manual/conf.texi

   1 @node System Configuration, Cryptographic Functions, System Management, Top
   2 @c %MENU% Parameters describing operating system limits
   3 @chapter System Configuration Parameters
   4
   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}).
   9
  10 @menu
  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.
  16 * Sysconf::                  Getting specific configuration values
  17                                 of general limits and system options.
  18 * Minimums::                 Minimum values for general limits.
  19
  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.
  26
  27 * Utility Limits::           Capacity limits of some POSIX.2 utility programs.
  28 * Utility Minimums::         Minimum allowable values of those limits.
  29
  30 * String Parameters::        Getting the default search path.
  31 @end menu
  32
  33 @node General Limits
  34 @section General Capacity Limits
  35 @cindex POSIX capacity limits
  36 @cindex limits, POSIX
  37 @cindex capacity limits, POSIX
  38
  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.
  45
  46 @pindex limits.h
  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}.
  53
  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}.
  57
  58 @cindex limits, program argument size
  59 @deftypevr Macro int ARG_MAX
  60 @standards{POSIX.1, limits.h}
  61 If defined, the unvarying maximum combined length of the @var{argv} and
  62 @var{environ} arguments that can be passed to the @code{exec} functions.
  63 @end deftypevr
  64
  65 @cindex limits, number of processes
  66 @deftypevr Macro int CHILD_MAX
  67 @standards{POSIX.1, limits.h}
  68 If defined, the unvarying maximum number of processes that can exist
  69 with the same real user ID at any one time.  In BSD and GNU, this is
  70 controlled by the @code{RLIMIT_NPROC} resource limit; @pxref{Limits on
  71 Resources}.
  72 @end deftypevr
  73
  74 @cindex limits, number of open files
  75 @deftypevr Macro int OPEN_MAX
  76 @standards{POSIX.1, limits.h}
  77 If defined, the unvarying maximum number of files that a single process
  78 can have open simultaneously.  In BSD and GNU, this is controlled
  79 by the @code{RLIMIT_NOFILE} resource limit; @pxref{Limits on Resources}.
  80 @end deftypevr
  81
  82 @deftypevr Macro int STREAM_MAX
  83 @standards{POSIX.1, limits.h}
  84 If defined, the unvarying maximum number of streams that a single
  85 process can have open simultaneously.  @xref{Opening Streams}.
  86 @end deftypevr
  87
  88 @cindex limits, time zone name length
  89 @deftypevr Macro int TZNAME_MAX
  90 @standards{POSIX.1, limits.h}
  91 If defined, the unvarying maximum length of a time zone name.
  92 @xref{Time Zone Functions}.
  93 @end deftypevr
  94
  95 These limit macros are always defined in @file{limits.h}.
  96
  97 @cindex limits, number of supplementary group IDs
  98 @deftypevr Macro int NGROUPS_MAX
  99 @standards{POSIX.1, limits.h}
 100 The maximum number of supplementary group IDs that one process can have.
 101
 102 The value of this macro is actually a lower bound for the maximum.  That
 103 is, you can count on being able to have that many supplementary group
 104 IDs, but a particular machine might let you have even more.  You can use
 105 @code{sysconf} to see whether a particular machine will let you have
 106 more (@pxref{Sysconf}).
 107 @end deftypevr
 108
 109 @deftypevr Macro ssize_t SSIZE_MAX
 110 @standards{POSIX.1, limits.h}
 111 The largest value that can fit in an object of type @code{ssize_t}.
 112 Effectively, this is the limit on the number of bytes that can be read
 113 or written in a single operation.
 114
 115 This macro is defined in all POSIX systems because this limit is never
 116 configurable.
 117 @end deftypevr
 118
 119 @deftypevr Macro int RE_DUP_MAX
 120 @standards{POSIX.2, limits.h}
 121 The largest number of repetitions you are guaranteed is allowed in the
 122 construct @samp{\@{@var{min},@var{max}\@}} in a regular expression.
 123
 124 The value of this macro is actually a lower bound for the maximum.  That
 125 is, you can count on being able to have that many repetitions, but a
 126 particular machine might let you have even more.  You can use
 127 @code{sysconf} to see whether a particular machine will let you have
 128 more (@pxref{Sysconf}).  And even the value that @code{sysconf} tells
 129 you is just a lower bound---larger values might work.
