except that if the ISO week number (see @code{%V}) belongs to the previous or next year, that year is used instead. This format is a GNU extension. @item %h The abbreviated month name according to the current locale. The action is the same as for @code{%b}. This format is a POSIX.2 extension. @item %H The hour as a decimal number, using a 24-hour clock (range @code{00} through @code{23}). @item %I The hour as a decimal number, using a 12-hour clock (range @code{01} through @code{12}). @item %j The day of the year as a decimal number (range @code{001} through @code{366}). @item %k The hour as a decimal number, using a 24-hour clock like @code{%H}, but padded with blank (range @code{ 0} through @code{23}). This format is a GNU extension. @item %l The hour as a decimal number, using a 12-hour clock like @code{%I}, but padded with blank (range @code{ 1} through @code{12}). This format is a GNU extension. @item %m The month as a decimal number (range @code{01} through @code{12}). @item %M The minute as a decimal number (range @code{00} through @code{59}). @item %n A single @samp{\n} (newline) character. This format is a POSIX.2 extension. @item %p Either @samp{AM} or @samp{PM}, according to the given time value; or the corresponding strings for the current locale. Noon is treated as @samp{PM} and midnight as @samp{AM}. @ignore We currently have a problem with makeinfo. Write @samp{AM} and @samp{am} both results in `am'. I.e., the difference in case is not visible anymore. @end ignore @item %P Either @samp{am} or @samp{pm}, according to the given time value; or the corresponding strings for the current locale, printed in lowercase characters. Noon is treated as @samp{pm} and midnight as @samp{am}. This format is a GNU extension. @item %r The complete time using the AM/PM format of the current locale. This format is a POSIX.2 extension. @item %R The hour and minute in decimal numbers using the format @code{%H:%M}. This format is a GNU extension. @item %s The number of seconds since the epoch, i.e., since 1970-01-01 00:00:00 UTC. Leap seconds are not counted unless leap second support is available. This format is a GNU extension. @item %S The seconds as a decimal number (range @code{00} through @code{60}). @item %t A single @samp{\t} (tabulator) character. This format is a POSIX.2 extension. @item %T The time using decimal numbers using the format @code{%H:%M:%S}. This format is a POSIX.2 extension. @item %u The day of the week as a decimal number (range @code{1} through @code{7}), Monday being @code{1}. This format is a POSIX.2 extension. @item %U The week number of the current year as a decimal number (range @code{00} through @code{53}), starting with the first Sunday as the first day of the first week. Days preceding the first Sunday in the year are considered to be in week @code{00}. @item %V The @w{ISO 8601:1988} week number as a decimal number (range @code{01} through @code{53}). ISO weeks start with Monday and end with Sunday. Week @code{01} of a year is the first week which has the majority of its days in that year; this is equivalent to the week containing the year's first Thursday, and it is also equivalent to the week containing January 4. Week @code{01} of a year can contain days from the previous year. The week before week @code{01} of a year is the last week (@code{52} or @code{53}) of the previous year even if it contains days from the new year. This format is a POSIX.2 extension. @item %w The day of the week as a decimal number (range @code{0} through @code{6}), Sunday being @code{0}. @item %W The week number of the current year as a decimal number (range @code{00} through @code{53}), starting with the first Monday as the first day of the first week. All days preceding the first Monday in the year are considered to be in week @code{00}. @item %x The preferred date representation for the current locale, but without the time. @item %X The preferred time representation for the current locale, but with no date. @item %y The year without a century as a decimal number (range @code{00} through @code{99}). This is equivalent to the year modulo 100. @item %Y The year as a decimal number, using the Gregorian calendar. Years before the year @code{1} are numbered @code{0}, @code{-1}, and so on. @item %z @w{RFC 822}/@w{ISO 8601:1988} style numeric time zone (e.g., @code{-0600} or @code{+0100}), or nothing if no time zone is determinable. This format is a GNU extension. A full @w{RFC 822} timestamp is generated by the format @w{@samp{"%a, %d %b %Y %H:%M:%S %z"}} (or the equivalent @w{@samp{"%a, %d %b %Y %T %z"}}). @item %Z The time zone abbreviation (empty if the time zone can't be determined). @item %% A literal @samp{%} character. @end table The @var{size} parameter can be used to specify the maximum number of characters to be stored in the array @var{s}, including the terminating null character. If the formatted time requires more than @var{size} characters, @code{strftime} returns zero and the content of the array @var{s} is indetermined. Otherwise the return value indicates the number of characters placed in the array @var{s}, not including the terminating null character. @emph{Warning:} This convention for the return value which is prescribed in @w{ISO C} can lead to problems in some situations. For certain format strings and certain locales the output really can be the empty string and this cannot be discovered by testing the return value only. E.g., in most locales the AM/PM time format is not supported (most of the world uses the 24 hour time representation). In such locales @code{"%p"} will return the empty string, i.e., the return value is zero. To detect situations like this something similar to the following code should be used: @smallexample buf[0] = '\1'; len = strftime (buf, bufsize, format, tp); if (len == 0 && buf[0] != '\0') @{ /* Something went wrong in the strftime call. */ @dots{} @} @end smallexample If @var{s} is a null pointer, @code{strftime} does not actually write anything, but instead returns the number of characters it would have written. According to POSIX.1 every call to @code{strftime} implies a call to @code{tzset}. So the contents of the environment variable @code{TZ} is examined before any output is produced. For an example of @code{strftime}, see @ref{Time Functions Example}. @end deftypefun @node Parsing Date and Time @subsection Convert textual time and date information back The @w{ISO C} standard does not specify any functions which can convert the output of the @code{strftime} function back into a binary format. This lead to variety of more or less successful implementations with different interfaces over the years. Then the Unix standard got extended by two functions: @code{strptime} and @code{getdate}. Both have kind of strange interfaces but at least they are widely available. @menu * Low-Level Time String Parsing:: Interpret string according to given format. * General Time String Parsing:: User-friendly function to parse data and time strings. @end menu @node Low-Level Time String Parsing @subsubsection Interpret string according to given format The first function is a rather low-level interface. It is nevertheless frequently used in user programs since it is better known. Its implementation and the interface though is heavily influenced by the @code{getdate} function which is defined and implemented in terms of calls to @code{strptime}. @comment time.h @comment XPG4 @deftypefun {char *} strptime (const char *@var{s}, const char *@var{fmt}, struct tm *@var{tp}) The @code{strptime} function parses the input string @var{s} according to the format string @var{fmt} and stores the found values in the structure @var{tp}. The input string can be retrieved in any way. It does not matter whether it was generated by a @code{strftime} call or made up directly by a program. It is also not necessary that the content is in any human-recognizable format. I.e., it is OK if a date is written like @code{"02:1999:9"} which is not understandable without context. As long the format string @var{fmt} matches the format of the input string everything goes. The format string consists of the same components as the format string for the @code{strftime} function. The only difference is that the flags @code{_}, @code{-}, @code{0}, and @code{^} are not allowed. @comment Is this really