about summary refs log tree commit diff
path: root/manual/lang.texi
blob: a151c9b690ebb7557197bbdefa2bc021875b01b5 (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
@c This node must have no pointers.
@node Language Features
@c @node Language Features, Library Summary, , Top
@c %MENU% C language features provided by the library
@appendix C Language Facilities in the Library

Some of the facilities implemented by the C library really should be
thought of as parts of the C language itself.  These facilities ought to
be documented in the C Language Manual, not in the library manual; but
since we don't have the language manual yet, and documentation for these
features has been written, we are publishing it here.

@menu
* Consistency Checking::        Using @code{assert} to abort if
				 something ``impossible'' happens.
* Variadic Functions::          Defining functions with varying numbers
                                 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.
@end menu

@node Consistency Checking
@section Explicitly Checking Internal Consistency
@cindex consistency checking
@cindex impossible events
@cindex assertions

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 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},
provides a convenient way to abort the program while printing a message
about where in the program the error was detected.

@vindex NDEBUG
Once you think your program is debugged, you can disable the error
checks performed by the @code{assert} macro by recompiling with the
macro @code{NDEBUG} defined.  This means you don't actually have to
change the program source code to disable these checks.

But disabling these consistency checks is undesirable unless they make
the program significantly slower.  All else being equal, more error
checking is good no matter who is running the program.  A wise user
would rather have a program crash, visibly, than have it return nonsense
without indicating anything might be wrong.

@comment assert.h
@comment ISO
@deftypefn Macro void assert (int @var{expression})
@safety{@prelim{}@mtsafe{}@asunsafe{@ascuheap{} @asucorrupt{}}@acunsafe{@acsmem{} @aculock{} @acucorrupt{}}}
@c assert_fail_base calls asprintf, and fflushes stderr.
Verify the programmer's belief that @var{expression} is nonzero at
this point in the program.

If @code{NDEBUG} is not defined, @code{assert} tests the value of
@var{expression}.  If it is false (zero), @code{assert} aborts the
program (@pxref{Aborting a Program}) after printing a message of the
form:

@smallexample
@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 made.  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 before
@file{assert.h} is included, the @code{assert} macro is defined to do
absolutely nothing.

@strong{Warning:} Even the argument expression @var{expression} is not
evaluated if @code{NDEBUG} is in effect.  So never use @code{assert}
with arguments that involve side effects.  For example, @code{assert
(++i > 0);} is a bad idea, because @code{i} will not be incremented if
@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})
@safety{@prelim{}@mtsafe{}@asunsafe{@ascuheap{} @asucorrupt{}}@acunsafe{@acsmem{} @aculock{} @acucorrupt{}}}
@c assert_fail_base calls asprintf, and fflushes stderr.
Similar to @code{assert}, but verifies that @var{errnum} is zero.

If @code{NDEBUG} is not defined, @code{assert_perror} tests the value of
@var{errnum}.  If it is nonzero, @code{assert_perror} aborts the program
after 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 just a 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 the @emph{user} of the
program.

The information in the diagnostic messages printed by the @code{assert}
and @code{assert_perror} 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.  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 Status}) after printing its error messages, or perhaps
read another command or move on to the next input file.

@xref{Error Messages}, for information on printing error messages for
problems that @emph{do not} represent bugs in the program.


@node Variadic Functions
@section Variadic Functions
@cindex variable number of arguments
@cindex variadic functions
@cindex optional arguments

@w{ISO C} defines a syntax for declaring a function to take a variable
number or type of arguments.  (Such functions are referred to as
@dfn{varargs functions} or @dfn{variadic functions}.)  However, the
language itself provides no mechanism for such functions to access their
non-required arguments; instead, you use the variable arguments macros
defined in @file{stdarg.h}.

This section describes how to declare variadic functions, how to write
them, and how to call them properly.

@strong{Compatibility Note:} Many older C dialects provide a similar,
but incompatible, mechanism for defining functions with variable numbers
of arguments, using @file{varargs.h}.

@menu
* Why Variadic::                Reasons for making functions take
                                 variable arguments.
* How Variadic::                How to define and call variadic functions.
* Variadic Example::            A complete example.
@end menu

@node Why Variadic
@subsection Why Variadic Functions are Used

Ordinary C functions take a fixed number of arguments.  When you define
a function, you specify the data type for each argument.  Every call to
the function should supply the expected number of arguments, with types
that can be converted to the specified ones.  Thus, if the function
@samp{foo} is declared with @code{int foo (int, char *);} then you must
call it with two arguments, a number (any kind will do) and a string
pointer.

But some functions perform operations that can meaningfully accept an
unlimited number of arguments.

In some cases a function can handle any number of values by operating on
all of them as a block.  For example, consider a function that allocates
a one-dimensional array with @code{malloc} to hold a specified set of
values.  This operation makes sense for any number of values, as long as
the length of the array corresponds to that number.  Without facilities
for variable arguments, you would have to define a separate function for
each possible array size.

The library function @code{printf} (@pxref{Formatted Output}) is an
example of another class of function where variable arguments are
useful.  This function prints its arguments (which can vary in type as
well as number) under the control of a format template string.

