1
@node System Configuration, Cryptographic Functions, System Management, Top
2
@c %MENU% Parameters describing operating system limits
3
@chapter System Configuration Parameters
5
The functions and macros listed in this chapter give information about
6
configuration parameters of the operating system---for example, capacity
7
limits, presence of optional POSIX features, and the default path for
8
executable files (@pxref{String Parameters}).
11
* General Limits:: Constants and functions that describe
12
various process-related limits that have
13
one uniform value for any given machine.
14
* System Options:: Optional POSIX features.
15
* Version Supported:: Version numbers of POSIX.1 and POSIX.2.
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* Sysconf:: Getting specific configuration values
17
of general limits and system options.
18
* Minimums:: Minimum values for general limits.
20
* Limits for Files:: Size limitations that pertain to individual files.
21
These can vary between file systems
22
or even from file to file.
23
* Options for Files:: Optional features that some files may support.
24
* File Minimums:: Minimum values for file limits.
25
* Pathconf:: Getting the limit values for a particular file.
27
* Utility Limits:: Capacity limits of some POSIX.2 utility programs.
28
* Utility Minimums:: Minimum allowable values of those limits.
30
* String Parameters:: Getting the default search path.
34
@section General Capacity Limits
35
@cindex POSIX capacity limits
37
@cindex capacity limits, POSIX
39
The POSIX.1 and POSIX.2 standards specify a number of parameters that
40
describe capacity limitations of the system. These limits can be fixed
41
constants for a given operating system, or they can vary from machine to
42
machine. For example, some limit values may be configurable by the
43
system administrator, either at run time or by rebuilding the kernel,
44
and this should not require recompiling application programs.
47
Each of the following limit parameters has a macro that is defined in
48
@file{limits.h} only if the system has a fixed, uniform limit for the
49
parameter in question. If the system allows different file systems or
50
files to have different limits, then the macro is undefined; use
51
@code{sysconf} to find out the limit that applies at a particular time
52
on a particular machine. @xref{Sysconf}.
54
Each of these parameters also has another macro, with a name starting
55
with @samp{_POSIX}, which gives the lowest value that the limit is
56
allowed to have on @emph{any} POSIX system. @xref{Minimums}.
58
@cindex limits, program argument size
61
@deftypevr Macro int ARG_MAX
62
If defined, the unvarying maximum combined length of the @var{argv} and
63
@var{environ} arguments that can be passed to the @code{exec} functions.
66
@cindex limits, number of processes
69
@deftypevr Macro int CHILD_MAX
70
If defined, the unvarying maximum number of processes that can exist
71
with the same real user ID at any one time. In BSD and GNU, this is
72
controlled by the @code{RLIMIT_NPROC} resource limit; @pxref{Limits on
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@cindex limits, number of open files
79
@deftypevr Macro int OPEN_MAX
80
If defined, the unvarying maximum number of files that a single process
81
can have open simultaneously. In BSD and GNU, this is controlled
82
by the @code{RLIMIT_NOFILE} resource limit; @pxref{Limits on Resources}.
87
@deftypevr Macro int STREAM_MAX
88
If defined, the unvarying maximum number of streams that a single
89
process can have open simultaneously. @xref{Opening Streams}.
92
@cindex limits, time zone name length
95
@deftypevr Macro int TZNAME_MAX
96
If defined, the unvarying maximum length of a time zone name.
97
@xref{Time Zone Functions}.
100
These limit macros are always defined in @file{limits.h}.
102
@cindex limits, number of supplementary group IDs
105
@deftypevr Macro int NGROUPS_MAX
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The maximum number of supplementary group IDs that one process can have.
108
The value of this macro is actually a lower bound for the maximum. That
109
is, you can count on being able to have that many supplementary group
110
IDs, but a particular machine might let you have even more. You can use
111
@code{sysconf} to see whether a particular machine will let you have
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more (@pxref{Sysconf}).
117
@deftypevr Macro int SSIZE_MAX
118
The largest value that can fit in an object of type @code{ssize_t}.
119
Effectively, this is the limit on the number of bytes that can be read
120
or written in a single operation.
122
This macro is defined in all POSIX systems because this limit is never
128
@deftypevr Macro int RE_DUP_MAX
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The largest number of repetitions you are guaranteed is allowed in the
130
construct @samp{\@{@var{min},@var{max}\@}} in a regular expression.
132
The value of this macro is actually a lower bound for the maximum. That
133
is, you can count on being able to have that many repetitions, but a
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particular machine might let you have even more. You can use
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@code{sysconf} to see whether a particular machine will let you have
136
more (@pxref{Sysconf}). And even the value that @code{sysconf} tells
137
you is just a lower bound---larger values might work.
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This macro is defined in all POSIX.2 systems, because POSIX.2 says it
140
should always be defined even if there is no specific imposed limit.
144
@section Overall System Options
145
@cindex POSIX optional features
146
@cindex optional POSIX features
148
POSIX defines certain system-specific options that not all POSIX systems
149
support. Since these options are provided in the kernel, not in the
150
library, simply using the GNU C library does not guarantee any of these
151
features is supported; it depends on the system you are using.
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You can test for the availability of a given option using the macros in
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this section, together with the function @code{sysconf}. The macros are
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defined only if you include @file{unistd.h}.
158
For the following macros, if the macro is defined in @file{unistd.h},
159
then the option is supported. Otherwise, the option may or may not be
160
supported; use @code{sysconf} to find out. @xref{Sysconf}.
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@deftypevr Macro int _POSIX_JOB_CONTROL
165
If this symbol is defined, it indicates that the system supports job
166
control. Otherwise, the implementation behaves as if all processes
167
within a session belong to a single process group. @xref{Job Control}.