the intention? --drepper Several of the formats which @code{strftime} handled differently do the same work in @code{strptime} since differences like case of the output do not matter. For symmetry reasons all formats are supported, though. The modifiers @code{E} and @code{O} are also allowed everywhere the @code{strftime} function allows them. The formats are: @table @code @item %a @itemx %A The weekday name according to the current locale, in abbreviated form or the full name. @item %b @itemx %B @itemx %h The month name according to the current locale, in abbreviated form or the full name. @item %c The date and time representation for the current locale. @item %Ec Like @code{%c} but the locale's alternative date and time format is used. @item %C The century of the year. It makes sense to use this format only if the format string also contains the @code{%y} format. @item %EC The locale's representation of the period. Unlike @code{%C} it makes sometimes sense to use this format since in some cultures it is required to specify years relative to periods instead of using the Gregorian years. @item %d @item %e The day of the month as a decimal number (range @code{1} through @code{31}). Leading zeroes are permitted but not required. @item %Od @itemx %Oe Same as @code{%d} but the locale's alternative numeric symbols are used. Leading zeroes are permitted but not required. @item %D Equivalent to the use of @code{%m/%d/%y} in this place. @item %F Equivalent to the use of @code{%Y-%m-%d} which is the @w{ISO 8601} date format. This is a GNU extension following an @w{ISO C 9X} extension to @code{strftime}. @item %g The year corresponding to the ISO week number, but without the century (range @code{00} through @code{99}). @emph{Note:} This is not really implemented currently. The format is recognized, input is consumed but no field in @var{tm} is set. This format is a GNU extension following a GNU extension of @code{strftime}. @item %G The year corresponding to the ISO week number. @emph{Note:} This is not really implemented currently. The format is recognized, input is consumed but no field in @var{tm} is set. This format is a GNU extension following a GNU extension of @code{strftime}. @item %H @itemx %k The hour as a decimal number, using a 24-hour clock (range @code{00} through @code{23}). @code{%k} is a GNU extension following a GNU extension of @code{strftime}. @item %OH Same as @code{%H} but using the locale's alternative numeric symbols are used. @item %I @itemx %l The hour as a decimal number, using a 12-hour clock (range @code{01} through @code{12}). @code{%l} is a GNU extension following a GNU extension of @code{strftime}. @item %OI Same as @code{%I} but using the locale's alternative numeric symbols are used. @item %j The day of the year as a decimal number (range @code{1} through @code{366}). Leading zeroes are permitted but not required. @item %m The month as a decimal number (range @code{1} through @code{12}). Leading zeroes are permitted but not required. @item %Om Same as @code{%m} but using the locale's alternative numeric symbols are used. @item %M The minute as a decimal number (range @code{0} through @code{59}). Leading zeroes are permitted but not required. @item %OM Same as @code{%M} but using the locale's alternative numeric symbols are used. @item %n @itemx %t Matches any white space. @item %p @item %P The locale-dependent equivalent to @samp{AM} or @samp{PM}. This format is not useful unless @code{%I} or @code{%l} is also used. Another complication is that the locale might not define these values at all and therefore the conversion fails. @code{%P} is a GNU extension following a GNU extension to @code{strftime}. @item %r The complete time using the AM/PM format of the current locale. A complication is that the locale might not define this format at all and therefore the conversion fails. @item %R The hour and minute in decimal numbers using the format @code{%H:%M}. @code{%R} is a GNU extension following a GNU extension to @code{strftime}. @item %s The number of seconds since the epoch, i.e., since 1970-01-01 00:00:00 UTC. Leap seconds are not counted unless leap second support is available. @code{%s} is a GNU extension following a GNU extension to @code{strftime}. @item %S The seconds as a decimal number (range @code{0} through @code{61}). Leading zeroes are permitted but not required. Please note the nonsense with @code{61} being allowed. This is what the Unix specification says. They followed the stupid decision once made to allow double leap seconds. These do not exist but the myth persists. @item %OS Same as @code{%S} but using the locale's alternative numeric symbols are used. @item %T Equivalent to the use of @code{%H:%M:%S} in this place. @item %u The day of the week as a decimal number (range @code{1} through @code{7}), Monday being @code{1}. Leading zeroes are permitted but not required. @emph{Note:} This is not really implemented currently. The format is recognized, input is consumed but no field in @var{tm} is set. @item %U The week number of the current year as a decimal number (range @code{0} through @code{53}). Leading zeroes are permitted but not required. @item %OU Same as @code{%U} but using the locale's alternative numeric symbols are used. @item %V The @w{ISO 8601:1988} week number as a decimal number (range @code{1} through @code{53}). Leading zeroes are permitted but not required. @emph{Note:} This is not really implemented currently. The format is recognized, input is consumed but no field in @var{tm} is set. @item %w The day of the week as a decimal number (range @code{0} through @code{6}), Sunday being @code{0}. Leading zeroes are permitted but not required. @emph{Note:} This is not really implemented currently. The format is recognized, input is consumed but no field in @var{tm} is set. @item %Ow Same as @code{%w} but using the locale's alternative numeric symbols are used. @item %W The week number of the current year as a decimal number (range @code{0} through @code{53}). Leading zeroes are permitted but not required. @emph{Note:} This is not really implemented currently. The format is recognized, input is consumed but no field in @var{tm} is set. @item %OW Same as @code{%W} but using the locale's alternative numeric symbols are used. @item %x The date using the locale's date format. @item %Ex Like @code{%x} but the locale's alternative data representation is used. @item %X The time using the locale's time format. @item %EX Like @code{%X} but the locale's alternative time representation is used. @item %y The year without a century as a decimal number (range @code{0} through @code{99}). Leading zeroes are permitted but not required. Please note that it is at least questionable to use this format without the @code{%C} format. The @code{strptime} function does regard input values in the range @math{68} to @math{99} as the years @math{1969} to @math{1999} and the values @math{0} to @math{68} as the years @math{2000} to @math{2068}. But maybe this heuristic fails for some input data. Therefore it is best to avoid @code{%y} completely and use @code{%Y} instead. @item %Ey The offset from @code{%EC} in the locale's alternative representation. @item %Oy The offset of the year (from @code{%C}) using the locale's alternative numeric symbols. @item %Y The year as a decimal number, using the Gregorian calendar. @item %EY The full alternative year representation. @item %z Equivalent to the use of @code{%a, %d %b %Y %H:%M:%S %z} in this place. This is the full @w{ISO 8601} date and time format. @item %Z The timezone name. @emph{Note:} This is not really implemented currently. The format is recognized, input is consumed but no field in @var{tm} is set. @item %% A literal @samp{%} character. @end table All other characters in the format string must have a matching character in the input string. Exceptions are white spaces in the input string which can match zero or more white space characters in the input string. The @code{strptime} function processes the input string from right to left. Each of the three possible input elements (white space, literal, or format) are handled one after the other. If the input cannot be matched to the format string the function stops. The remainder of the format and input strings are not processed. The return value of the function is a pointer to