 130
 131 This macro is defined in all POSIX.2 systems, because POSIX.2 says it
 132 should always be defined even if there is no specific imposed limit.
 133 @end deftypevr
 134
 135 @node System Options
 136 @section Overall System Options
 137 @cindex POSIX optional features
 138 @cindex optional POSIX features
 139
 140 POSIX defines certain system-specific options that not all POSIX systems
 141 support.  Since these options are provided in the kernel, not in the
 142 library, simply using @theglibc{} does not guarantee any of these
 143 features are supported; it depends on the system you are using.
 144
 145 @pindex unistd.h
 146 You can test for the availability of a given option using the macros in
 147 this section, together with the function @code{sysconf}.  The macros are
 148 defined only if you include @file{unistd.h}.
 149
 150 For the following macros, if the macro is defined in @file{unistd.h},
 151 then the option is supported.  Otherwise, the option may or may not be
 152 supported; use @code{sysconf} to find out.  @xref{Sysconf}.
 153
 154 @deftypevr Macro int _POSIX_JOB_CONTROL
 155 @standards{POSIX.1, unistd.h}
 156 If this symbol is defined, it indicates that the system supports job
 157 control.  Otherwise, the implementation behaves as if all processes
 158 within a session belong to a single process group.  @xref{Job Control}.
 159 Systems conforming to the 2001 revision of POSIX, or newer, will
 160 always define this symbol.
 161 @end deftypevr
 162
 163 @deftypevr Macro int _POSIX_SAVED_IDS
 164 @standards{POSIX.1, unistd.h}
 165 If this symbol is defined, it indicates that the system remembers the
 166 effective user and group IDs of a process before it executes an
 167 executable file with the set-user-ID or set-group-ID bits set, and that
 168 explicitly changing the effective user or group IDs back to these values
 169 is permitted.  If this option is not defined, then if a nonprivileged
 170 process changes its effective user or group ID to the real user or group
 171 ID of the process, it can't change it back again.  @xref{Enable/Disable
 172 Setuid}.
 173 @end deftypevr
 174
 175 For the following macros, if the macro is defined in @file{unistd.h},
 176 then its value indicates whether the option is supported.  A value of
 177 @code{-1} means no, and any other value means yes.  If the macro is not
 178 defined, then the option may or may not be supported; use @code{sysconf}
 179 to find out.  @xref{Sysconf}.
 180
 181 @deftypevr Macro int _POSIX2_C_DEV
 182 @standards{POSIX.2, unistd.h}
 183 If this symbol is defined, it indicates that the system has the POSIX.2
 184 C compiler command, @code{c89}.  @Theglibc{} always defines this
 185 as @code{1}, on the assumption that you would not have installed it if
 186 you didn't have a C compiler.
 187 @end deftypevr
 188
 189 @deftypevr Macro int _POSIX2_FORT_DEV
 190 @standards{POSIX.2, unistd.h}
 191 If this symbol is defined, it indicates that the system has the POSIX.2
 192 Fortran compiler command, @code{fort77}.  @Theglibc{} never
 193 defines this, because we don't know what the system has.
 194 @end deftypevr
 195
 196 @deftypevr Macro int _POSIX2_FORT_RUN
 197 @standards{POSIX.2, unistd.h}
 198 If this symbol is defined, it indicates that the system has the POSIX.2
 199 @code{asa} command to interpret Fortran carriage control.  @Theglibc{}
 200 never defines this, because we don't know what the system has.
 201 @end deftypevr
 202
 203 @deftypevr Macro int _POSIX2_LOCALEDEF
 204 @standards{POSIX.2, unistd.h}
 205 If this symbol is defined, it indicates that the system has the POSIX.2
 206 @code{localedef} command.  @Theglibc{} never defines this, because
 207 we don't know what the system has.
 208 @end deftypevr
 209
 210 @deftypevr Macro int _POSIX2_SW_DEV
 211 @standards{POSIX.2, unistd.h}
 212 If this symbol is defined, it indicates that the system has the POSIX.2
 213 commands @code{ar}, @code{make}, and @code{strip}.  @Theglibc{}
 214 always defines this as @code{1}, on the assumption that you had to have
 215 @code{ar} and @code{make} to install the library, and it's unlikely that
 216 @code{strip} would be absent when those are present.