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@deftypevr Macro int _POSIX_SAVED_IDS
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If this symbol is defined, it indicates that the system remembers the
174
effective user and group IDs of a process before it executes an
175
executable file with the set-user-ID or set-group-ID bits set, and that
176
explicitly changing the effective user or group IDs back to these values
177
is permitted. If this option is not defined, then if a nonprivileged
178
process changes its effective user or group ID to the real user or group
179
ID of the process, it can't change it back again. @xref{Enable/Disable
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For the following macros, if the macro is defined in @file{unistd.h},
184
then its value indicates whether the option is supported. A value of
185
@code{-1} means no, and any other value means yes. If the macro is not
186
defined, then the option may or may not be supported; use @code{sysconf}
187
to find out. @xref{Sysconf}.
191
@deftypevr Macro int _POSIX2_C_DEV
192
If this symbol is defined, it indicates that the system has the POSIX.2
193
C compiler command, @code{c89}. The GNU C library always defines this
194
as @code{1}, on the assumption that you would not have installed it if
195
you didn't have a C compiler.
200
@deftypevr Macro int _POSIX2_FORT_DEV
201
If this symbol is defined, it indicates that the system has the POSIX.2
202
Fortran compiler command, @code{fort77}. The GNU C library never
203
defines this, because we don't know what the system has.
208
@deftypevr Macro int _POSIX2_FORT_RUN
209
If this symbol is defined, it indicates that the system has the POSIX.2
210
@code{asa} command to interpret Fortran carriage control. The GNU C
211
library never defines this, because we don't know what the system has.
216
@deftypevr Macro int _POSIX2_LOCALEDEF
217
If this symbol is defined, it indicates that the system has the POSIX.2
218
@code{localedef} command. The GNU C library never defines this, because
219
we don't know what the system has.
224
@deftypevr Macro int _POSIX2_SW_DEV
225
If this symbol is defined, it indicates that the system has the POSIX.2
226
commands @code{ar}, @code{make}, and @code{strip}. The GNU C library
227
always defines this as @code{1}, on the assumption that you had to have
228
@code{ar} and @code{make} to install the library, and it's unlikely that
229
@code{strip} would be absent when those are present.
232
@node Version Supported
233
@section Which Version of POSIX is Supported
237
@deftypevr Macro {long int} _POSIX_VERSION
238
This constant represents the version of the POSIX.1 standard to which
239
the implementation conforms. For an implementation conforming to the
240
1995 POSIX.1 standard, the value is the integer @code{199506L}.
242
@code{_POSIX_VERSION} is always defined (in @file{unistd.h}) in any
245
@strong{Usage Note:} Don't try to test whether the system supports POSIX
246
by including @file{unistd.h} and then checking whether
247
@code{_POSIX_VERSION} is defined. On a non-POSIX system, this will
248
probably fail because there is no @file{unistd.h}. We do not know of
249
@emph{any} way you can reliably test at compilation time whether your
250
target system supports POSIX or whether @file{unistd.h} exists.
252
The GNU C compiler predefines the symbol @code{__POSIX__} if the target
253
system is a POSIX system. Provided you do not use any other compilers
254
on POSIX systems, testing @code{defined (__POSIX__)} will reliably
260
@deftypevr Macro {long int} _POSIX2_C_VERSION
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This constant represents the version of the POSIX.2 standard which the
262
library and system kernel support. We don't know what value this will
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be for the first version of the POSIX.2 standard, because the value is
264
based on the year and month in which the standard is officially adopted.
266
The value of this symbol says nothing about the utilities installed on
269
@strong{Usage Note:} You can use this macro to tell whether a POSIX.1
270
system library supports POSIX.2 as well. Any POSIX.1 system contains
271
@file{unistd.h}, so include that file and then test @code{defined
272
(_POSIX2_C_VERSION)}.
276
@section Using @code{sysconf}
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When your system has configurable system limits, you can use the
279
@code{sysconf} function to find out the value that applies to any
280
particular machine. The function and the associated @var{parameter}
281
constants are declared in the header file @file{unistd.h}.
284
* Sysconf Definition:: Detailed specifications of @code{sysconf}.
285
* Constants for Sysconf:: The list of parameters @code{sysconf} can read.
286
* Examples of Sysconf:: How to use @code{sysconf} and the parameter
287
macros properly together.
290
@node Sysconf Definition
291
@subsection Definition of @code{sysconf}
295
@deftypefun {long int} sysconf (int @var{parameter})
296
This function is used to inquire about runtime system parameters. The
297
@var{parameter} argument should be one of the @samp{_SC_} symbols listed
300
The normal return value from @code{sysconf} is the value you requested.
301
A value of @code{-1} is returned both if the implementation does not
302
impose a limit, and in case of an error.
304
The following @code{errno} error conditions are defined for this function:
308
The value of the @var{parameter} is invalid.
312
@node Constants for Sysconf
313
@subsection Constants for @code{sysconf} Parameters
315
Here are the symbolic constants for use as the @var{parameter} argument
316
to @code{sysconf}. The values are all integer constants (more
317
specifically, enumeration type values).
323
Inquire about the parameter corresponding to @code{ARG_MAX}.
328
Inquire about the parameter corresponding to @code{CHILD_MAX}.
333
Inquire about the parameter corresponding to @code{OPEN_MAX}.
338
Inquire about the parameter corresponding to @code{STREAM_MAX}.
343
Inquire about the parameter corresponding to @code{TZNAME_MAX}.
347
@item _SC_NGROUPS_MAX
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Inquire about the parameter corresponding to @code{NGROUPS_MAX}.
352
@item _SC_JOB_CONTROL
353
Inquire about the parameter corresponding to @code{_POSIX_JOB_CONTROL}.
358
Inquire about the parameter corresponding to @code{_POSIX_SAVED_IDS}.
363
Inquire about the parameter corresponding to @code{_POSIX_VERSION}.