These are good reasons to define a @dfn{variadic} function which can
handle as many arguments as the caller chooses to pass.

Some functions such as @code{open} take a fixed set of arguments, but
occasionally ignore the last few.  Strict adherence to @w{ISO C} requires
these functions to be defined as variadic; in practice, however, the GNU
C compiler and most other C compilers let you define such a function to
take a fixed set of arguments---the most it can ever use---and then only
@emph{declare} the function as variadic (or not declare its arguments
at all!).

@node How Variadic
@subsection How Variadic Functions are Defined and Used

Defining and using a variadic function involves three steps:

@itemize @bullet
@item
@emph{Define} the function as variadic, using an ellipsis
(@samp{@dots{}}) in the argument list, and using special macros to
access the variable arguments.  @xref{Receiving Arguments}.

@item
@emph{Declare} the function as variadic, using a prototype with an
ellipsis (@samp{@dots{}}), in all the files which call it.
@xref{Variadic Prototypes}.

@item
@emph{Call} the function by writing the fixed arguments followed by the
additional variable arguments.  @xref{Calling Variadics}.
@end itemize

@menu
* Variadic Prototypes::  How to make a prototype for a function
			  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.
* Calling Variadics::    Things you need to know about calling
			  variable arguments functions.
* Argument Macros::      Detailed specification of the macros
        		  for accessing variable arguments.
@end menu

@node Variadic Prototypes
@subsubsection Syntax for Variable Arguments
@cindex function prototypes (variadic)
@cindex prototypes for variadic functions
@cindex variadic function prototypes

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
additional arguments.  The syntax of @w{ISO C} requires at least one fixed
argument before the @samp{@dots{}}.  For example,

@smallexample
int
func (const char *a, int b, @dots{})
@{
  @dots{}
@}
@end smallexample

@noindent
defines a function @code{func} which returns an @code{int} and takes two
required arguments, a @code{const char *} and an @code{int}.  These are
followed by any number of anonymous arguments.

@strong{Portability note:} For some C compilers, the last required
argument must not be declared @code{register} in the function
definition.  Furthermore, this argument's type must be
@dfn{self-promoting}: that is, the default promotions must not change
its type.  This rules out array and function types, as well as
@code{float}, @code{char} (whether signed or not) and @w{@code{short int}}
(whether signed or not).  This is actually an @w{ISO C} requirement.

@node Receiving Arguments
@subsubsection Receiving the Argument Values
@cindex variadic function argument access
@cindex arguments (variadic functions)

Ordinary fixed arguments have individual names, and you can use these
names to access their values.  But optional arguments have no
names---nothing but @samp{@dots{}}.  How can you access them?

@pindex stdarg.h
The only way to access them is sequentially, in the order they were
written, and you must use special macros from @file{stdarg.h} in the
following three step process:

@enumerate
@item
You initialize an argument pointer variable of type @code{va_list} using
@code{va_start}.  The argument pointer when initialized points to the
first optional argument.

@item
You access the optional arguments by successive calls to @code{va_arg}.
The first call to @code{va_arg} gives you the first optional argument,
the next call gives you the second, and so on.

You can stop at any time if you wish to ignore any remaining optional
arguments.  It is perfectly all right for a function to access fewer
arguments than were supplied in the call, but you will get garbage
values if you try to access too many arguments.

@item
You indicate that you are finished with the argument pointer variable by
calling @code{va_end}.

(In practice, with most C compilers, calling @code{va_end} does nothing.
This is always true in the GNU C 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},
@code{va_arg} and @code{va_end}.

Steps 1 and 3 must be performed in the function that accepts the
optional arguments.  However, you can pass the @code{va_list} variable
as an argument to another function and perform all or part of step 2
there.

You can perform the entire sequence of three steps multiple times
within a single function invocation.  If you want to ignore the optional
arguments, you can do these steps zero times.

You can have more than one argument pointer variable if you like.  You
can initialize each variable with @code{va_start} when you wish, and
then you can fetch arguments with each argument pointer as you wish.
Each argument pointer variable will sequence through the same set of
argument values, but at its own pace.

@strong{Portability note:} With some compilers, once you pass an
argument pointer value to a subroutine, you must not keep using the same
argument pointer value after that subroutine returns.  For full
portability, you should just pass it to @code{va_end}.  This is actually
an @w{ISO C} requirement, but most ANSI C compilers work happily
regardless.

@node How Many Arguments
@subsubsection How Many Arguments Were Supplied
@cindex number of arguments passed
@cindex how many arguments
@cindex arguments, how many

There is no general way for a function to determine the number and type
of the optional arguments it was called with.  So whoever designs the
function typically designs a convention for the caller to specify the number
and type of arguments.  It is up to you to define an appropriate calling
convention for each variadic function, and write all calls accordingly.

One kind of calling convention is to pass the number of optional
arguments as one of the fixed arguments.  This convention works provided
all of the optional arguments are of the same type.

A similar alternative is to have one of the required arguments be a bit
mask, with a bit for each possible purpose for which an optional
argument might be supplied.  You would test the bits in a predefined
sequence; if the bit is set, fetch the value of the next argument,
otherwise use a default value.

A required argument can be used as a pattern to specify both the number
and types of the optional arguments.  The format string argument to
@code{printf} is one example of this (@pxref{Formatted Output Functions}).