 217 @end deftypevr
 218
 219 @node Version Supported
 220 @section Which Version of POSIX is Supported
 221
 222 @deftypevr Macro {long int} _POSIX_VERSION
 223 @standards{POSIX.1, unistd.h}
 224 This constant represents the version of the POSIX.1 standard to which
 225 the implementation conforms.  For an implementation conforming to the
 226 1995 POSIX.1 standard, the value is the integer @code{199506L}.
 227
 228 @code{_POSIX_VERSION} is always defined (in @file{unistd.h}) in any
 229 POSIX system.
 230
 231 @strong{Usage Note:} Don't try to test whether the system supports POSIX
 232 by including @file{unistd.h} and then checking whether
 233 @code{_POSIX_VERSION} is defined.  On a non-POSIX system, this will
 234 probably fail because there is no @file{unistd.h}.  We do not know of
 235 @emph{any} way you can reliably test at compilation time whether your
 236 target system supports POSIX or whether @file{unistd.h} exists.
 237 @end deftypevr
 238
 239 @deftypevr Macro {long int} _POSIX2_C_VERSION
 240 @standards{POSIX.2, unistd.h}
 241 This constant represents the version of the POSIX.2 standard which the
 242 library and system kernel support.  We don't know what value this will
 243 be for the first version of the POSIX.2 standard, because the value is
 244 based on the year and month in which the standard is officially adopted.
 245
 246 The value of this symbol says nothing about the utilities installed on
 247 the system.
 248
 249 @strong{Usage Note:} You can use this macro to tell whether a POSIX.1
 250 system library supports POSIX.2 as well.  Any POSIX.1 system contains
 251 @file{unistd.h}, so include that file and then test @code{defined
 252 (_POSIX2_C_VERSION)}.
 253 @end deftypevr
 254
 255 @node Sysconf
 256 @section Using @code{sysconf}
 257
 258 When your system has configurable system limits, you can use the
 259 @code{sysconf} function to find out the value that applies to any
 260 particular machine.  The function and the associated @var{parameter}
 261 constants are declared in the header file @file{unistd.h}.
 262
 263 @menu
 264 * Sysconf Definition::        Detailed specifications of @code{sysconf}.
 265 * Constants for Sysconf::     The list of parameters @code{sysconf} can read.
 266 * Examples of Sysconf::       How to use @code{sysconf} and the parameter
 267                                  macros properly together.
 268 @end menu
 269
 270 @node Sysconf Definition
 271 @subsection Definition of @code{sysconf}
 272
 273 @deftypefun {long int} sysconf (int @var{parameter})
 274 @standards{POSIX.1, unistd.h}
 275 @safety{@prelim{}@mtsafe{@mtsenv{}}@asunsafe{@asulock{} @ascuheap{}}@acunsafe{@aculock{} @acsmem{} @acsfd{}}}
 276 @c Some parts of the implementation open /proc and /sys files and dirs
 277 @c to collect system details, using fd and stream I/O depending on the
 278 @c case.  The returned max value may change over time for NPROCS,
 279 @c NPROCS_CONF, PHYS_PAGES, AVPHYS_PAGES, NGROUPS_MAX, SIGQUEUE_MAX,
 280 @c depending on variable values read from /proc at each call, and from
 281 @c rlimit-obtained values CHILD_MAX, OPEN_MAX, ARG_MAX, SIGQUEUE_MAX.
 282 This function is used to inquire about runtime system parameters.  The
 283 @var{parameter} argument should be one of the @samp{_SC_} symbols listed
 284 below.
 285
 286 The normal return value from @code{sysconf} is the value you requested.
 287 A value of @code{-1} is returned both if the implementation does not
 288 impose a limit, and in case of an error.
 289
 290 The following @code{errno} error conditions are defined for this function:
 291
 292 @table @code
 293 @item EINVAL
 294 The value of the @var{parameter} is invalid.
 295 @end table
 296 @end deftypefun
 297
 298 @node Constants for Sysconf
 299 @subsection Constants for @code{sysconf} Parameters
 300
 301 Here are the symbolic constants for use as the @var{parameter} argument
 302 to @code{sysconf}.  The values are all integer constants (more
 303 specifically, enumeration type values).