368
Inquire about the parameter corresponding to @code{CLOCKS_PER_SEC};
373
@item _SC_CHARCLASS_NAME_MAX
374
Inquire about the parameter corresponding to maximal length allowed for
375
a character class name in an extended locale specification. These
376
extensions are not yet standardized and so this option is not standardized
381
@item _SC_REALTIME_SIGNALS
382
Inquire about the parameter corresponding to @code{_POSIX_REALTIME_SIGNALS}.
386
@item _SC_PRIORITY_SCHEDULING
387
Inquire about the parameter corresponding to @code{_POSIX_PRIORITY_SCHEDULING}.
392
Inquire about the parameter corresponding to @code{_POSIX_TIMERS}.
396
@item _SC_ASYNCHRONOUS_IO
397
Inquire about the parameter corresponding to @code{_POSIX_ASYNCHRONOUS_IO}.
401
@item _SC_PRIORITIZED_IO
402
Inquire about the parameter corresponding to @code{_POSIX_PRIORITIZED_IO}.
406
@item _SC_SYNCHRONIZED_IO
407
Inquire about the parameter corresponding to @code{_POSIX_SYNCHRONIZED_IO}.
412
Inquire about the parameter corresponding to @code{_POSIX_FSYNC}.
416
@item _SC_MAPPED_FILES
417
Inquire about the parameter corresponding to @code{_POSIX_MAPPED_FILES}.
422
Inquire about the parameter corresponding to @code{_POSIX_MEMLOCK}.
426
@item _SC_MEMLOCK_RANGE
427
Inquire about the parameter corresponding to @code{_POSIX_MEMLOCK_RANGE}.
431
@item _SC_MEMORY_PROTECTION
432
Inquire about the parameter corresponding to @code{_POSIX_MEMORY_PROTECTION}.
436
@item _SC_MESSAGE_PASSING
437
Inquire about the parameter corresponding to @code{_POSIX_MESSAGE_PASSING}.
442
Inquire about the parameter corresponding to @code{_POSIX_SEMAPHORES}.
446
@item _SC_SHARED_MEMORY_OBJECTS
447
Inquire about the parameter corresponding to@*
448
@code{_POSIX_SHARED_MEMORY_OBJECTS}.
452
@item _SC_AIO_LISTIO_MAX
453
Inquire about the parameter corresponding to @code{_POSIX_AIO_LISTIO_MAX}.
458
Inquire about the parameter corresponding to @code{_POSIX_AIO_MAX}.
462
@item _SC_AIO_PRIO_DELTA_MAX
463
Inquire the value by which a process can decrease its asynchronous I/O
464
priority level from its own scheduling priority. This corresponds to the
465
run-time invariant value @code{AIO_PRIO_DELTA_MAX}.
469
@item _SC_DELAYTIMER_MAX
470
Inquire about the parameter corresponding to @code{_POSIX_DELAYTIMER_MAX}.
474
@item _SC_MQ_OPEN_MAX
475
Inquire about the parameter corresponding to @code{_POSIX_MQ_OPEN_MAX}.
479
@item _SC_MQ_PRIO_MAX
480
Inquire about the parameter corresponding to @code{_POSIX_MQ_PRIO_MAX}.
485
Inquire about the parameter corresponding to @code{_POSIX_RTSIG_MAX}.
489
@item _SC_SEM_NSEMS_MAX
490
Inquire about the parameter corresponding to @code{_POSIX_SEM_NSEMS_MAX}.
494
@item _SC_SEM_VALUE_MAX
495
Inquire about the parameter corresponding to @code{_POSIX_SEM_VALUE_MAX}.
499
@item _SC_SIGQUEUE_MAX
500
Inquire about the parameter corresponding to @code{_POSIX_SIGQUEUE_MAX}.
505
Inquire about the parameter corresponding to @code{_POSIX_TIMER_MAX}.
510
Inquire about the parameter corresponding to @code{_POSIX_PII}.
515
Inquire about the parameter corresponding to @code{_POSIX_PII_XTI}.
520
Inquire about the parameter corresponding to @code{_POSIX_PII_SOCKET}.
524
@item _SC_PII_INTERNET
525
Inquire about the parameter corresponding to @code{_POSIX_PII_INTERNET}.
530
Inquire about the parameter corresponding to @code{_POSIX_PII_OSI}.
535
Inquire about the parameter corresponding to @code{_POSIX_SELECT}.
540
Inquire about the parameter corresponding to @code{_POSIX_UIO_MAXIOV}.
544
@item _SC_PII_INTERNET_STREAM
545
Inquire about the parameter corresponding to @code{_POSIX_PII_INTERNET_STREAM}.
549
@item _SC_PII_INTERNET_DGRAM
550
Inquire about the parameter corresponding to @code{_POSIX_PII_INTERNET_DGRAM}.
554
@item _SC_PII_OSI_COTS
555
Inquire about the parameter corresponding to @code{_POSIX_PII_OSI_COTS}.
559
@item _SC_PII_OSI_CLTS
560
Inquire about the parameter corresponding to @code{_POSIX_PII_OSI_CLTS}.
565
Inquire about the parameter corresponding to @code{_POSIX_PII_OSI_M}.
570
Inquire the value of the value associated with the @code{T_IOV_MAX}
576
Inquire about the parameter corresponding to @code{_POSIX_THREADS}.
580
@item _SC_THREAD_SAFE_FUNCTIONS
581
Inquire about the parameter corresponding to@*
582
@code{_POSIX_THREAD_SAFE_FUNCTIONS}.
586
@item _SC_GETGR_R_SIZE_MAX
587
Inquire about the parameter corresponding to @code{_POSIX_GETGR_R_SIZE_MAX}.