Another possibility is to pass an ``end marker'' value as the last
optional argument.  For example, for a function that manipulates an
arbitrary number of pointer arguments, a null pointer might indicate the
end of the argument list.  (This assumes that a null pointer isn't
otherwise meaningful to the function.)  The @code{execl} function works
in just this way; see @ref{Executing a File}.


@node Calling Variadics
@subsubsection Calling Variadic Functions
@cindex variadic functions, calling
@cindex calling variadic functions
@cindex declaring variadic functions

You don't have to do anything special to call a variadic function.
Just put the arguments (required arguments, followed by optional ones)
inside parentheses, separated by commas, as usual.  But you must declare
the function with a prototype and know how the argument values are converted.

In principle, functions that are @emph{defined} to be variadic must also
be @emph{declared} to be variadic using a function prototype whenever
you call them.  (@xref{Variadic Prototypes}, for how.)  This is because
some C compilers use a different calling convention to pass the same set
of argument values to a function depending on whether that function
takes variable arguments or fixed arguments.

In practice, the GNU C compiler always passes a given set of argument
types in the same way regardless of whether they are optional or
required.  So, as long as the argument types are self-promoting, you can
safely omit declaring them.  Usually it is a good idea to declare the
argument types for variadic functions, and indeed for all functions.
But there are a few functions which it is extremely convenient not to
have to declare as variadic---for example, @code{open} and
@code{printf}.

@cindex default argument promotions
@cindex argument promotion
Since the prototype doesn't specify types for optional arguments, in a
call to a variadic function the @dfn{default argument promotions} are
performed on the optional argument values.  This means the objects of
type @code{char} or @w{@code{short int}} (whether signed or not) are
promoted to either @code{int} or @w{@code{unsigned int}}, as
appropriate; and that objects of type @code{float} are promoted to type
@code{double}.  So, if the caller passes a @code{char} as an optional
argument, it is promoted to an @code{int}, and the function can access
it with @code{va_arg (@var{ap}, int)}.

Conversion of the required arguments is controlled by the function
prototype in the usual way: the argument expression is converted to the
declared argument type as if it were being assigned to a variable of
that type.

@node Argument Macros
@subsubsection Argument Access Macros

Here are descriptions of the macros used to retrieve variable arguments.
These macros are defined in the header file @file{stdarg.h}.
@pindex stdarg.h

@comment stdarg.h
@comment ISO
@deftp {Data Type} va_list
The type @code{va_list} is used for argument pointer variables.
@end deftp

@comment stdarg.h
@comment ISO
@deftypefn {Macro} void va_start (va_list @var{ap}, @var{last-required})
@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
@c This is no longer provided by glibc, but rather by the compiler.
This macro initializes the argument pointer variable @var{ap} to point
to the first of the optional arguments of the current function;
@var{last-required} must be the last required argument to the function.
@end deftypefn

@comment stdarg.h
@comment ISO
@deftypefn {Macro} @var{type} va_arg (va_list @var{ap}, @var{type})
@safety{@prelim{}@mtsafe{@mtsrace{:ap}}@assafe{}@acunsafe{@acucorrupt{}}}
@c This is no longer provided by glibc, but rather by the compiler.
@c Unlike the other va_ macros, that either start/end the lifetime of
@c the va_list object or don't modify it, this one modifies ap, and it
@c may leave it in a partially updated state.
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
arguments.

The type of the value returned by @code{va_arg} is @var{type} as
specified in the call.  @var{type} must be a self-promoting type (not
@code{char} or @code{short int} or @code{float}) that matches the type
of the actual argument.
@end deftypefn

@comment stdarg.h
@comment ISO
@deftypefn {Macro} void va_end (va_list @var{ap})
@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
@c This is no longer provided by glibc, but rather by the compiler.
This ends the use of @var{ap}.  After a @code{va_end} call, further
@code{va_arg} calls with the same @var{ap} may not work.  You should invoke
@code{va_end} before returning from the function in which @code{va_start}
was invoked with the same @var{ap} argument.

In @theglibc{}, @code{va_end} does nothing, and you need not ever
use it except for reasons of portability.
@refill
@end deftypefn

Sometimes it is necessary to parse the list of parameters more than once
or one wants to remember a certain position in the parameter list.  To
do this, one will have to make a copy of the current value of the
argument.  But @code{va_list} is an opaque type and one cannot necessarily
assign the value of one variable of type @code{va_list} to another variable
of the same type.

@comment stdarg.h
@comment ISO
@deftypefn {Macro} void va_copy (va_list @var{dest}, va_list @var{src})
@deftypefnx {Macro} void __va_copy (va_list @var{dest}, va_list @var{src})
@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
@c This is no longer provided by glibc, but rather by the compiler.
The @code{va_copy} macro allows copying of objects of type
@code{va_list} even if this is not an integral type.  The argument pointer
in @var{dest} is initialized to point to the same argument as the
pointer in @var{src}.