 304
 305 @vtable @code
 306 @item _SC_ARG_MAX
 307 @standards{POSIX.1, unistd.h}
 308 Inquire about the parameter corresponding to @code{ARG_MAX}.
 309
 310 @item _SC_CHILD_MAX
 311 @standards{POSIX.1, unistd.h}
 312 Inquire about the parameter corresponding to @code{CHILD_MAX}.
 313
 314 @item _SC_OPEN_MAX
 315 @standards{POSIX.1, unistd.h}
 316 Inquire about the parameter corresponding to @code{OPEN_MAX}.
 317
 318 @item _SC_STREAM_MAX
 319 @standards{POSIX.1, unistd.h}
 320 Inquire about the parameter corresponding to @code{STREAM_MAX}.
 321
 322 @item _SC_TZNAME_MAX
 323 @standards{POSIX.1, unistd.h}
 324 Inquire about the parameter corresponding to @code{TZNAME_MAX}.
 325
 326 @item _SC_NGROUPS_MAX
 327 @standards{POSIX.1, unistd.h}
 328 Inquire about the parameter corresponding to @code{NGROUPS_MAX}.
 329
 330 @item _SC_JOB_CONTROL
 331 @standards{POSIX.1, unistd.h}
 332 Inquire about the parameter corresponding to @code{_POSIX_JOB_CONTROL}.
 333
 334 @item _SC_SAVED_IDS
 335 @standards{POSIX.1, unistd.h}
 336 Inquire about the parameter corresponding to @code{_POSIX_SAVED_IDS}.
 337
 338 @item _SC_VERSION
 339 @standards{POSIX.1, unistd.h}
 340 Inquire about the parameter corresponding to @code{_POSIX_VERSION}.
 341
 342 @item _SC_CLK_TCK
 343 @standards{POSIX.1, unistd.h}
 344 Inquire about the number of clock ticks per second; @pxref{CPU Time}.
 345 The corresponding parameter @code{CLK_TCK} is obsolete.
 346
 347 @item _SC_CHARCLASS_NAME_MAX
 348 @standards{GNU, unistd.h}
 349 Inquire about the parameter corresponding to maximal length allowed for
 350 a character class name in an extended locale specification.  These
 351 extensions are not yet standardized and so this option is not standardized
 352 as well.