591
@item _SC_GETPW_R_SIZE_MAX
592
Inquire about the parameter corresponding to @code{_POSIX_GETPW_R_SIZE_MAX}.
596
@item _SC_LOGIN_NAME_MAX
597
Inquire about the parameter corresponding to @code{_POSIX_LOGIN_NAME_MAX}.
601
@item _SC_TTY_NAME_MAX
602
Inquire about the parameter corresponding to @code{_POSIX_TTY_NAME_MAX}.
606
@item _SC_THREAD_DESTRUCTOR_ITERATIONS
607
Inquire about the parameter corresponding to
608
@code{_POSIX_THREAD_DESTRUCTOR_ITERATIONS}.
612
@item _SC_THREAD_KEYS_MAX
613
Inquire about the parameter corresponding to @code{_POSIX_THREAD_KEYS_MAX}.
617
@item _SC_THREAD_STACK_MIN
618
Inquire about the parameter corresponding to @code{_POSIX_THREAD_STACK_MIN}.
622
@item _SC_THREAD_THREADS_MAX
623
Inquire about the parameter corresponding to @code{_POSIX_THREAD_THREADS_MAX}.
627
@item _SC_THREAD_ATTR_STACKADDR
628
Inquire about the parameter corresponding to@*a
629
@code{_POSIX_THREAD_ATTR_STACKADDR}.
633
@item _SC_THREAD_ATTR_STACKSIZE
634
Inquire about the parameter corresponding to@*
635
@code{_POSIX_THREAD_ATTR_STACKSIZE}.
639
@item _SC_THREAD_PRIORITY_SCHEDULING
640
Inquire about the parameter corresponding to
641
@code{_POSIX_THREAD_PRIORITY_SCHEDULING}.
645
@item _SC_THREAD_PRIO_INHERIT
646
Inquire about the parameter corresponding to @code{_POSIX_THREAD_PRIO_INHERIT}.
650
@item _SC_THREAD_PRIO_PROTECT
651
Inquire about the parameter corresponding to @code{_POSIX_THREAD_PRIO_PROTECT}.
655
@item _SC_THREAD_PROCESS_SHARED
656
Inquire about the parameter corresponding to
657
@code{_POSIX_THREAD_PROCESS_SHARED}.
662
Inquire about whether the system has the POSIX.2 C compiler command,
668
Inquire about whether the system has the POSIX.2 Fortran compiler
669
command, @code{fort77}.
674
Inquire about whether the system has the POSIX.2 @code{asa} command to
675
interpret Fortran carriage control.
679
@item _SC_2_LOCALEDEF
680
Inquire about whether the system has the POSIX.2 @code{localedef}
686
Inquire about whether the system has the POSIX.2 commands @code{ar},
687
@code{make}, and @code{strip}.
691
@item _SC_BC_BASE_MAX
692
Inquire about the maximum value of @code{obase} in the @code{bc}
698
Inquire about the maximum size of an array in the @code{bc}
703
@item _SC_BC_SCALE_MAX
704
Inquire about the maximum value of @code{scale} in the @code{bc}
709
@item _SC_BC_STRING_MAX
710
Inquire about the maximum size of a string constant in the
715
@item _SC_COLL_WEIGHTS_MAX
716
Inquire about the maximum number of weights that can necessarily
717
be used in defining the collating sequence for a locale.
721
@item _SC_EXPR_NEST_MAX
722
Inquire about the maximum number of expressions nested within
723
parentheses when using the @code{expr} utility.
728
Inquire about the maximum size of a text line that the POSIX.2 text
729
utilities can handle.
733
@item _SC_EQUIV_CLASS_MAX
734
Inquire about the maximum number of weights that can be assigned to an
735
entry of the @code{LC_COLLATE} category @samp{order} keyword in a locale
736
definition. The GNU C library does not presently support locale
742
Inquire about the version number of POSIX.1 that the library and kernel
748
Inquire about the version number of POSIX.2 that the system utilities
754
Inquire about the virtual memory page size of the machine.
755
@code{getpagesize} returns the same value (@pxref{Query Memory Parameters}).
759
@item _SC_NPROCESSORS_CONF
760
Inquire about the number of configured processors.
764
@item _SC_NPROCESSORS_ONLN
765
Inquire about the number of processors online.
770
Inquire about the number of physical pages in the system.
774
@item _SC_AVPHYS_PAGES
775
Inquire about the number of available physical pages in the system.
780
Inquire about the number of functions which can be registered as termination
781
functions for @code{atexit}; @pxref{Cleanups on Exit}.
785
@item _SC_XOPEN_VERSION
786
Inquire about the parameter corresponding to @code{_XOPEN_VERSION}.
790
@item _SC_XOPEN_XCU_VERSION
791
Inquire about the parameter corresponding to @code{_XOPEN_XCU_VERSION}.
796
Inquire about the parameter corresponding to @code{_XOPEN_UNIX}.
800
@item _SC_XOPEN_REALTIME
801
Inquire about the parameter corresponding to @code{_XOPEN_REALTIME}.
805
@item _SC_XOPEN_REALTIME_THREADS
806
Inquire about the parameter corresponding to @code{_XOPEN_REALTIME_THREADS}.
810
@item _SC_XOPEN_LEGACY
811
Inquire about the parameter corresponding to @code{_XOPEN_LEGACY}.
815
@item _SC_XOPEN_CRYPT
816
Inquire about the parameter corresponding to @code{_XOPEN_CRYPT}.
820
@item _SC_XOPEN_ENH_I18N
821
Inquire about the parameter corresponding to @code{_XOPEN_ENH_I18N}.
826
Inquire about the parameter corresponding to @code{_XOPEN_SHM}.
831
Inquire about the parameter corresponding to @code{_XOPEN_XPG2}.
836
Inquire about the parameter corresponding to @code{_XOPEN_XPG3}.