This macro was added in ISO C99.  When building for strict conformance
to ISO C90 (@samp{gcc -ansi}), it is not available.  The macro
@code{__va_copy} is available as a GNU extension in any standards
mode; before GCC 3.0, it was the only macro for this functionality.
@end deftypefn

If you want to use @code{va_copy} and be portable to pre-C99 systems,
you should always be prepared for the
possibility that this macro will not be available.  On architectures where a
simple assignment is invalid, hopefully @code{va_copy} @emph{will} be available,
so one should always write something like this if concerned about
pre-C99 portability:

@smallexample
@{
  va_list ap, save;
  @dots{}
#ifdef va_copy
  va_copy (save, ap);
#else
  save = ap;
#endif
  @dots{}
@}
@end smallexample


@node Variadic Example
@subsection Example of a Variadic Function

Here is a complete sample function that accepts a variable number of
arguments.  The first argument to the function is the count of remaining
arguments, which are added up and the result returned.  While trivial,
this function is sufficient to illustrate how to use the variable
arguments facility.

@comment Yes, this example has been tested.
@smallexample
@include add.c.texi
@end smallexample

@node Null Pointer Constant
@section Null Pointer Constant
@cindex null pointer constant

The null pointer constant is guaranteed not to point to any real object.
You can assign it to any pointer variable since it has type @code{void
*}.  The preferred way to write a null pointer constant is with
@code{NULL}.

@comment stddef.h
@comment ISO
@deftypevr Macro {void *} NULL
This is a null pointer constant.
@end deftypevr

You can also use @code{0} or @code{(void *)0} as a null pointer
constant, but using @code{NULL} is cleaner because it makes the purpose
of the constant more evident.

If you use the null pointer constant as a function argument, then for
complete portability you should make sure that the function has a
prototype declaration.  Otherwise, if the target machine has two
different pointer representations, the compiler won't know which
representation to use for that argument.  You can avoid the problem by
explicitly casting the constant to the proper pointer type, but we
recommend instead adding a prototype for the function you are calling.

@node Important Data Types
@section Important Data Types

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.
ISO C defines standard aliases for these two types, so you can refer to
them in a portable fashion.  They are defined in the header file
@file{stddef.h}.
@pindex stddef.h

@comment stddef.h
@comment ISO
@deftp {Data Type} ptrdiff_t
This is the signed integer type of the result of subtracting two
pointers.  For example, with the declaration @code{char *p1, *p2;}, the
expression @code{p2 - p1} is of type @code{ptrdiff_t}.  This will
probably be one of the standard signed integer types (@w{@code{short
int}}, @code{int} or @w{@code{long int}}), but might be a nonstandard
type that exists only for this purpose.
@end deftp

@comment stddef.h
@comment ISO
@deftp {Data Type} size_t
This is an unsigned integer type used to represent the sizes of objects.
The result of the @code{sizeof} operator is of this type, and functions
such as @code{malloc} (@pxref{Unconstrained Allocation}) and
@code{memcpy} (@pxref{Copying Strings and Arrays}) accept arguments of
this type to specify object sizes.  On systems using @theglibc{}, this
will be @w{@code{unsigned int}} or @w{@code{unsigned long int}}.

@strong{Usage Note:} @code{size_t} is the preferred way to declare any
arguments or variables that hold the size of an object.
@end deftp

@strong{Compatibility Note:} Implementations of C before the advent of
@w{ISO C} generally used @code{unsigned int} for representing object sizes
and @code{int} for pointer subtraction results.  They did not
necessarily define either @code{size_t} or @code{ptrdiff_t}.  Unix
systems did define @code{size_t}, in @file{sys/types.h}, but the
definition was usually a signed type.

@node Data Type Measurements
@section Data Type Measurements

Most of the time, if you choose the proper C data type for each object
in your program, you need not be concerned with just how it is
represented or how many bits it uses.  When you do need such
information, the C language itself does not provide a way to get it.
The header files @file{limits.h} and @file{float.h} contain macros
which give you this information in full detail.

@menu
* 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.
* Structure Measurement::   Getting measurements on structure types.
@end menu

@node Width of Type
@subsection Computing the Width of an Integer Data Type
@cindex integer type width
@cindex width of integer type
@cindex type measurements, integer

The most common reason that a program needs to know how many bits are in
an integer type is for using an array of @code{long int} as a bit vector.
You can access the bit at index @var{n} with

@smallexample
vector[@var{n} / LONGBITS] & (1 << (@var{n} % LONGBITS))
@end smallexample

@noindent
provided you define @code{LONGBITS} as the number of bits in a
@code{long int}.

@pindex limits.h
There is no operator in the C language that can give you the number of
bits in an integer data type.  But you can compute it from the macro
@code{CHAR_BIT}, defined in the header file @file{limits.h}.

@table @code
@comment limits.h
@comment ISO
@item CHAR_BIT
This is the number of bits in a @code{char}---eight, on most systems.
The value has type @code{int}.

You can compute the number of bits in any data type @var{type} like
this:

@smallexample
sizeof (@var{type}) * CHAR_BIT
@end smallexample
@end table

That expression includes padding bits as well as value and sign bits.
On all systems supported by @theglibc{}, standard integer types other
than @code{_Bool} do not have any padding bits.  TS 18661-1:2014
defines additional macros for the width of integer types (the number
of value and sign bits); these macros can also be used in @code{#if}
preprocessor directives, whereas @code{sizeof} cannot.  The following
macros are defined in @file{limits.h}.