the first character not processed in this function call. In the case of an error the return value points to the first character not matched. In case the input string contains more than required by the format string the return value points right after the last consumed input character. In case the whole input string is consumed the return value points to the NUL byte at the end of the string. @end deftypefun The specification of the function in the XPG standard is rather vague. It leaves out a few important pieces of information. Most important it does not specify what happens to those elements of @var{tm} which are not directly initialized by the different formats. Various implementations on different Unix systems vary here. The GNU libc implementation does not touch those fields which are not directly initialized. Exceptions are the @code{tm_wday} and @code{tm_yday} elements which are recomputed if any of the year, month, or date elements changed. This has two implications: @itemize @bullet @item Before calling the @code{strptime} function for a new input string one has to prepare the structure passed in as the @var{tm}. Normally this will mean that all values are initialized to zero. Alternatively one can use all fields to values like @code{INT_MAX} which allows to determine which elements were set by the function call. Zero does not work here since it is a valid value for many of the fields. Careful initialization is necessary if one wants to find out whether a certain field in @var{tm} was initialized by the function call. @item One can construct a @code{struct tm} value in several @code{strptime} calls in a row. A useful application of this is for example the parsing of two separate strings, one containing the date information, the other the time information. By parsing both one after the other without clearing the structure in between one can construct a complete broken-down time. @end itemize The following example shows a function which parses a string which is supposed to contain the date information in either US style or @w{ISO 8601} form. @smallexample const char * parse_date (const char *input, struct tm *tm) @{ const char *cp; /* @r{First clear the result structure.} */ memset (tm, '\0', sizeof (*tm)); /* @r{Try the ISO format first.} */ cp = strptime (input, "%F", tm); if (cp == NULL) @{ /* @r{Does not match. Try the US form.} */ cp = strptime (input, "%D", tm); @} return cp; @} @end smallexample @node General Time String Parsing @subsubsection A user-friendlier way to parse times and dates The Unix standard defines another function to parse date strings. The interface is, mildly said, weird. But if this function fits into the application to be written it is just fine. It is a problem when using this function in multi-threaded programs or in libraries since it returns a pointer to a static variable, uses a global variable, and a global state (an environment variable). @comment time.h @comment Unix98 @defvar getdate_err This variable of type @code{int} will contain the error code of the last unsuccessful call of the @code{getdate} function. Defined values are: @table @math @item 1 The environment variable @code{DATEMSK} is not defined or null. @item 2 The template file denoted by the @code{DATEMSK} environment variable cannot be opened. @item 3 Information about the template file cannot retrieved. @item 4 The template file is no regular file. @item 5 An I/O error occurred while reading the template file. @item 6 Not enough memory available to execute the function. @item 7 The template file contains no matching template. @item 8 The input string is invalid for a template which would match otherwise. This includes error like February 31st, or return values which can be represented using @code{time_t}. @end table @end defvar @comment time.h @comment Unix98 @deftypefun {struct tm *} getdate (const char *@var{string}) The interface of the @code{getdate} function is the simplest possible for a function to parse a string and return the value. @var{string} is the input string and the result is passed to the user in a statically allocated variable. The details about how the string is processed is hidden from the user. In fact, it can be outside the control of the program. Which formats are recognized is controlled by the file named by the environment variable @code{DATEMSK}. The content of the named file should contain lines of valid format strings which could be passed to @code{strptime}. The @code{getdate} function reads these format strings one after the other and tries to match the input string. The first line which completely matches the input string is used. Elements which were not initialized through the format string get assigned the values of the time the @code{getdate} function is called. The format elements recognized by @code{getdate} are the same as for @code{strptime}. See above for an explanation. There are only a few extension to the @code{strptime} behavior: @itemize @bullet @item If the @code{%Z} format is given the broken-down time is based on the current time in the timezone matched, not in the current timezone of the runtime environment. @emph{Note}: This is not implemented (currently). The problem is that timezone names are not unique. If a fixed timezone is assumed for a given string (say @code{EST} meaning US East Coast time) uses for countries other than the USA will fail. So far we have found no good solution for this. @item If only the weekday is specified the selected day depends on the current date. If the current weekday is greater or equal to the @code{tm_wday} value this weeks day is selected. Otherwise next weeks day. @item A similar heuristic is used if only the month is given, not the year. For value corresponding to the current or a later month the current year s used. Otherwise the next year. The first day of the month is assumed if it is not explicitly specified. @item The current hour, minute, and second is used if the appropriate value is not set through the format. @item If no date is given the date for the next day is used if the time is smaller than the current time. Otherwise it is the same day. @end itemize It should be noted that the format in the template file need not only contain format elements. The following is a list of possible format strings (taken from the Unix standard): @smallexample %m %A %B %d, %Y %H:%M:%S %A %B %m/%d/%y %I %p %d,%m,%Y %H:%M at %A the %dst of %B in %Y run job at %I %p,%B %dnd %A den %d. %B %Y %H.%M Uhr @end smallexample As one can see the template list can contain very specific strings like @code{run job at %I %p,%B %dnd}. Using the above list of templates and assuming the current time is Mon Sep 22 12:19:47 EDT 1986 we can get the The results for the given input. @multitable {xxxxxxxxxxxx} {xxxxxxxx} {xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx} @item Mon @tab %a @tab Mon Sep 22 12:19:47 EDT 1986 @item Sun @tab %a @tab Sun Sep 28 12:19:47 EDT 1986 @item Fri @tab %a @tab Fri Sep 26 12:19:47 EDT 1986 @item September @tab %B @tab Mon Sep 1 12:19:47 EDT 1986 @item January @tab %B @tab Thu Jan 1 12:19:47 EST 1987 @item December @tab %B @tab Mon Dec 1 12:19:47 EST 1986 @item Sep Mon @tab %b %a @tab Mon Sep 1 12:19:47 EDT 1986 @item Jan Fri @tab %b %a @tab Fri Jan 2 12:19:47 EST 1987 @item Dec Mon @tab %b %a @tab Mon Dec 1 12:19:47 EST 1986 @item Jan Wed 1989 @tab %b %a %Y @tab Wed Jan 4 12:19:47 EST 1989 @item Fri 9 @tab %a %H @tab Fri Sep 26 09:00:00 EDT 1986 @item Feb 10:30 @tab %b %H:%S @tab Sun Feb 1 10:00:30 EST 1987 @item 10:30 @tab %H:%M @tab Tue Sep 23 10:30:00 EDT 1986 @item 13:30 @tab %H:%M @tab Mon Sep 22 13:30:00 EDT 1986 @end multitable The return value of the function is a pointer to a static variable of type @w{@code{struct tm}} or a null pointer if an error occurred. The result in the variable pointed to by the return value is only valid until the next @code{getdate} call which makes this function unusable in multi-threaded applications. The @code{errno} variable is @emph{not} changed. Error conditions are signalled using the global variable @code{getdate_err}. See the description above for a list of the possible error values. @emph{Warning:} The @code{getdate} function should @emph{never} be used in SUID-programs. The reason is obvious: using the @code{DATEMSK} environment variable one can get