841
Inquire about the parameter corresponding to @code{_XOPEN_XPG4}.
846
Inquire about the number of bits in a variable of type @code{char}.
851
Inquire about the maximum value which can be stored in a variable of type
857
Inquire about the minimum value which can be stored in a variable of type
863
Inquire about the maximum value which can be stored in a variable of type
869
Inquire about the minimum value which can be stored in a variable of type
875
Inquire about the number of bits in a variable of type @code{long int}.
880
Inquire about the number of bits in a variable of a register word.
885
Inquire the maximum length of a multi-byte representation of a wide
891
Inquire about the value used to internally represent the zero priority level for
892
the process execution.
897
Inquire about the maximum value which can be stored in a variable of type
903
Inquire about the maximum value which can be stored in a variable of type
909
Inquire about the minimum value which can be stored in a variable of type
915
Inquire about the maximum value which can be stored in a variable of type
921
Inquire about the minimum value which can be stored in a variable of type
927
Inquire about the maximum value which can be stored in a variable of type
928
@code{unsigned char}.
933
Inquire about the maximum value which can be stored in a variable of type
939
Inquire about the maximum value which can be stored in a variable of type
940
@code{unsigned long int}.
945
Inquire about the maximum value which can be stored in a variable of type
946
@code{unsigned short int}.
951
Inquire about the parameter corresponding to @code{NL_ARGMAX}.
956
Inquire about the parameter corresponding to @code{NL_LANGMAX}.
961
Inquire about the parameter corresponding to @code{NL_MSGMAX}.
966
Inquire about the parameter corresponding to @code{NL_NMAX}.
971
Inquire about the parameter corresponding to @code{NL_SETMAX}.
976
Inquire about the parameter corresponding to @code{NL_TEXTMAX}.
979
@node Examples of Sysconf
980
@subsection Examples of @code{sysconf}
982
We recommend that you first test for a macro definition for the
983
parameter you are interested in, and call @code{sysconf} only if the
984
macro is not defined. For example, here is how to test whether job
985
control is supported:
990
have_job_control (void)
992
#ifdef _POSIX_JOB_CONTROL
995
int value = sysconf (_SC_JOB_CONTROL);
997
/* @r{If the system is that badly wedged,}
998
@r{there's no use trying to go on.} */
999
fatal (strerror (errno));
1006
Here is how to get the value of a numeric limit:
1015
int value = sysconf (_SC_CHILD_MAX);
1017
fatal (strerror (errno));
1024
@section Minimum Values for General Capacity Limits
1026
Here are the names for the POSIX minimum upper bounds for the system
1027
limit parameters. The significance of these values is that you can
1028
safely push to these limits without checking whether the particular
1029
system you are using can go that far.
1034
@item _POSIX_AIO_LISTIO_MAX
1035
The most restrictive limit permitted by POSIX for the maximum number of
1036
I/O operations that can be specified in a list I/O call. The value of
1037
this constant is @code{2}; thus you can add up to two new entries
1038
of the list of outstanding operations.
1042
@item _POSIX_AIO_MAX
1043
The most restrictive limit permitted by POSIX for the maximum number of
1044
outstanding asynchronous I/O operations. The value of this constant is
1045
@code{1}. So you cannot expect that you can issue more than one
1046
operation and immediately continue with the normal work, receiving the
1047
notifications asynchronously.
1051
@item _POSIX_ARG_MAX
1052
The value of this macro is the most restrictive limit permitted by POSIX
1053
for the maximum combined length of the @var{argv} and @var{environ}
1054
arguments that can be passed to the @code{exec} functions.
1055
Its value is @code{4096}.
1059
@item _POSIX_CHILD_MAX
1060
The value of this macro is the most restrictive limit permitted by POSIX
1061
for the maximum number of simultaneous processes per real user ID. Its
1066
@item _POSIX_NGROUPS_MAX
1067
The value of this macro is the most restrictive limit permitted by POSIX
1068
for the maximum number of supplementary group IDs per process. Its
1073
@item _POSIX_OPEN_MAX
1074
The value of this macro is the most restrictive limit permitted by POSIX
1075
for the maximum number of files that a single process can have open
1076
simultaneously. Its value is @code{16}.
1080
@item _POSIX_SSIZE_MAX
1081
The value of this macro is the most restrictive limit permitted by POSIX
1082
for the maximum value that can be stored in an object of type
1083
@code{ssize_t}. Its value is @code{32767}.
1087
@item _POSIX_STREAM_MAX
1088
The value of this macro is the most restrictive limit permitted by POSIX
1089
for the maximum number of streams that a single process can have open
1090
simultaneously. Its value is @code{8}.
1094
@item _POSIX_TZNAME_MAX
1095
The value of this macro is the most restrictive limit permitted by POSIX
1096
for the maximum length of a time zone name. Its value is @code{3}.
1100
@item _POSIX2_RE_DUP_MAX
1101
The value of this macro is the most restrictive limit permitted by POSIX
1102
for the numbers used in the @samp{\@{@var{min},@var{max}\@}} construct
1103
in a regular expression. Its value is @code{255}.
1106
@node Limits for Files
1107
@section Limits on File System Capacity
1109
The POSIX.1 standard specifies a number of parameters that describe the
1110
limitations of the file system. It's possible for the system to have a
1111
fixed, uniform limit for a parameter, but this isn't the usual case. On
1112
most systems, it's possible for different file systems (and, for some
1113
parameters, even different files) to have different maximum limits. For
1114
example, this is very likely if you use NFS to mount some of the file
1115
systems from other machines.
1118
Each of the following macros is defined in @file{limits.h} only if the
1119
system has a fixed, uniform limit for the parameter in question. If the
1120
system allows different file systems or files to have different limits,
1121
then the macro is undefined; use @code{pathconf} or @code{fpathconf} to
1122
find out the limit that applies to a particular file. @xref{Pathconf}.