 353
 354 @item _SC_REALTIME_SIGNALS
 355 @standards{POSIX.1, unistdh.h}
 356 Inquire about the parameter corresponding to @code{_POSIX_REALTIME_SIGNALS}.
 357
 358 @item _SC_PRIORITY_SCHEDULING
 359 @standards{POSIX.1, unistd.h}
 360 Inquire about the parameter corresponding to @code{_POSIX_PRIORITY_SCHEDULING}.
 361
 362 @item _SC_TIMERS
 363 @standards{POSIX.1, unistd.h}
 364 Inquire about the parameter corresponding to @code{_POSIX_TIMERS}.
 365
 366 @item _SC_ASYNCHRONOUS_IO
 367 @standards{POSIX.1, unistd.h}
 368 Inquire about the parameter corresponding to @code{_POSIX_ASYNCHRONOUS_IO}.
 369
 370 @item _SC_PRIORITIZED_IO
 371 @standards{POSIX.1, unistd.h}
 372 Inquire about the parameter corresponding to @code{_POSIX_PRIORITIZED_IO}.
 373
 374 @item _SC_SYNCHRONIZED_IO
 375 @standards{POSIX.1, unistd.h}
 376 Inquire about the parameter corresponding to @code{_POSIX_SYNCHRONIZED_IO}.
 377
 378 @item _SC_FSYNC
 379 @standards{POSIX.1, unistd.h}
 380 Inquire about the parameter corresponding to @code{_POSIX_FSYNC}.
 381
 382 @item _SC_MAPPED_FILES
 383 @standards{POSIX.1, unistd.h}
 384 Inquire about the parameter corresponding to @code{_POSIX_MAPPED_FILES}.
 385
 386 @item _SC_MEMLOCK
 387 @standards{POSIX.1, unistd.h}
 388 Inquire about the parameter corresponding to @code{_POSIX_MEMLOCK}.
 389
 390 @item _SC_MEMLOCK_RANGE
 391 @standards{POSIX.1, unistd.h}
 392 Inquire about the parameter corresponding to @code{_POSIX_MEMLOCK_RANGE}.
 393
 394 @item _SC_MEMORY_PROTECTION
 395 @standards{POSIX.1, unistd.h}
 396 Inquire about the parameter corresponding to @code{_POSIX_MEMORY_PROTECTION}.
 397
 398 @item _SC_MESSAGE_PASSING
 399 @standards{POSIX.1, unistd.h}
 400 Inquire about the parameter corresponding to @code{_POSIX_MESSAGE_PASSING}.
 401
 402 @item _SC_SEMAPHORES
 403 @standards{POSIX.1, unistd.h}
 404 Inquire about the parameter corresponding to @code{_POSIX_SEMAPHORES}.
 405
 406 @item _SC_SHARED_MEMORY_OBJECTS
 407 @standards{POSIX.1, unistd.h}
 408 Inquire about the parameter corresponding to@*
 409 @code{_POSIX_SHARED_MEMORY_OBJECTS}.
 410
 411 @item _SC_AIO_LISTIO_MAX
 412 @standards{POSIX.1, unistd.h}
 413 Inquire about the parameter corresponding to @code{_POSIX_AIO_LISTIO_MAX}.
 414
 415 @item _SC_AIO_MAX
 416 @standards{POSIX.1, unistd.h}
 417 Inquire about the parameter corresponding to @code{_POSIX_AIO_MAX}.
 418
 419 @item _SC_AIO_PRIO_DELTA_MAX
 420 @standards{POSIX.1, unistd.h}
 421 Inquire about the value by which a process can decrease its asynchronous I/O
 422 priority level from its own scheduling priority.  This corresponds to the
 423 run-time invariant value @code{AIO_PRIO_DELTA_MAX}.