the function to open any arbitrary file and changes are high that with some bogus input (such as a binary file) the program will crash. @end deftypefun @comment time.h @comment GNU @deftypefun int getdate_r (const char *@var{string}, struct tm *@var{tp}) The @code{getdate_r} function is the reentrant counterpart of @code{getdate}. It does not use the global variable @code{getdate_err} to signal the error but instead the return value now is this error code. The same error codes as described in the @code{getdate_err} documentation above are used. @code{getdate_r} also does not store the broken-down time in a static variable. Instead it takes an second argument which must be a pointer to a variable of type @code{struct tm} where the broken-down can be stored. This function is not defined in the Unix standard. Nevertheless it is available on some other Unix systems as well. As for @code{getdate} the warning for using this function in SUID-programs applies to @code{getdate_r} as well. @end deftypefun @node TZ Variable @subsection Specifying the Time Zone with @code{TZ} In POSIX systems, a user can specify the time zone by means of the @code{TZ} environment variable. For information about how to set environment variables, see @ref{Environment Variables}. The functions for accessing the time zone are declared in @file{time.h}. @pindex time.h @cindex time zone You should not normally need to set @code{TZ}. If the system is configured properly, the default time zone will be correct. You might set @code{TZ} if you are using a computer over the network from a different time zone, and would like times reported to you in the time zone that local for you, rather than what is local for the computer. In POSIX.1 systems the value of the @code{TZ} variable can be of one of three formats. With the GNU C library, the most common format is the last one, which can specify a selection from a large database of time zone information for many regions of the world. The first two formats are used to describe the time zone information directly, which is both more cumbersome and less precise. But the POSIX.1 standard only specifies the details of the first two formats, so it is good to be familiar with them in case you come across a POSIX.1 system that doesn't support a time zone information database. The first format is used when there is no Daylight Saving Time (or summer time) in the local time zone: @smallexample @r{@var{std} @var{offset}} @end smallexample The @var{std} string specifies the name of the time zone. It must be three or more characters long and must not contain a leading colon or embedded digits, commas, or plus or minus signs. There is no space character separating the time zone name from the @var{offset}, so these restrictions are necessary to parse the specification correctly. The @var{offset} specifies the time value one must add to the local time to get a Coordinated Universal Time value. It has syntax like [@code{+}|@code{-}]@var{hh}[@code{:}@var{mm}[@code{:}@var{ss}]]. This is positive if the local time zone is west of the Prime Meridian and negative if it is east. The hour must be between @code{0} and @code{23}, and the minute and seconds between @code{0} and @code{59}. For example, here is how we would specify Eastern Standard Time, but without any daylight saving time alternative: @smallexample EST+5 @end smallexample The second format is used when there is Daylight Saving Time: @smallexample @r{@var{std} @var{offset} @var{dst} [@var{offset}]@code{,}@var{start}[@code{/}@var{time}]@code{,}@var{end}[@code{/}@var{time}]} @end smallexample The initial @var{std} and @var{offset} specify the standard time zone, as described above. The @var{dst} string and @var{offset} specify the name and offset for the corresponding daylight saving time zone; if the @var{offset} is omitted, it defaults to one hour ahead of standard time. The remainder of the specification describes when daylight saving time is in effect. The @var{start} field is when daylight saving time goes into effect and the @var{end} field is when the change is made back to standard time. The following formats are recognized for these fields: @table @code @item J@var{n} This specifies the Julian day, with @var{n} between @code{1} and @code{365}. February 29 is never counted, even in leap years. @item @var{n} This specifies the Julian day, with @var{n} between @code{0} and @code{365}. February 29 is counted in leap years. @item M@var{m}.@var{w}.@var{d} This specifies day @var{d} of week @var{w} of month @var{m}. The day @var{d} must be between @code{0} (Sunday) and @code{6}. The week @var{w} must be between @code{1} and @code{5}; week @code{1} is the first week in which day @var{d} occurs, and week @code{5} specifies the @emph{last} @var{d} day in the month. The month @var{m} should be between @code{1} and @code{12}. @end table The @var{time} fields specify when, in the local time currently in effect, the change to the other time occurs. If omitted, the default is @code{02:00:00}. For example, here is how one would specify the Eastern time zone in the United States, including the appropriate daylight saving time and its dates of applicability. The normal offset from UTC is 5 hours; since this is west of the prime meridian, the sign is positive. Summer time begins on the first Sunday in April at 2:00am, and ends on the last Sunday in October at 2:00am. @smallexample EST+5EDT,M4.1.0/2,M10.5.0/2 @end smallexample The schedule of daylight saving time in any particular jurisdiction has changed over the years. To be strictly correct, the conversion of dates and times in the past should be based on the schedule that was in effect then. However, this format has no facilities to let you specify how the schedule has changed from year to year. The most you can do is specify one particular schedule---usually the present day schedule---and this is used to convert any date, no matter when. For precise time zone specifications, it is best to use the time zone information database (see below). The third format looks like this: @smallexample :@var{characters} @end smallexample Each operating system interprets this format differently; in the GNU C library, @var{characters} is the name of a file which describes the time zone. @pindex /etc/localtime @pindex localtime If the @code{TZ} environment variable does not have a value, the operation chooses a time zone by default. In the GNU C library, the default time zone is like the specification @samp{TZ=:/etc/localtime} (or @samp{TZ=:/usr/local/etc/localtime}, depending on how GNU C library was configured; @pxref{Installation}). Other C libraries use their own rule for choosing the default time zone, so there is little we can say about them. @cindex time zone database @pindex /share/lib/zoneinfo @pindex zoneinfo If @var{characters} begins with a slash, it is an absolute file name; otherwise the library looks for the file @w{@file{/share/lib/zoneinfo/@var{characters}}}. The @file{zoneinfo} directory contains data files describing local time zones in many different parts of the world. The names represent major cities, with subdirectories for geographical areas; for example, @file{America/New_York}, @file{Europe/London}, @file{Asia/Hong_Kong}. These data files are installed by the system administrator, who also sets @file{/etc/localtime} to point to the data file for the local time zone. The GNU C library comes with a large database of time zone information for most regions of the world, which is maintained by a community of volunteers and put in the public domain. @node Time Zone Functions @subsection Functions and Variables for Time Zones @comment time.h @comment POSIX.1 @deftypevar {char *} tzname [2] The array @code{tzname} contains two strings, which are the standard names of the pair of time zones (standard and daylight saving) that the user has selected. @code{tzname[0]} is the name of the standard time zone (for example, @code{"EST"}), and @code{tzname[1]} is the name for the time zone when daylight saving time is in use (for example, @code{"EDT"}). These correspond to the @var{std} and @var{dst} strings (respectively) from the @code{TZ} environment variable. If daylight saving time is never used, @code{tzname[1]} is the empty string. The @code{tzname} array is initialized from the @code{TZ} environment variable whenever @code{tzset}, @code{ctime}, @code{strftime}, @code{mktime}, or @code{localtime} is called. If multiple abbreviations have been used (e.g. @code{"EWT"} and @code{"EDT"} for U.S. Eastern War Time and Eastern Daylight Time), the array contains the most recent abbreviation. The @code{tzname} array is required for POSIX.1 compatibility, but in GNU programs it is better to use the @code{tm_zone} member of the broken-down time structure, since @code{tm_zone} reports the correct abbreviation even when it is not the latest one. Though the strings are declared as @code{char *} the user must stay away from modifying these strings. Modifying the strings will almost certainly lead to trouble. @end deftypevar @comment time.h @comment POSIX.1 @deftypefun void tzset (void) The @code{tzset} function initializes the @code{tzname} variable from the value of the @code{TZ} environment variable. It is not usually necessary for your program to call this function, because it is called automatically when you use the other time conversion functions that depend on the time zone. @end deftypefun The following variables are defined for compatibility with System V Unix. Like @code{tzname}, these variables are set by calling @code{tzset} or the other time conversion functions. @comment time.h @comment SVID @deftypevar {long int} timezone This contains the difference between UTC and the latest local standard time, in seconds west of UTC. For example, in the U.S. Eastern time zone, the value is @code{5*60*60}. Unlike the @code{tm_gmtoff} member of the broken-down time structure, this value is not adjusted for daylight saving, and its sign is reversed. In GNU programs it is better to use @code{tm_gmtoff}, since it contains the correct offset even when it is not the latest one. @end deftypevar @comment time.h @comment SVID @deftypevar int daylight This variable has a nonzero value if daylight savings time rules apply. A nonzero value does not necessarily mean that daylight savings time is now in effect; it means only that daylight savings time is sometimes in effect. @end deftypevar @node Time Functions Example @subsection Time Functions Example Here is an example program showing the use of some of the local time and calendar time functions. @smallexample @include strftim.c.texi @end smallexample It produces output like this: @smallexample Wed Jul 31 13:02:36 1991 Today is Wednesday, July 31. The time is 01:02 PM. @end smallexample @node Precision Time @section Precision Time @cindex time, high precision @pindex sys/timex.h The @code{net_gettime} and @code{ntp_adjtime} functions provide an interface to monitor and manipulate high precision time. These functions are declared in @file{sys/timex.h}. @tindex struct ntptimeval @deftp {Data Type} {struct ntptimeval} This structure is used to monitor kernel time. It contains the following members: @table @code @item struct timeval time This is the current time. The @code{struct timeval} data type is described in @ref{High-Resolution Calendar}. @item long int maxerror This is the maximum error, measured in microseconds. Unless updated via @code{ntp_adjtime} periodically, this value will reach some platform-specific maximum value. @item long int esterror This is the estimated error, measured in microseconds. This value can be set by @code{ntp_adjtime} to indicate the estimated offset of the local clock against the true time. @end table @end deftp @comment sys/timex,h @comment GNU @deftypefun int ntp_gettime (struct ntptimeval *@var{tptr}) The @code{ntp_gettime} function sets the structure pointed to by @var{tptr} to current values. The elements of the structure afterwards contain the values the timer implementation in the kernel assumes. They might or might not be correct. If they are not a @code{ntp_adjtime} call is necessary. The return value is @code{0} on success and other values on failure. The following @code{errno} error conditions are defined for this function: @table @code @item TIME_ERROR The precision clock model is not properly set up at the moment, thus the clock must be considered unsynchronized, and the values should be treated with care. @end table @end deftypefun @tindex struct timex @deftp {Data Type} {struct timex} This structure is used to control and monitor kernel time in a greater level of detail. It contains the following members: @table @code @item unsigned int modes This variable controls whether and which values are set. Several symbolic constants have to be combined with @emph{binary or} to specify the effective mode. These constants start with @code{MOD_}. @item long int offset This value indicates the current offset of the local clock from the true time. The value is given in microseconds. If bit @code{MOD_OFFSET} is set in @code{modes}, the offset (and possibly other dependent values) can be set. The offset's absolute value must not exceed @code{MAXPHASE}. @item long int frequency This value indicates the difference in frequency between the true time and the local clock. The value is expressed as scaled PPM (parts per million, 0.0001%). The scaling is @code{1 << SHIFT_USEC}. The value can be set with bit @code{MOD_FREQUENCY}, but the absolute value must not exceed @code{MAXFREQ}. @item long int maxerror This is the maximum error, measured in microseconds. A new value can be set using bit @code{MOD_MAXERROR}. Unless updated via @code{ntp_adjtime} periodically, this value will increase steadily and reach some platform-specific maximum value. @item long int esterror This is the estimated error, measured in microseconds. This value can be set using bit @code{MOD_ESTERROR}. @item int status This valiable reflects the various states of the clock machinery. There are symbolic constants for the significant bits, starting with @code{STA_}. Some of these flags can be updated using the @code{MOD_STATUS} bit. @item long int constant This value represents the bandwidth or stiffness of the PLL (phase locked loop) implemented in the kernel. The value can be changed using bit @code{MOD_TIMECONST}. @item long int precision This value represents the accuracy or the maximum error when reading the system clock. The value is expressed in microseconds and can't be changed. @item long int tolerance This value represents the maximum frequency error of the system clock in scaled PPM. This value is used to increase the @code{maxerror} every second. @item long int ppsfreq This is the first of a few optional variables that are present only if the system clock can use a PPS (pulse per second) signal to discipline the local clock. The value is expressed in scaled PPM and it denotes the difference in frequency between the local clock and the PPS signal. @item long int jitter This value expresses a median filtered average of the PPS signal's dispersion in microseconds. @item int int shift This value is a binary exponent for the duration of the PPS calibration interval, ranging from @code{PPS_SHIFT} to @code{PPS_SHIFTMAX}. @item long int stabil This value represents the median filtered dispersion of the PPS frequency in scaled PPM. @item long int jitcnt This counter represents the numer of pulses where the jitter exceeded the allowed maximum @code{MAXTIME}. @item long int calcnt This counter reflects the number of successful calibration intervals. @item long int errcnt This counter represents the number of calibration errors (caused by large offsets or jitter). @item long int stbcnt This counter denotes the number of of calibrations where the stability exceeded the threshold. @end table @end deftp @comment sys/timex.h @comment GNU @deftypefun int ntp_adjtime (struct timex *@var{tptr}) The @code{ntp_adjtime} function sets the structure specified by @var{tptr} to current values. In addition, values passed in @var{tptr} can be used to replace existing settings. To do this the @code{modes} element of the @code{struct timex} must be set appropriately. Setting it to zero selects reading the current state. The return value is @code{0} on success and other values on failure. The following @code{errno} error conditions are defined for this function: @table @code @item TIME_ERROR The precision clock model is not properly set up at the moment, thus the clock must be considered unsynchronized, and the values should be treated with care. Another reason could be that the specified new values are not