@vtable @code
@comment limits.h
@comment ISO
@item CHAR_WIDTH
@comment limits.h
@comment ISO
@itemx SCHAR_WIDTH
@comment limits.h
@comment ISO
@itemx UCHAR_WIDTH
@comment limits.h
@comment ISO
@itemx SHRT_WIDTH
@comment limits.h
@comment ISO
@itemx USHRT_WIDTH
@comment limits.h
@comment ISO
@itemx INT_WIDTH
@comment limits.h
@comment ISO
@itemx UINT_WIDTH
@comment limits.h
@comment ISO
@itemx LONG_WIDTH
@comment limits.h
@comment ISO
@itemx ULONG_WIDTH
@comment limits.h
@comment ISO
@itemx LLONG_WIDTH
@comment limits.h
@comment ISO
@itemx ULLONG_WIDTH

These are the widths of the types @code{char}, @code{signed char},
@code{unsigned char}, @code{short int}, @code{unsigned short int},
@code{int}, @code{unsigned int}, @code{long int}, @code{unsigned long
int}, @code{long long int} and @code{unsigned long long int},
respectively.
@end vtable

Further such macros are defined in @file{stdint.h}.  Apart from those
for types specified by width (@pxref{Integers}), the following are
defined.

@vtable @code
@comment stdint.h
@comment ISO
@item INTPTR_WIDTH
@comment stdint.h
@comment ISO
@itemx UINTPTR_WIDTH
@comment stdint.h
@comment ISO
@itemx PTRDIFF_WIDTH
@comment stdint.h
@comment ISO
@itemx SIG_ATOMIC_WIDTH
@comment stdint.h
@comment ISO
@itemx SIZE_WIDTH
@comment stdint.h
@comment ISO
@itemx WCHAR_WIDTH
@comment stdint.h
@comment ISO
@itemx WINT_WIDTH

These are the widths of the types @code{intptr_t}, @code{uintptr_t},
@code{ptrdiff_t}, @code{sig_atomic_t}, @code{size_t}, @code{wchar_t}
and @code{wint_t}, respectively.
@end vtable

@node Range of Type
@subsection Range of an Integer Type
@cindex integer type range
@cindex range of integer type
@cindex limits, integer types

Suppose you need to store an integer value which can range from zero to
one million.  Which is the smallest type you can use?  There is no
general rule; it depends on the C compiler and target machine.  You can
use the @samp{MIN} and @samp{MAX} macros in @file{limits.h} to determine
which type will work.

Each signed integer type has a pair of macros which give the smallest
and largest values that it can hold.  Each unsigned integer type has one
such macro, for the maximum value; the minimum value is, of course,
zero.

The values of these macros are all integer constant expressions.  The
@samp{MAX} and @samp{MIN} macros for @code{char} and @w{@code{short
int}} types have values of type @code{int}.  The @samp{MAX} and
@samp{MIN} macros for the other types have values of the same type
described by the macro---thus, @code{ULONG_MAX} has type
@w{@code{unsigned long int}}.

@comment Extra blank lines make it look better.
@vtable @code
@comment limits.h
@comment ISO
@item SCHAR_MIN

This is the minimum value that can be represented by a @w{@code{signed char}}.

@comment limits.h
@comment ISO
@item SCHAR_MAX
@comment limits.h
@comment ISO
@itemx UCHAR_MAX

These are the maximum values that can be represented by a
@w{@code{signed char}} and @w{@code{unsigned char}}, respectively.

@comment limits.h
@comment ISO
@item CHAR_MIN

This is the minimum value that can be represented by a @code{char}.
It's equal to @code{SCHAR_MIN} if @code{char} is signed, or zero
otherwise.

@comment limits.h
@comment ISO
@item CHAR_MAX

This is the maximum value that can be represented by a @code{char}.
It's equal to @code{SCHAR_MAX} if @code{char} is signed, or
@code{UCHAR_MAX} otherwise.

@comment limits.h
@comment ISO
@item SHRT_MIN

This is the minimum value that can be represented by a @w{@code{signed
short int}}.  On most machines that @theglibc{} runs on,
@code{short} integers are 16-bit quantities.

@comment limits.h
@comment ISO
@item SHRT_MAX
@comment limits.h
@comment ISO
@itemx USHRT_MAX

These are the maximum values that can be represented by a
@w{@code{signed short int}} and @w{@code{unsigned short int}},
respectively.

@comment limits.h
@comment ISO
@item INT_MIN

This is the minimum value that can be represented by a @w{@code{signed
int}}.  On most machines that @theglibc{} runs on, an @code{int} is
a 32-bit quantity.

@comment limits.h
@comment ISO
@item INT_MAX
@comment limits.h
@comment ISO
@itemx UINT_MAX

These are the maximum values that can be represented by, respectively,
the type @w{@code{signed int}} and the type @w{@code{unsigned int}}.

@comment limits.h
@comment ISO
@item LONG_MIN

This is the minimum value that can be represented by a @w{@code{signed
long int}}.  On most machines that @theglibc{} runs on, @code{long}
integers are 32-bit quantities, the same size as @code{int}.