1124
Each parameter also has another macro, with a name starting with
1125
@samp{_POSIX}, which gives the lowest value that the limit is allowed to
1126
have on @emph{any} POSIX system. @xref{File Minimums}.
1128
@cindex limits, link count of files
1131
@deftypevr Macro int LINK_MAX
1132
The uniform system limit (if any) for the number of names for a given
1133
file. @xref{Hard Links}.
1136
@cindex limits, terminal input queue
1139
@deftypevr Macro int MAX_CANON
1140
The uniform system limit (if any) for the amount of text in a line of
1141
input when input editing is enabled. @xref{Canonical or Not}.
1146
@deftypevr Macro int MAX_INPUT
1147
The uniform system limit (if any) for the total number of characters
1148
typed ahead as input. @xref{I/O Queues}.
1151
@cindex limits, file name length
1154
@deftypevr Macro int NAME_MAX
1155
The uniform system limit (if any) for the length of a file name component.
1160
@deftypevr Macro int PATH_MAX
1161
The uniform system limit (if any) for the length of an entire file name (that
1162
is, the argument given to system calls such as @code{open}).
1165
@cindex limits, pipe buffer size
1168
@deftypevr Macro int PIPE_BUF
1169
The uniform system limit (if any) for the number of bytes that can be
1170
written atomically to a pipe. If multiple processes are writing to the
1171
same pipe simultaneously, output from different processes might be
1172
interleaved in chunks of this size. @xref{Pipes and FIFOs}.
1175
These are alternative macro names for some of the same information.
1179
@deftypevr Macro int MAXNAMLEN
1180
This is the BSD name for @code{NAME_MAX}. It is defined in
1186
@deftypevr Macro int FILENAME_MAX
1187
The value of this macro is an integer constant expression that
1188
represents the maximum length of a file name string. It is defined in
1191
Unlike @code{PATH_MAX}, this macro is defined even if there is no actual
1192
limit imposed. In such a case, its value is typically a very large
1193
number. @strong{This is always the case on the GNU system.}
1195
@strong{Usage Note:} Don't use @code{FILENAME_MAX} as the size of an
1196
array in which to store a file name! You can't possibly make an array
1197
that big! Use dynamic allocation (@pxref{Memory Allocation}) instead.
1200
@node Options for Files
1201
@section Optional Features in File Support
1203
POSIX defines certain system-specific options in the system calls for
1204
operating on files. Some systems support these options and others do
1205
not. Since these options are provided in the kernel, not in the
1206
library, simply using the GNU C library does not guarantee that any of these
1207
features is supported; it depends on the system you are using. They can
1208
also vary between file systems on a single machine.
1211
This section describes the macros you can test to determine whether a
1212
particular option is supported on your machine. If a given macro is
1213
defined in @file{unistd.h}, then its value says whether the
1214
corresponding feature is supported. (A value of @code{-1} indicates no;
1215
any other value indicates yes.) If the macro is undefined, it means
1216
particular files may or may not support the feature.
1218
Since all the machines that support the GNU C library also support NFS,
1219
one can never make a general statement about whether all file systems
1220
support the @code{_POSIX_CHOWN_RESTRICTED} and @code{_POSIX_NO_TRUNC}
1221
features. So these names are never defined as macros in the GNU C
1226
@deftypevr Macro int _POSIX_CHOWN_RESTRICTED
1227
If this option is in effect, the @code{chown} function is restricted so
1228
that the only changes permitted to nonprivileged processes is to change
1229
the group owner of a file to either be the effective group ID of the
1230
process, or one of its supplementary group IDs. @xref{File Owner}.
1235
@deftypevr Macro int _POSIX_NO_TRUNC
1236
If this option is in effect, file name components longer than
1237
@code{NAME_MAX} generate an @code{ENAMETOOLONG} error. Otherwise, file
1238
name components that are too long are silently truncated.
1243
@deftypevr Macro {unsigned char} _POSIX_VDISABLE
1244
This option is only meaningful for files that are terminal devices.
1245
If it is enabled, then handling for special control characters can
1246
be disabled individually. @xref{Special Characters}.
1250
If one of these macros is undefined, that means that the option might be
1251
in effect for some files and not for others. To inquire about a
1252
particular file, call @code{pathconf} or @code{fpathconf}.
1256
@section Minimum Values for File System Limits
1258
Here are the names for the POSIX minimum upper bounds for some of the
1259
above parameters. The significance of these values is that you can
1260
safely push to these limits without checking whether the particular
1261
system you are using can go that far. In most cases GNU systems do not
1262
have these strict limitations. The actual limit should be requested if
1268
@item _POSIX_LINK_MAX
1269
The most restrictive limit permitted by POSIX for the maximum value of a
1270
file's link count. The value of this constant is @code{8}; thus, you
1271
can always make up to eight names for a file without running into a
1276
@item _POSIX_MAX_CANON
1277
The most restrictive limit permitted by POSIX for the maximum number of
1278
bytes in a canonical input line from a terminal device. The value of
1279
this constant is @code{255}.
1283
@item _POSIX_MAX_INPUT
1284
The most restrictive limit permitted by POSIX for the maximum number of
1285
bytes in a terminal device input queue (or typeahead buffer).
1286
@xref{Input Modes}. The value of this constant is @code{255}.
1290
@item _POSIX_NAME_MAX
1291
The most restrictive limit permitted by POSIX for the maximum number of
1292
bytes in a file name component. The value of this constant is
1297
@item _POSIX_PATH_MAX
1298
The most restrictive limit permitted by POSIX for the maximum number of
1299
bytes in a file name. The value of this constant is @code{256}.
1303
@item _POSIX_PIPE_BUF
1304
The most restrictive limit permitted by POSIX for the maximum number of
1305
bytes that can be written atomically to a pipe. The value of this
1306
constant is @code{512}.