allowed. @end table For more details see RFC1305 (Network Time Protocol, Version 3) and related documents. @end deftypefun @node Setting an Alarm @section Setting an Alarm The @code{alarm} and @code{setitimer} functions provide a mechanism for a process to interrupt itself at some future time. They do this by setting a timer; when the timer expires, the process receives a signal. @cindex setting an alarm @cindex interval timer, setting @cindex alarms, setting @cindex timers, setting Each process has three independent interval timers available: @itemize @bullet @item A real-time timer that counts clock time. This timer sends a @code{SIGALRM} signal to the process when it expires. @cindex real-time timer @cindex timer, real-time @item A virtual timer that counts CPU time used by the process. This timer sends a @code{SIGVTALRM} signal to the process when it expires. @cindex virtual timer @cindex timer, virtual @item A profiling timer that counts both CPU time used by the process, and CPU time spent in system calls on behalf of the process. This timer sends a @code{SIGPROF} signal to the process when it expires. @cindex profiling timer @cindex timer, profiling This timer is useful for profiling in interpreters. The interval timer mechanism does not have the fine granularity necessary for profiling native code. @c @xref{profil} !!! @end itemize You can only have one timer of each kind set at any given time. If you set a timer that has not yet expired, that timer is simply reset to the new value. You should establish a handler for the appropriate alarm signal using @code{signal} or @code{sigaction} before issuing a call to @code{setitimer} or @code{alarm}. Otherwise, an unusual chain of events could cause the timer to expire before your program establishes the handler, and in that case it would be terminated, since that is the default action for the alarm signals. @xref{Signal Handling}. The @code{setitimer} function is the primary means for setting an alarm. This facility is declared in the header file @file{sys/time.h}. The @code{alarm} function, declared in @file{unistd.h}, provides a somewhat simpler interface for setting the real-time timer. @pindex unistd.h @pindex sys/time.h @comment sys/time.h @comment BSD @deftp {Data Type} {struct itimerval} This structure is used to specify when a timer should expire. It contains the following members: @table @code @item struct timeval it_interval This is the interval between successive timer interrupts. If zero, the alarm will only be sent once. @item struct timeval it_value This is the interval to the first timer interrupt. If zero, the alarm is disabled. @end table The @code{struct timeval} data type is described in @ref{High-Resolution Calendar}. @end deftp @comment sys/time.h @comment BSD @deftypefun int setitimer (int @var{which}, struct itimerval *@var{new}, struct itimerval *@var{old}) The @code{setitimer} function sets the timer specified by @var{which} according to @var{new}. The @var{which} argument can have a value of @code{ITIMER_REAL}, @code{ITIMER_VIRTUAL}, or @code{ITIMER_PROF}. If @var{old} is not a null pointer, @code{setitimer} returns information about any previous unexpired timer of the same kind in the structure it points to. The return value is @code{0} on success and @code{-1} on failure. The following @code{errno} error conditions are defined for this function: @table @code @item EINVAL The timer interval was too large. @end table @end deftypefun @comment sys/time.h @comment BSD @deftypefun int getitimer (int @var{which}, struct itimerval *@var{old}) The @code{getitimer} function stores information about the timer specified by @var{which} in the structure pointed at by @var{old}. The return value and error conditions are the same as for @code{setitimer}. @end deftypefun @comment sys/time.h @comment BSD @table @code @item ITIMER_REAL @findex ITIMER_REAL This constant can be used as the @var{which} argument to the @code{setitimer} and @code{getitimer} functions to specify the real-time timer. @comment sys/time.h @comment BSD @item ITIMER_VIRTUAL @findex ITIMER_VIRTUAL This constant can be used as the @var{which} argument to the @code{setitimer} and @code{getitimer} functions to specify the virtual timer. @comment sys/time.h @comment BSD @item ITIMER_PROF @findex ITIMER_PROF This constant can be used as the @var{which} argument to the @code{setitimer} and @code{getitimer} functions to specify the profiling timer. @end table @comment unistd.h @comment POSIX.1 @deftypefun {unsigned int} alarm (unsigned int @var{seconds}) The @code{alarm} function sets the real-time timer to expire in @var{seconds} seconds. If you want to cancel any existing alarm, you can do this by calling @code{alarm} with a @var{seconds} argument of zero. The return value indicates how many seconds remain before the previous alarm would have been sent. If there is no previous alarm, @code{alarm} returns zero. @end deftypefun The @code{alarm} function could be defined in terms of @code{setitimer} like this: @smallexample unsigned int alarm (unsigned int seconds) @{ struct itimerval old, new; new.it_interval.tv_usec = 0; new.it_interval.tv_sec = 0; new.it_value.tv_usec = 0; new.it_value.tv_sec = (long int) seconds; if (setitimer (ITIMER_REAL, &new, &old) < 0) return 0; else return old.it_value.tv_sec; @} @end smallexample There is an example showing the use of the @code{alarm} function in @ref{Handler Returns}. If you simply want your process to wait for a given number of seconds, you should use the @code{sleep} function. @xref{Sleeping}. You shouldn't count on the signal arriving precisely when the timer expires. In a multiprocessing environment there is typically some amount of delay involved. @strong{Portability Note:} The @code{setitimer} and @code{getitimer} functions are derived from BSD Unix, while the @code{alarm} function is specified by the POSIX.1 standard. @code{setitimer} is more powerful than @code{alarm}, but @code{alarm} is more widely used. @node Sleeping @section Sleeping The function @code{sleep} gives a simple way to make the program wait for short periods of time. If your program doesn't use signals (except to terminate), then you can expect @code{sleep} to wait reliably for the specified amount of time. Otherwise, @code{sleep} can return sooner if a signal arrives; if you want to wait for a given period regardless of signals, use @code{select} (@pxref{Waiting for I/O}) and don't specify any descriptors to wait for. @c !!! select can get EINTR; using SA_RESTART makes sleep win too. @comment unistd.h @comment POSIX.1 @deftypefun {unsigned int} sleep (unsigned int @var{seconds}) The @code{sleep} function waits for @var{seconds} or until a signal is delivered, whichever happens first. If @code{sleep} function returns because the requested time has elapsed, it returns a value of zero. If it returns because of delivery of a signal, its return value is the remaining time in the sleep period. The @code{sleep} function is declared in @file{unistd.h}. @end deftypefun Resist the temptation to implement a sleep for a fixed amount of time by using the return value of @code{sleep}, when nonzero, to call @code{sleep} again. This will work with a certain amount of accuracy as long as signals arrive infrequently. But each signal can cause the eventual wakeup time to be off by an additional second or so. Suppose a few signals happen to arrive in rapid succession by bad luck---there is no limit on how much this could shorten or lengthen the wait. Instead, compute the time at which the program should stop waiting, and keep trying to wait until that time. This won't be off by more than a second. With just a little more work, you can use @code{select} and make the waiting period quite accurate. (Of course, heavy system load can cause unavoidable additional delays---unless the machine is dedicated to one application, there is no way you can avoid this.) On some systems, @code{sleep} can do strange things if your program uses @code{SIGALRM} explicitly. Even if @code{SIGALRM} signals are being ignored or blocked when @code{sleep} is called, @code{sleep} might return prematurely on delivery of a @code{SIGALRM} signal. If you have established a handler for @code{SIGALRM} signals and a @code{SIGALRM} signal is delivered while the process is sleeping, the action taken might be just