@comment limits.h
@comment ISO
@item LONG_MAX
@comment limits.h
@comment ISO
@itemx ULONG_MAX

These are the maximum values that can be represented by a
@w{@code{signed long int}} and @code{unsigned long int}, respectively.

@comment limits.h
@comment ISO
@item LLONG_MIN

This is the minimum value that can be represented by a @w{@code{signed
long long int}}.  On most machines that @theglibc{} runs on,
@w{@code{long long}} integers are 64-bit quantities.

@comment limits.h
@comment ISO
@item LLONG_MAX
@comment limits.h
@comment ISO
@itemx ULLONG_MAX

These are the maximum values that can be represented by a @code{signed
long long int} and @code{unsigned long long int}, respectively.

@comment limits.h
@comment GNU
@item LONG_LONG_MIN
@comment limits.h
@comment GNU
@itemx LONG_LONG_MAX
@comment limits.h
@comment GNU
@itemx ULONG_LONG_MAX
These are obsolete names for @code{LLONG_MIN}, @code{LLONG_MAX}, and
@code{ULLONG_MAX}.  They are only available if @code{_GNU_SOURCE} is
defined (@pxref{Feature Test Macros}).  In GCC versions prior to 3.0,
these were the only names available.

@comment limits.h
@comment GNU
@item WCHAR_MAX

This is the maximum value that can be represented by a @code{wchar_t}.
@xref{Extended Char Intro}.
@end vtable

The header file @file{limits.h} also defines some additional constants
that parameterize various operating system and file system limits.  These
constants are described in @ref{System Configuration}.

@node Floating Type Macros
@subsection Floating Type Macros
@cindex floating type measurements
@cindex measurements of floating types
@cindex type measurements, floating
@cindex limits, floating types

The specific representation of floating point numbers varies from
machine to machine.  Because floating point numbers are represented
internally as approximate quantities, algorithms for manipulating
floating point data often need to take account of the precise details of
the machine's floating point representation.

Some of the functions in the C library itself need this information; for
example, the algorithms for printing and reading floating point numbers
(@pxref{I/O on Streams}) and for calculating trigonometric and
irrational functions (@pxref{Mathematics}) use it to avoid round-off
error and loss of accuracy.  User programs that implement numerical
analysis techniques also often need this information in order to
minimize or compute error bounds.

The header file @file{float.h} describes the format used by your
machine.

@menu
* Floating Point Concepts::     Definitions of terminology.
* Floating Point Parameters::   Details of specific macros.
* IEEE Floating Point::         The measurements for one common
                                 representation.
@end menu

@node Floating Point Concepts
@subsubsection Floating Point Representation Concepts

This section introduces the terminology for describing floating point
representations.

You are probably already familiar with most of these concepts in terms
of scientific or exponential notation for floating point numbers.  For
example, the number @code{123456.0} could be expressed in exponential
notation as @code{1.23456e+05}, a shorthand notation indicating that the
mantissa @code{1.23456} is multiplied by the base @code{10} raised to
power @code{5}.

More formally, the internal representation of a floating point number
can be characterized in terms of the following parameters:

@itemize @bullet
@item
@cindex sign (of floating point number)
The @dfn{sign} is either @code{-1} or @code{1}.

@item
@cindex base (of floating point number)
@cindex radix (of floating point number)
The @dfn{base} or @dfn{radix} for exponentiation, an integer greater
than @code{1}.  This is a constant for a particular representation.

@item
@cindex exponent (of floating point number)
The @dfn{exponent} to which the base is raised.  The upper and lower
bounds of the exponent value are constants for a particular
representation.

@cindex bias (of floating point number exponent)
Sometimes, in the actual bits representing the floating point number,
the exponent is @dfn{biased} by adding a constant to it, to make it
always be represented as an unsigned quantity.  This is only important
if you have some reason to pick apart the bit fields making up the
floating point number by hand, which is something for which @theglibc{}
provides no support.  So this is ignored in the discussion that
follows.

@item
@cindex mantissa (of floating point number)
@cindex significand (of floating point number)
The @dfn{mantissa} or @dfn{significand} is an unsigned integer which is a
part of each floating point number.

@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
the mantissa.  This is a constant for a particular representation.

@cindex hidden bit (of floating point number mantissa)
Many floating point representations have an implicit @dfn{hidden bit} in
the mantissa.  This is a bit which is present virtually in the mantissa,
but not stored in memory because its value is always 1 in a normalized
number.  The precision figure (see above) includes any hidden bits.

Again, @theglibc{} provides no facilities for dealing with such
low-level aspects of the representation.
@end itemize

The mantissa of a floating point number represents an implicit fraction
whose denominator is the base raised to the power of the precision.  Since
the largest representable mantissa is one less than this denominator, the
value of the fraction is always strictly less than @code{1}.  The
mathematical value of a floating point number is then the product of this
fraction, the sign, and the base raised to the exponent.

@cindex normalized floating point number
We say that the floating point number is @dfn{normalized} if the
fraction is at least @code{1/@var{b}}, where @var{b} is the base.  In
other words, the mantissa would be too large to fit if it were
multiplied by the base.  Non-normalized numbers are sometimes called
@dfn{denormal}; they contain less precision than the representation
normally can hold.