1311
Maximum number of bytes in a symbolic link.
1315
@item POSIX_REC_INCR_XFER_SIZE
1316
Recommended increment for file transfer sizes between the
1317
@code{POSIX_REC_MIN_XFER_SIZE} and @code{POSIX_REC_MAX_XFER_SIZE}
1322
@item POSIX_REC_MAX_XFER_SIZE
1323
Maximum recommended file transfer size.
1327
@item POSIX_REC_MIN_XFER_SIZE
1328
Minimum recommended file transfer size.
1332
@item POSIX_REC_XFER_ALIGN
1333
Recommended file transfer buffer alignment.
1337
@section Using @code{pathconf}
1339
When your machine allows different files to have different values for a
1340
file system parameter, you can use the functions in this section to find
1341
out the value that applies to any particular file.
1343
These functions and the associated constants for the @var{parameter}
1344
argument are declared in the header file @file{unistd.h}.
1348
@deftypefun {long int} pathconf (const char *@var{filename}, int @var{parameter})
1349
This function is used to inquire about the limits that apply to
1350
the file named @var{filename}.
1352
The @var{parameter} argument should be one of the @samp{_PC_} constants
1355
The normal return value from @code{pathconf} is the value you requested.
1356
A value of @code{-1} is returned both if the implementation does not
1357
impose a limit, and in case of an error. In the former case,
1358
@code{errno} is not set, while in the latter case, @code{errno} is set
1359
to indicate the cause of the problem. So the only way to use this
1360
function robustly is to store @code{0} into @code{errno} just before
1363
Besides the usual file name errors (@pxref{File Name Errors}),
1364
the following error condition is defined for this function:
1368
The value of @var{parameter} is invalid, or the implementation doesn't
1369
support the @var{parameter} for the specific file.
1375
@deftypefun {long int} fpathconf (int @var{filedes}, int @var{parameter})
1376
This is just like @code{pathconf} except that an open file descriptor
1377
is used to specify the file for which information is requested, instead
1380
The following @code{errno} error conditions are defined for this function:
1384
The @var{filedes} argument is not a valid file descriptor.
1387
The value of @var{parameter} is invalid, or the implementation doesn't
1388
support the @var{parameter} for the specific file.
1392
Here are the symbolic constants that you can use as the @var{parameter}
1393
argument to @code{pathconf} and @code{fpathconf}. The values are all
1400
Inquire about the value of @code{LINK_MAX}.
1405
Inquire about the value of @code{MAX_CANON}.
1410
Inquire about the value of @code{MAX_INPUT}.
1415
Inquire about the value of @code{NAME_MAX}.
1420
Inquire about the value of @code{PATH_MAX}.
1425
Inquire about the value of @code{PIPE_BUF}.
1429
@item _PC_CHOWN_RESTRICTED
1430
Inquire about the value of @code{_POSIX_CHOWN_RESTRICTED}.
1435
Inquire about the value of @code{_POSIX_NO_TRUNC}.
1440
Inquire about the value of @code{_POSIX_VDISABLE}.
1445
Inquire about the value of @code{_POSIX_SYNC_IO}.
1450
Inquire about the value of @code{_POSIX_ASYNC_IO}.
1455
Inquire about the value of @code{_POSIX_PRIO_IO}.
1459
@item _PC_FILESIZEBITS
1460
Inquire about the availability of large files on the filesystem.
1464
@item _PC_REC_INCR_XFER_SIZE
1465
Inquire about the value of @code{POSIX_REC_INCR_XFER_SIZE}.
1469
@item _PC_REC_MAX_XFER_SIZE
1470
Inquire about the value of @code{POSIX_REC_MAX_XFER_SIZE}.
1474
@item _PC_REC_MIN_XFER_SIZE
1475
Inquire about the value of @code{POSIX_REC_MIN_XFER_SIZE}.
1479
@item _PC_REC_XFER_ALIGN
1480
Inquire about the value of @code{POSIX_REC_XFER_ALIGN}.
1483
@node Utility Limits
1484
@section Utility Program Capacity Limits
1486
The POSIX.2 standard specifies certain system limits that you can access
1487
through @code{sysconf} that apply to utility behavior rather than the
1488
behavior of the library or the operating system.
1490
The GNU C library defines macros for these limits, and @code{sysconf}
1491
returns values for them if you ask; but these values convey no
1492
meaningful information. They are simply the smallest values that
1497
@deftypevr Macro int BC_BASE_MAX
1498
The largest value of @code{obase} that the @code{bc} utility is
1499
guaranteed to support.
1504
@deftypevr Macro int BC_DIM_MAX
1505
The largest number of elements in one array that the @code{bc} utility
1506
is guaranteed to support.
1511
@deftypevr Macro int BC_SCALE_MAX
1512
The largest value of @code{scale} that the @code{bc} utility is
1513
guaranteed to support.
1518
@deftypevr Macro int BC_STRING_MAX
1519
The largest number of characters in one string constant that the
1520
@code{bc} utility is guaranteed to support.
1525
@deftypevr Macro int COLL_WEIGHTS_MAX
1526
The largest number of weights that can necessarily be used in defining
1527
the collating sequence for a locale.
1532
@deftypevr Macro int EXPR_NEST_MAX
1533
The maximum number of expressions that can be nested within parenthesis
1534
by the @code{expr} utility.
1539
@deftypevr Macro int LINE_MAX
1540
The largest text line that the text-oriented POSIX.2 utilities can
1541
support. (If you are using the GNU versions of these utilities, then
1542
there is no actual limit except that imposed by the available virtual
1543
memory, but there is no way that the library can tell you this.)
