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Diffstat (limited to 'manual/lang.texi')
| -rw-r--r-- | manual/lang.texi | 77 |
1 files changed, 56 insertions, 21 deletions
diff --git a/manual/lang.texi b/manual/lang.texi index 66d41846d2..18a1da3b22 100644 --- a/manual/lang.texi +++ b/manual/lang.texi @@ -11,7 +11,7 @@ features has been written, we are publishing it here. * Consistency Checking:: Using @code{assert} to abort if something ``impossible'' happens. * Variadic Functions:: Defining functions with varying numbers - of args. + of args. * Null Pointer Constant:: The macro @code{NULL}. * Important Data Types:: Data types for object sizes. * Data Type Measurements:: Parameters of data type representations. @@ -25,8 +25,8 @@ features has been written, we are publishing it here. When you're writing a program, it's often a good idea to put in checks at strategic places for ``impossible'' errors or violations of basic -assumptions. These checks are helpful in debugging problems due to -misunderstandings between different parts of the program. +assumptions. These kinds of checks are helpful in debugging problems +with the interfaces between different parts of the program, for example. @pindex assert.h The @code{assert} macro, defined in the header file @file{assert.h}, @@ -57,16 +57,19 @@ program (@pxref{Aborting a Program}) after printing a message of the form: @smallexample -@file{@var{file}}:@var{linenum}: Assertion `@var{expression}' failed. +@file{@var{file}}:@var{linenum}: @var{function}: Assertion `@var{expression}' failed. @end smallexample @noindent on the standard error stream @code{stderr} (@pxref{Standard Streams}). The filename and line number are taken from the C preprocessor macros @code{__FILE__} and @code{__LINE__} and specify where the call to -@code{assert} was written. +@code{assert} was written. When using the GNU C compiler, the name of +the function which calls @code{assert} is taken from the built-in +variable @code{__PRETTY_FUNCTION__}; with older compilers, the function +name and following colon are omitted. -If the preprocessor macro @code{NDEBUG} is defined at the point where +If the preprocessor macro @code{NDEBUG} is defined before @file{assert.h} is included, the @code{assert} macro is defined to do absolutely nothing. @@ -77,6 +80,38 @@ with arguments that involve side effects. For example, @code{assert @code{NDEBUG} is defined. @end deftypefn +Sometimes the ``impossible'' condition you want to check for is an error +return from an operating system function. Then it is useful to display +not only where the program crashes, but also what error was returned. +The @code{assert_perror} macro makes this easy. + +@comment assert.h +@comment GNU +@deftypefn Macro void assert_perror (int @var{errnum}) +Similar to @code{assert}, but verifies that @var{errnum} is zero. + +If @code{NDEBUG} is defined, @code{assert_perror} tests the value of +@var{errnum}. If it is nonzero, @code{assert_perror} aborts the program +after a printing a message of the form: + +@smallexample +@file{@var{file}}:@var{linenum}: @var{function}: @var{error text} +@end smallexample + +@noindent +on the standard error stream. The file name, line number, and function +name are as for @code{assert}. The error text is the result of +@w{@code{strerror (@var{errnum})}}. @xref{Error Messages}. + +Like @code{assert}, if @code{NDEBUG} is defined before @file{assert.h} +is included, the @code{assert_perror} macro does absolutely nothing. It +does not evaluate the argument, so @var{errnum} should not have any side +effects. It is best for @var{errnum} to be a just simple variable +reference; often it will be @code{errno}. + +This macro is a GNU extension. +@end deftypefn + @strong{Usage note:} The @code{assert} facility is designed for detecting @emph{internal inconsistency}; it is not suitable for reporting invalid input or improper usage by @emph{the user} of the @@ -86,8 +121,8 @@ The information in the diagnostic messages printed by the @code{assert} macro is intended to help you, the programmer, track down the cause of a bug, but is not really useful for telling a user of your program why his or her input was invalid or why a command could not be carried out. So -you can't use @code{assert} to print the error messages for these -eventualities. +you can't use @code{assert} or @code{assert_perror} to print the error +messages for