If the number is not normalized, then you can subtract @code{1} from the
exponent while multiplying the mantissa by the base, and get another
floating point number with the same value.  @dfn{Normalization} consists
of doing this repeatedly until the number is normalized.  Two distinct
normalized floating point numbers cannot be equal in value.

(There is an exception to this rule: if the mantissa is zero, it is
considered normalized.  Another exception happens on certain machines
where the exponent is as small as the representation can hold.  Then
it is impossible to subtract @code{1} from the exponent, so a number
may be normalized even if its fraction is less than @code{1/@var{b}}.)

@node Floating Point Parameters
@subsubsection Floating Point Parameters

@pindex float.h
These macro definitions can be accessed by including the header file
@file{float.h} in your program.

Macro names starting with @samp{FLT_} refer to the @code{float} type,
while names beginning with @samp{DBL_} refer to the @code{double} type
and names beginning with @samp{LDBL_} refer to the @code{long double}
type.  (If GCC does not support @code{long double} as a distinct data
type on a target machine then the values for the @samp{LDBL_} constants
are equal to the corresponding constants for the @code{double} type.)

Of these macros, only @code{FLT_RADIX} is guaranteed to be a constant
expression.  The other macros listed here cannot be reliably used in
places that require constant expressions, such as @samp{#if}
preprocessing directives or in the dimensions of static arrays.

Although the @w{ISO C} standard specifies minimum and maximum values for
most of these parameters, the GNU C implementation uses whatever values
describe the floating point representation of the target machine.  So in
principle GNU C actually satisfies the @w{ISO C} requirements only if the
target machine is suitable.  In practice, all the machines currently
supported are suitable.

@vtable @code
@comment float.h
@comment ISO
@item FLT_ROUNDS
This value characterizes the rounding mode for floating point addition.
The following values indicate standard rounding modes:

@need 750

@table @code
@item -1
The mode is indeterminable.
@item 0
Rounding is towards zero.
@item 1
Rounding is to the nearest number.
@item 2
Rounding is towards positive infinity.
@item 3
Rounding is towards negative infinity.
@end table

@noindent
Any other value represents a machine-dependent nonstandard rounding
mode.

On most machines, the value is @code{1}, in accordance with the IEEE
standard for floating point.

Here is a table showing how certain values round for each possible value
of @code{FLT_ROUNDS}, if the other aspects of the representation match
the IEEE single-precision standard.

@smallexample
                0      1             2             3
 1.00000003    1.0    1.0           1.00000012    1.0
 1.00000007    1.0    1.00000012    1.00000012    1.0
-1.00000003   -1.0   -1.0          -1.0          -1.00000012
-1.00000007   -1.0   -1.00000012   -1.0          -1.00000012
@end smallexample

@comment float.h
@comment ISO
@item FLT_RADIX
This is the value of the base, or radix, of the exponent representation.
This is guaranteed to be a constant expression, unlike the other macros
described in this section.  The value is 2 on all machines we know of
except the IBM 360 and derivatives.

@comment float.h
@comment ISO
@item FLT_MANT_DIG
This is the number of base-@code{FLT_RADIX} digits in the floating point
mantissa for the @code{float} data type.  The following expression
yields @code{1.0} (even though mathematically it should not) due to the
limited number of mantissa digits:

@smallexample
float radix = FLT_RADIX;

1.0f + 1.0f / radix / radix / @dots{} / radix
@end smallexample

@noindent
where @code{radix} appears @code{FLT_MANT_DIG} times.

@comment float.h
@comment ISO
@item DBL_MANT_DIG
@itemx LDBL_MANT_DIG
This is the number of base-@code{FLT_RADIX} digits in the floating point
mantissa for the data types @code{double} and @code{long double},
respectively.

@comment Extra blank lines make it look better.
@comment float.h
@comment ISO
@item FLT_DIG

This is the number of decimal digits of precision for the @code{float}
data type.  Technically, if @var{p} and @var{b} are the precision and
base (respectively) for the representation, then the decimal precision
@var{q} is the maximum number of decimal digits such that any floating
point number with @var{q} base 10 digits can be rounded to a floating
point number with @var{p} base @var{b} digits and back again, without
change to the @var{q} decimal digits.

The value of this macro is supposed to be at least @code{6}, to satisfy
@w{ISO C}.

@comment float.h
@comment ISO
@item DBL_DIG
@itemx LDBL_DIG

These are similar to @code{FLT_DIG}, but for the data types
@code{double} and @code{long double}, respectively.  The values of these
macros are supposed to be at least @code{10}.

@comment float.h
@comment ISO
@item FLT_MIN_EXP
This is the smallest possible exponent value for type @code{float}.
More precisely, it is the minimum negative integer such that the value
@code{FLT_RADIX} raised to this power minus 1 can be represented as a
normalized floating point number of type @code{float}.

@comment float.h
@comment ISO
@item DBL_MIN_EXP
@itemx LDBL_MIN_EXP

These are similar to @code{FLT_MIN_EXP}, but for the data types
@code{double} and @code{long double}, respectively.