1548
@deftypevr Macro int EQUIV_CLASS_MAX
1549
The maximum number of weights that can be assigned to an entry of the
1550
@code{LC_COLLATE} category @samp{order} keyword in a locale definition.
1551
The GNU C library does not presently support locale definitions.
1554
@node Utility Minimums
1555
@section Minimum Values for Utility Limits
1560
@item _POSIX2_BC_BASE_MAX
1561
The most restrictive limit permitted by POSIX.2 for the maximum value of
1562
@code{obase} in the @code{bc} utility. Its value is @code{99}.
1566
@item _POSIX2_BC_DIM_MAX
1567
The most restrictive limit permitted by POSIX.2 for the maximum size of
1568
an array in the @code{bc} utility. Its value is @code{2048}.
1572
@item _POSIX2_BC_SCALE_MAX
1573
The most restrictive limit permitted by POSIX.2 for the maximum value of
1574
@code{scale} in the @code{bc} utility. Its value is @code{99}.
1578
@item _POSIX2_BC_STRING_MAX
1579
The most restrictive limit permitted by POSIX.2 for the maximum size of
1580
a string constant in the @code{bc} utility. Its value is @code{1000}.
1584
@item _POSIX2_COLL_WEIGHTS_MAX
1585
The most restrictive limit permitted by POSIX.2 for the maximum number
1586
of weights that can necessarily be used in defining the collating
1587
sequence for a locale. Its value is @code{2}.
1591
@item _POSIX2_EXPR_NEST_MAX
1592
The most restrictive limit permitted by POSIX.2 for the maximum number
1593
of expressions nested within parenthesis when using the @code{expr} utility.
1594
Its value is @code{32}.
1598
@item _POSIX2_LINE_MAX
1599
The most restrictive limit permitted by POSIX.2 for the maximum size of
1600
a text line that the text utilities can handle. Its value is
1605
@item _POSIX2_EQUIV_CLASS_MAX
1606
The most restrictive limit permitted by POSIX.2 for the maximum number
1607
of weights that can be assigned to an entry of the @code{LC_COLLATE}
1608
category @samp{order} keyword in a locale definition. Its value is
1609
@code{2}. The GNU C library does not presently support locale
1613
@node String Parameters
1614
@section String-Valued Parameters
1616
POSIX.2 defines a way to get string-valued parameters from the operating
1617
system with the function @code{confstr}:
1621
@deftypefun size_t confstr (int @var{parameter}, char *@var{buf}, size_t @var{len})
1622
This function reads the value of a string-valued system parameter,
1623
storing the string into @var{len} bytes of memory space starting at
1624
@var{buf}. The @var{parameter} argument should be one of the
1625
@samp{_CS_} symbols listed below.
1627
The normal return value from @code{confstr} is the length of the string
1628
value that you asked for. If you supply a null pointer for @var{buf},
1629
then @code{confstr} does not try to store the string; it just returns
1630
its length. A value of @code{0} indicates an error.
1632
If the string you asked for is too long for the buffer (that is, longer
1633
than @code{@var{len} - 1}), then @code{confstr} stores just that much
1634
(leaving room for the terminating null character). You can tell that
1635
this has happened because @code{confstr} returns a value greater than or
1638
The following @code{errno} error conditions are defined for this function:
1642
The value of the @var{parameter} is invalid.
1646
Currently there is just one parameter you can read with @code{confstr}:
1652
This parameter's value is the recommended default path for searching for
1653
executable files. This is the path that a user has by default just
1658
@item _CS_LFS_CFLAGS
1659
The returned string specifies which additional flags must be given to
1660
the C compiler if a source is compiled using the
1661
@code{_LARGEFILE_SOURCE} feature select macro; @pxref{Feature Test Macros}.
1665
@item _CS_LFS_LDFLAGS
1666
The returned string specifies which additional flags must be given to
1667
the linker if a source is compiled using the
1668
@code{_LARGEFILE_SOURCE} feature select macro; @pxref{Feature Test Macros}.
1673
The returned string specifies which additional libraries must be linked
1674
to the application if a source is compiled using the
1675
@code{_LARGEFILE_SOURCE} feature select macro; @pxref{Feature Test Macros}.
1679
@item _CS_LFS_LINTFLAGS
1680
The returned string specifies which additional flags must be given to
1681
the lint tool if a source is compiled using the
1682
@code{_LARGEFILE_SOURCE} feature select macro; @pxref{Feature Test Macros}.
1686
@item _CS_LFS64_CFLAGS
1687
The returned string specifies which additional flags must be given to
1688
the C compiler if a source is compiled using the
1689
@code{_LARGEFILE64_SOURCE} feature select macro; @pxref{Feature Test Macros}.
1693
@item _CS_LFS64_LDFLAGS
1694
The returned string specifies which additional flags must be given to
1695
the linker if a source is compiled using the
1696
@code{_LARGEFILE64_SOURCE} feature select macro; @pxref{Feature Test Macros}.
1700
@item _CS_LFS64_LIBS
1701
The returned string specifies which additional libraries must be linked
1702
to the application if a source is compiled using the
1703
@code{_LARGEFILE64_SOURCE} feature select macro; @pxref{Feature Test Macros}.
1707
@item _CS_LFS64_LINTFLAGS
1708
The returned string specifies which additional flags must be given to
1709
the lint tool if a source is compiled using the
1710
@code{_LARGEFILE64_SOURCE} feature select macro; @pxref{Feature Test Macros}.
1713
The way to use @code{confstr} without any arbitrary limit on string size
1714
is to call it twice: first call it to get the length, allocate the
1715
buffer accordingly, and then call @code{confstr} again to fill the
1721
get_default_path (void)
1723
size_t len = confstr (_CS_PATH, NULL, 0);
1724
char *buffer = (char *) xmalloc (len);
1726
if (confstr (_CS_PATH, buf, len + 1) == 0)