these eventualities. What's more, your program should not abort when given invalid input, as @code{assert} would do---it should exit with nonzero status (@pxref{Exit @@ -120,7 +155,7 @@ of arguments, using @file{varargs.h}. @menu * Why Variadic:: Reasons for making functions take - variable arguments. + variable arguments. * How Variadic:: How to define and call variadic functions. * Variadic Example:: A complete example. @end menu @@ -189,7 +224,7 @@ additional variable arguments. @xref{Calling Variadics}. with variable arguments. * Receiving Arguments:: Steps you must follow to access the optional argument values. -* How Many Arguments:: How to decide whether there are more arguments. +* How Many Arguments:: How to decide whether there are more arguments. * Calling Variadics:: Things you need to know about calling variable arguments functions. * Argument Macros:: Detailed specification of the macros @@ -205,16 +240,16 @@ additional variable arguments. @xref{Calling Variadics}. A function that accepts a variable number of arguments must be declared with a prototype that says so. You write the fixed arguments as usual, -and then tack on @samp{@dots{}} to indicate the possibility of +and then tack on @samp{@dots{}} to indicate the possibility of additional arguments. The syntax of ANSI C requires at least one fixed argument before the @samp{@dots{}}. For example, @smallexample -int +int func (const char *a, int b, @dots{}) @{ @dots{} -@} +@} @end smallexample @noindent @@ -271,7 +306,7 @@ compiler. But you might as well call @code{va_end} just in case your program is someday compiled with a peculiar compiler.) @end enumerate -@xref{Argument Macros}, for the full definitions of @code{va_start}, +@xref{Argument Macros}, for the full definitions of @code{va_start}, @code{va_arg} and @code{va_end}. Steps 1 and 3 must be performed in the function that accepts the @@ -405,7 +440,7 @@ found in the header file @file{varargs.h}. @deftypefn {Macro} @var{type} va_arg (va_list @var{ap}, @var{type}) The @code{va_arg} macro returns the value of the next optional argument, and modifies the value of @var{ap} to point to the subsequent argument. -Thus, successive uses of @code{va_arg} return successive optional +Thus, successive uses of @code{va_arg} return successive optional arguments. The type of the value returned by @code{va_arg} is @var{type} as @@ -478,7 +513,7 @@ functions: @comment Unix @deffn Macro va_alist This macro stands for the argument name list required in a variadic -function. +function. @end deffn @comment varargs.h @@ -537,7 +572,7 @@ The result of subtracting two pointers in C is always an integer, but the precise data type varies from C compiler to C compiler. Likewise, the data type of the result of @code{sizeof} also varies between compilers. ANSI defines standard aliases for these two types, so you can refer to -them in a portable fashion. They are defined in the header file +them in a portable fashion. They are defined in the header file @file{stddef.h}. @pindex stddef.h @@ -601,7 +636,7 @@ which give you this information in full detail. * Width of Type:: How many bits does an integer type hold? * Range of Type:: What are the largest and smallest values that an integer type can hold? -* Floating Type Macros:: Parameters that measure the floating point types. +* Floating Type Macros:: Parameters that measure the floating point types. * Structure Measurement:: Getting measurements on structure types. @end menu @@ -814,7 +849,7 @@ machine. * Floating Point Concepts:: Definitions of terminology. * Floating Point Parameters:: Details of specific macros. * IEEE Floating Point:: The measurements for one common - representation. + representation. @end menu @node Floating Point Concepts @@ -865,7 +900,7 @@ follows. The @dfn{mantissa} or @dfn{significand}, an unsigned integer which is a part of each floating point number. -@item +@item @cindex precision (of floating point number) The @dfn{precision} of the mantissa. If the base of the representation is @var{b}, then the precision is the number of base-@var{b} digits in @@ -1149,7 +1184,7 @@ supposed to be greater than @code{1E-9}. @node IEEE Floating Point @subsubsection IEEE Floating Point -@cindex IEEE floating point representation +@cindex IEEE floating point representation @cindex floating point, IEEE Here is an example showing how the floating type measurements come out |