to cause @code{sleep} to return instead of invoking your handler. And, if @code{sleep} is interrupted by delivery of a signal whose handler requests an alarm or alters the handling of @code{SIGALRM}, this handler and @code{sleep} will interfere. On the GNU system, it is safe to use @code{sleep} and @code{SIGALRM} in the same program, because @code{sleep} does not work by means of @code{SIGALRM}. @comment time.h @comment POSIX.1 @deftypefun int nanosleep (const struct timespec *@var{requested_time}, struct timespec *@var{remaining}) If the resolution of seconds is not enough the @code{nanosleep} function can be used. As the name suggests the sleeping period can be specified in nanoseconds. The actual period of waiting time might be longer since the requested time in the @var{requested_time} parameter is rounded up to the next integer multiple of the actual resolution of the system. If the function returns because the time has elapsed the return value is zero. If the function return @math{-1} the global variable @var{errno} is set to the following values: @table @code @item EINTR The call was interrupted because a signal was delivered to the thread. If the @var{remaining} parameter is not the null pointer the structure pointed to by @var{remaining} is updated to contain the remaining time. @item EINVAL The nanosecond value in the @var{requested_time} parameter contains an illegal value. Either the value is negative or greater than or equal to 1000 million. @end table This function is a cancelation point in multi-threaded programs. This is a problem if the thread allocates some resources (like memory, file descriptors, semaphores or whatever) at the time @code{nanosleep} is called. If the thread gets canceled these resources stay allocated until the program ends. To avoid this calls to @code{nanosleep} should be protected using cancelation handlers. @c ref pthread_cleanup_push / pthread_cleanup_pop The @code{nanosleep} function is declared in @file{time.h}. @end deftypefun @node Resource Usage @section Resource Usage @pindex sys/resource.h The function @code{getrusage} and the data type @code{struct rusage} are used for examining the usage figures of a process. They are declared in @file{sys/resource.h}. @comment sys/resource.h @comment BSD @deftypefun int getrusage (int @var{processes}, struct rusage *@var{rusage}) This function reports the usage totals for processes specified by @var{processes}, storing the information in @code{*@var{rusage}}. In most systems, @var{processes} has only two valid values: @table @code @comment sys/resource.h @comment BSD @item RUSAGE_SELF Just the current process. @comment sys/resource.h @comment BSD @item RUSAGE_CHILDREN All child processes (direct and indirect) that have terminated already. @end table In the GNU system, you can also inquire about a particular child process by specifying its process ID. The return value of @code{getrusage} is zero for success, and @code{-1} for failure. @table @code @item EINVAL The argument @var{processes} is not valid. @end table @end deftypefun One way of getting usage figures for a particular child process is with the function @code{wait4}, which returns totals for a child when it terminates. @xref{BSD Wait Functions}. @comment sys/resource.h @comment BSD @deftp {Data Type} {struct rusage} This data type records a collection usage amounts for various sorts of resources. It has the following members, and possibly others: @table @code @item struct timeval ru_utime Time spent executing user instructions. @item struct timeval ru_stime Time spent in operating system code on behalf of @var{processes}. @item long int ru_maxrss The maximum resident set size used, in kilobytes. That is, the maximum number of kilobytes that @var{processes} used in real memory simultaneously. @item long int ru_ixrss An integral value expressed in kilobytes times ticks of execution, which indicates the amount of memory used by text that was shared with other processes. @item long int ru_idrss An integral value expressed the same way, which is the amount of unshared memory used in data. @item long int ru_isrss An integral value expressed the same way, which is the amount of unshared memory used in stack space. @item long int ru_minflt The number of page faults which were serviced without requiring any I/O. @item long int ru_majflt The number of page faults which were serviced by doing I/O. @item long int ru_nswap The number of times @var{processes} was swapped entirely out of main memory. @item long int ru_inblock The number of times the file system had to read from the disk on behalf of @var{processes}. @item long int ru_oublock The number of times the file system had to write to the disk on behalf of @var{processes}. @item long int ru_msgsnd Number of IPC messages sent. @item long ru_msgrcv Number of IPC messages received. @item long int ru_nsignals Number of signals received. @item long int ru_nvcsw The number of times @var{processes} voluntarily invoked a context switch (usually to wait for some service). @item long int ru_nivcsw The number of times an involuntary context switch took place (because the time slice expired, or another process of higher priority became runnable). @end table @end deftp An additional historical function for examining usage figures, @code{vtimes}, is supported but not documented here. It is declared in @file{sys/vtimes.h}. @node Limits on Resources @section Limiting Resource Usage @cindex resource limits @cindex limits on resource usage @cindex usage limits You can specify limits for the resource usage of a process. When the process tries to exceed a limit, it may get a signal, or the system call by which it tried to do so may fail, depending on the limit. Each process initially inherits its limit values from its parent, but it can subsequently change them. @pindex sys/resource.h The symbols in this section are defined in @file{sys/resource.h}. @comment sys/resource.h @comment BSD @deftypefun int getrlimit (int @var{resource}, struct rlimit *@var{rlp}) Read the current value and the maximum value of resource @var{resource} and store them in @code{*@var{rlp}}. The return value is @code{0} on success and @code{-1} on failure. The only possible @code{errno} error condition is @code{EFAULT}. When the sources are compiled with @code{_FILE_OFFSET_BITS == 64} on a 32 bits system this function is in fact @code{getrlimit64}. I.e., the LFS interface transparently replaces the old interface. @end deftypefun @comment sys/resource.h @comment Unix98 @deftypefun int getrlimit64 (int @var{resource}, struct rlimit64 *@var{rlp}) This function is similar to the @code{getrlimit} but its second parameter is a pointer to a variable of type @code{struct rlimit64} which allows this function to read values which wouldn't fit in the member of a @code{struct rlimit}. If the sources are compiled with @code{_FILE_OFFSET_BITS == 64} on a 32 bits machine this function is available under the name @code{getrlimit} and so transparently replaces the old interface. @end deftypefun @comment sys/resource.h @comment BSD @deftypefun int setrlimit (int @var{resource}, const struct rlimit *@var{rlp}) Store the current value and the maximum value of resource @var{resource} in @code{*@var{rlp}}. The return value is @code{0} on success and @code{-1} on failure. The following @code{errno} error condition is possible: @table @code @item EPERM You tried to change the maximum permissible limit value, but you don't have privileges to do so. @end table When the sources are compiled with @code{_FILE_OFFSET_BITS == 64} on a 32 bits system this function is in fact @code{setrlimit64}. I.e., the LFS interface transparently replaces the old interface. @end deftypefun @comment sys/resource.h @comment Unix98 @deftypefun int setrlimit64 (int @var{resource}, const struct rlimit64 *@var{rlp}) This function is similar to the @code{setrlimit} but its second parameter is a pointer to a variable of type @code{struct rlimit64} which allows this function to set values which wouldn't fit in the member of a @code{struct rlimit}. If the sources are compiled with @code{_FILE_OFFSET_BITS == 64} on a 32 bits machine this function is available under the name @code{setrlimit} and so transparently replaces the old interface. @end deftypefun @comment sys/resource.h @comment BSD @deftp {Data Type} {struct