@comment float.h
@comment ISO
@item FLT_MIN_10_EXP
This is the minimum negative integer such that @code{10} raised to this
power minus 1 can be represented as a normalized floating point number
of type @code{float}.  This is supposed to be @code{-37} or even less.

@comment float.h
@comment ISO
@item DBL_MIN_10_EXP
@itemx LDBL_MIN_10_EXP
These are similar to @code{FLT_MIN_10_EXP}, but for the data types
@code{double} and @code{long double}, respectively.

@comment float.h
@comment ISO
@item FLT_MAX_EXP
This is the largest possible exponent value for type @code{float}.  More
precisely, this is the maximum positive integer such that value
@code{FLT_RADIX} raised to this power minus 1 can be represented as a
floating point number of type @code{float}.

@comment float.h
@comment ISO
@item DBL_MAX_EXP
@itemx LDBL_MAX_EXP
These are similar to @code{FLT_MAX_EXP}, but for the data types
@code{double} and @code{long double}, respectively.

@comment float.h
@comment ISO
@item FLT_MAX_10_EXP
This is the maximum positive integer such that @code{10} raised to this
power minus 1 can be represented as a normalized floating point number
of type @code{float}.  This is supposed to be at least @code{37}.

@comment float.h
@comment ISO
@item DBL_MAX_10_EXP
@itemx LDBL_MAX_10_EXP
These are similar to @code{FLT_MAX_10_EXP}, but for the data types
@code{double} and @code{long double}, respectively.

@comment float.h
@comment ISO
@item FLT_MAX

The value of this macro is the maximum number representable in type
@code{float}.  It is supposed to be at least @code{1E+37}.  The value
has type @code{float}.

The smallest representable number is @code{- FLT_MAX}.

@comment float.h
@comment ISO
@item DBL_MAX
@itemx LDBL_MAX

These are similar to @code{FLT_MAX}, but for the data types
@code{double} and @code{long double}, respectively.  The type of the
macro's value is the same as the type it describes.

@comment float.h
@comment ISO
@item FLT_MIN

The value of this macro is the minimum normalized positive floating
point number that is representable in type @code{float}.  It is supposed
to be no more than @code{1E-37}.

@comment float.h
@comment ISO
@item DBL_MIN
@itemx LDBL_MIN

These are similar to @code{FLT_MIN}, but for the data types
@code{double} and @code{long double}, respectively.  The type of the
macro's value is the same as the type it describes.

@comment float.h
@comment ISO
@item FLT_EPSILON

This is the difference between 1 and the smallest floating point
number of type @code{float} that is greater than 1.  It's supposed to
be no greater than @code{1E-5}.

@comment float.h
@comment ISO
@item DBL_EPSILON
@itemx LDBL_EPSILON

These are similar to @code{FLT_EPSILON}, but for the data types
@code{double} and @code{long double}, respectively.  The type of the
macro's value is the same as the type it describes.  The values are not
supposed to be greater than @code{1E-9}.
@end vtable

@node IEEE Floating Point
@subsubsection IEEE Floating Point
@cindex IEEE floating point representation
@cindex floating point, IEEE

Here is an example showing how the floating type measurements come out
for the most common floating point representation, specified by the
@cite{IEEE Standard for Binary Floating Point Arithmetic (ANSI/IEEE Std
754-1985)}.  Nearly all computers designed since the 1980s use this
format.

The IEEE single-precision float representation uses a base of 2.  There
is a sign bit, a mantissa with 23 bits plus one hidden bit (so the total
precision is 24 base-2 digits), and an 8-bit exponent that can represent
values in the range -125 to 128, inclusive.

So, for an implementation that uses this representation for the
@code{float} data type, appropriate values for the corresponding
parameters are:

@smallexample
FLT_RADIX                             2
FLT_MANT_DIG                         24
FLT_DIG                               6
FLT_MIN_EXP                        -125
FLT_MIN_10_EXP                      -37
FLT_MAX_EXP                         128
FLT_MAX_10_EXP                      +38
FLT_MIN                 1.17549435E-38F
FLT_MAX                 3.40282347E+38F
FLT_EPSILON             1.19209290E-07F
@end smallexample

Here are the values for the @code{double} data type:

@smallexample
DBL_MANT_DIG                         53
DBL_DIG                              15
DBL_MIN_EXP                       -1021
DBL_MIN_10_EXP                     -307
DBL_MAX_EXP                        1024
DBL_MAX_10_EXP                      308
DBL_MAX         1.7976931348623157E+308
DBL_MIN         2.2250738585072014E-308
DBL_EPSILON     2.2204460492503131E-016
@end smallexample

@node Structure Measurement
@subsection Structure Field Offset Measurement

You can use @code{offsetof} to measure the location within a structure
type of a particular structure member.

@comment stddef.h
@comment ISO
@deftypefn {Macro} size_t offsetof (@var{type}, @var{member})
@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
@c This is no longer provided by glibc, but rather by the compiler.
This expands to an integer constant expression that is the offset of the
structure member named @var{member} in the structure type @var{type}.
For example, @code{offsetof (struct s, elem)} is the offset, in bytes,
of the member @code{elem} in a @code{struct s}.

This macro won't work if @var{member} is a bit field; you get an error
from the C compiler in that case.
@end deftypefn