rlimit} This structure is used with @code{getrlimit} to receive limit values, and with @code{setrlimit} to specify limit values. It has two fields: @table @code @item rlim_t rlim_cur The current value of the limit in question. This is also called the ``soft limit''. @cindex soft limit @item rlim_t rlim_max The maximum permissible value of the limit in question. You cannot set the current value of the limit to a larger number than this maximum. Only the super user can change the maximum permissible value. This is also called the ``hard limit''. @cindex hard limit @end table In @code{getrlimit}, the structure is an output; it receives the current values. In @code{setrlimit}, it specifies the new values. @end deftp For the LFS functions a similar type is defined in @file{sys/resource.h}. @comment sys/resource.h @comment Unix98 @deftp {Data Type} {struct rlimit64} This structure is used with @code{getrlimit64} to receive limit values, and with @code{setrlimit64} to specify limit values. It has two fields: @table @code @item rlim64_t rlim_cur The current value of the limit in question. This is also called the ``soft limit''. @item rlim64_t rlim_max The maximum permissible value of the limit in question. You cannot set the current value of the limit to a larger number than this maximum. Only the super user can change the maximum permissible value. This is also called the ``hard limit''. @end table In @code{getrlimit64}, the structure is an output; it receives the current values. In @code{setrlimit64}, it specifies the new values. @end deftp Here is a list of resources that you can specify a limit for. Those that are sizes are measured in bytes. @table @code @comment sys/resource.h @comment BSD @item RLIMIT_CPU @vindex RLIMIT_CPU The maximum amount of cpu time the process can use. If it runs for longer than this, it gets a signal: @code{SIGXCPU}. The value is measured in seconds. @xref{Operation Error Signals}. @comment sys/resource.h @comment BSD @item RLIMIT_FSIZE @vindex RLIMIT_FSIZE The maximum size of file the process can create. Trying to write a larger file causes a signal: @code{SIGXFSZ}. @xref{Operation Error Signals}. @comment sys/resource.h @comment BSD @item RLIMIT_DATA @vindex RLIMIT_DATA The maximum size of data memory for the process. If the process tries to allocate data memory beyond this amount, the allocation function fails. @comment sys/resource.h @comment BSD @item RLIMIT_STACK @vindex RLIMIT_STACK The maximum stack size for the process. If the process tries to extend its stack past this size, it gets a @code{SIGSEGV} signal. @xref{Program Error Signals}. @comment sys/resource.h @comment BSD @item RLIMIT_CORE @vindex RLIMIT_CORE The maximum size core file that this process can create. If the process terminates and would dump a core file larger than this maximum size, then no core file is created. So setting this limit to zero prevents core files from ever being created. @comment sys/resource.h @comment BSD @item RLIMIT_RSS @vindex RLIMIT_RSS The maximum amount of physical memory that this process should get. This parameter is a guide for the system's scheduler and memory allocator; the system may give the process more memory when there is a surplus. @comment sys/resource.h @comment BSD @item RLIMIT_MEMLOCK The maximum amount of memory that can be locked into physical memory (so it will never be paged out). @comment sys/resource.h @comment BSD @item RLIMIT_NPROC The maximum number of processes that can be created with the same user ID. If you have reached the limit for your user ID, @code{fork} will fail with @code{EAGAIN}. @xref{Creating a Process}. @comment sys/resource.h @comment BSD @item RLIMIT_NOFILE @vindex RLIMIT_NOFILE @itemx RLIMIT_OFILE @vindex RLIMIT_OFILE The maximum number of files that the process can open. If it tries to open more files than this, it gets error code @code{EMFILE}. @xref{Error Codes}. Not all systems support this limit; GNU does, and 4.4 BSD does. @comment sys/resource.h @comment BSD @item RLIM_NLIMITS @vindex RLIM_NLIMITS The number of different resource limits. Any valid @var{resource} operand must be less than @code{RLIM_NLIMITS}. @end table @comment sys/resource.h @comment BSD @deftypevr Constant int RLIM_INFINITY This constant stands for a value of ``infinity'' when supplied as the limit value in @code{setrlimit}. @end deftypevr @c ??? Someone want to finish these? Two historical functions for setting resource limits, @code{ulimit} and @code{vlimit}, are not documented here. The latter is declared in @file{sys/vlimit.h} and comes from BSD. @node Priority @section Process Priority @cindex process priority @cindex priority of a process @pindex sys/resource.h When several processes try to run, their respective priorities determine what share of the CPU each process gets. This section describes how you can read and set the priority of a process. All these functions and macros are declared in @file{sys/resource.h}. The range of valid priority values depends on the operating system, but typically it runs from @code{-20} to @code{20}. A lower priority value means the process runs more often. These constants describe the range of priority values: @table @code @comment sys/resource.h @comment BSD @item PRIO_MIN @vindex PRIO_MIN The smallest valid priority value. @comment sys/resource.h @comment BSD @item PRIO_MAX @vindex PRIO_MAX The largest valid priority value. @end table @comment sys/resource.h @comment BSD @deftypefun int getpriority (int @var{class}, int @var{id}) Read the priority of a class of processes; @var{class} and @var{id} specify which ones (see below). If the processes specified do not all have the same priority, this returns the smallest value that any of them has. The return value is the priority value on success, and @code{-1} on failure. The following @code{errno} error condition are possible for this function: @table @code @item ESRCH The combination of @var{class} and @var{id} does not match any existing process. @item EINVAL The value of @var{class} is not valid. @end table When the return value is @code{-1}, it could indicate failure, or it could be the priority value. The only way to make certain is to set @code{errno = 0} before calling @code{getpriority}, then use @code{errno != 0} afterward as the criterion for failure. @end deftypefun @comment sys/resource.h @comment BSD @deftypefun int setpriority (int @var{class}, int @var{id}, int @var{priority}) Set the priority of a class of processes to @var{priority}; @var{class} and @var{id} specify which ones (see below). The return value is @code{0} on success and @code{-1} on failure. The following @code{errno} error condition are defined for this function: @table @code @item ESRCH The combination of @var{class} and @var{id} does not match any existing process. @item EINVAL The value of @var{class} is not valid. @item EPERM You tried to set the priority of some other user's process, and you don't have privileges for that. @item EACCES You tried to lower the priority of a process, and you don't have privileges for that. @end table @end deftypefun The arguments @var{class} and @var{id} together specify a set of processes you are interested in. These are the possible values for @var{class}: @table @code @comment sys/resource.h @comment BSD @item PRIO_PROCESS @vindex PRIO_PROCESS Read or set the priority of one process. The argument @var{id} is a process ID. @comment sys/resource.h @comment BSD @item PRIO_PGRP @vindex PRIO_PGRP Read or set the priority of one process group. The argument @var{id} is a process group ID. @comment sys/resource.h @comment BSD @item PRIO_USER @vindex PRIO_USER Read or set the priority of one user's processes. The argument @var{id} is a user ID. @end table If the argument @var{id} is 0, it stands for the current process, current process group, or the current user, according to @var{class}. @c ??? I don't know where we should say this comes from. @comment Unix @comment dunno.h @deftypefun int nice (int @var{increment}) Increment the priority of the current process by @var{increment}. The return value is the same as for @code{setpriority}. Here is an equivalent definition for @code{nice}: @smallexample int nice (int increment) @{ int old = getpriority (PRIO_PROCESS, 0); return setpriority (PRIO_PROCESS, 0, old + increment); @} @end smallexample @end deftypefun