 424
 425 @item _SC_DELAYTIMER_MAX
 426 @standards{POSIX.1, unistd.h}
 427 Inquire about the parameter corresponding to @code{_POSIX_DELAYTIMER_MAX}.
 428
 429 @item _SC_MQ_OPEN_MAX
 430 @standards{POSIX.1, unistd.h}
 431 Inquire about the parameter corresponding to @code{_POSIX_MQ_OPEN_MAX}.
 432
 433 @item _SC_MQ_PRIO_MAX
 434 @standards{POSIX.1, unistd.h}
 435 Inquire about the parameter corresponding to @code{_POSIX_MQ_PRIO_MAX}.
 436
 437 @item _SC_RTSIG_MAX
 438 @standards{POSIX.1, unistd.h}
 439 Inquire about the parameter corresponding to @code{_POSIX_RTSIG_MAX}.
 440
 441 @item _SC_SEM_NSEMS_MAX
 442 @standards{POSIX.1, unistd.h}
 443 Inquire about the parameter corresponding to @code{_POSIX_SEM_NSEMS_MAX}.
 444
 445 @item _SC_SEM_VALUE_MAX
 446 @standards{POSIX.1, unistd.h}
 447 Inquire about the parameter corresponding to @code{_POSIX_SEM_VALUE_MAX}.
 448
 449 @item _SC_SIGQUEUE_MAX
 450 @standards{POSIX.1, unistd.h}
 451 Inquire about the parameter corresponding to @code{_POSIX_SIGQUEUE_MAX}.
 452
 453 @item _SC_TIMER_MAX
 454 @standards{POSIX.1, unistd.h}
 455 Inquire about the parameter corresponding to @code{_POSIX_TIMER_MAX}.
 456
 457 @item _SC_PII
 458 @standards{POSIX.1g, unistd.h}
 459 Inquire about the parameter corresponding to @code{_POSIX_PII}.
 460
 461 @item _SC_PII_XTI
 462 @standards{POSIX.1g, unistd.h}
 463 Inquire about the parameter corresponding to @code{_POSIX_PII_XTI}.
 464
 465 @item _SC_PII_SOCKET
 466 @standards{POSIX.1g, unistd.h}
 467 Inquire about the parameter corresponding to @code{_POSIX_PII_SOCKET}.
 468
 469 @item _SC_PII_INTERNET
 470 @standards{POSIX.1g, unistd.h}
 471 Inquire about the parameter corresponding to @code{_POSIX_PII_INTERNET}.
 472
 473 @item _SC_PII_OSI
 474 @standards{POSIX.1g, unistd.h}
 475 Inquire about the parameter corresponding to @code{_POSIX_PII_OSI}.
 476
 477 @item _SC_SELECT
 478 @standards{POSIX.1g, unistd.h}
 479 Inquire about the parameter corresponding to @code{_POSIX_SELECT}.
 480
 481 @item _SC_UIO_MAXIOV
 482 @standards{POSIX.1g, unistd.h}
 483 Inquire about the parameter corresponding to @code{_POSIX_UIO_MAXIOV}.
 484
 485 @item _SC_PII_INTERNET_STREAM
 486 @standards{POSIX.1g, unistd.h}
 487 Inquire about the parameter corresponding to @code{_POSIX_PII_INTERNET_STREAM}.
 488
 489 @item _SC_PII_INTERNET_DGRAM
 490 @standards{POSIX.1g, unistd.h}
 491 Inquire about the parameter corresponding to @code{_POSIX_PII_INTERNET_DGRAM}.
 492
 493 @item _SC_PII_OSI_COTS
 494 @standards{POSIX.1g, unistd.h}
 495 Inquire about the parameter corresponding to @code{_POSIX_PII_OSI_COTS}.
 496
 497 @item _SC_PII_OSI_CLTS
 498 @standards{POSIX.1g, unistd.h}
 499 Inquire about the parameter corresponding to @code{_POSIX_PII_OSI_CLTS}.
 500
 501 @item _SC_PII_OSI_M
 502 @standards{POSIX.1g, unistd.h}
 503 Inquire about the parameter corresponding to @code{_POSIX_PII_OSI_M}.
 504
 505 @item _SC_T_IOV_MAX
 506 @standards{POSIX.1g, unistd.h}
 507 Inquire about the value associated with the @code{T_IOV_MAX}
 508 variable.
 509
 510 @item _SC_THREADS
 511 @standards{POSIX.1, unistd.h}
 512 Inquire about the parameter corresponding to @code{_POSIX_THREADS}.
 513
 514 @item _SC_THREAD_SAFE_FUNCTIONS
 515 @standards{POSIX.1, unistd.h}
 516 Inquire about the parameter corresponding to@*
 517 @code{_POSIX_THREAD_SAFE_FUNCTIONS}.
 518
 519 @item _SC_GETGR_R_SIZE_MAX
 520 @standards{POSIX.1, unistd.h}
 521 Inquire about the parameter corresponding to @code{_POSIX_GETGR_R_SIZE_MAX}.
 522
 523 @item _SC_GETPW_R_SIZE_MAX
 524 @standards{POSIX.1, unistd.h}
 525 Inquire about the parameter corresponding to @code{_POSIX_GETPW_R_SIZE_MAX}.
 526
 527 @item _SC_LOGIN_NAME_MAX
 528 @standards{POSIX.1, unistd.h}
 529 Inquire about the parameter corresponding to @code{_POSIX_LOGIN_NAME_MAX}.
 530
 531 @item _SC_TTY_NAME_MAX
 532 @standards{POSIX.1, unistd.h}
 533 Inquire about the parameter corresponding to @code{_POSIX_TTY_NAME_MAX}.
 534
 535 @item _SC_THREAD_DESTRUCTOR_ITERATIONS
 536 @standards{POSIX.1, unistd.h}
 537 Inquire about the parameter corresponding to
 538 @code{_POSIX_THREAD_DESTRUCTOR_ITERATIONS}.
 539
 540 @item _SC_THREAD_KEYS_MAX
 541 @standards{POSIX.1, unistd.h}
 542 Inquire about the parameter corresponding to @code{_POSIX_THREAD_KEYS_MAX}.
 543
 544 @item _SC_THREAD_STACK_MIN
 545 @standards{POSIX.1, unistd.h}
 546 Inquire about the parameter corresponding to @code{_POSIX_THREAD_STACK_MIN}.
 547
 548 @item _SC_THREAD_THREADS_MAX
 549 @standards{POSIX.1, unistd.h}
 550 Inquire about the parameter corresponding to @code{_POSIX_THREAD_THREADS_MAX}.
 551
 552 @item _SC_THREAD_ATTR_STACKADDR
 553 @standards{POSIX.1, unistd.h}
 554 Inquire about the parameter corresponding to@*a
 555 @code{_POSIX_THREAD_ATTR_STACKADDR}.
 556
 557 @item _SC_THREAD_ATTR_STACKSIZE
 558 @standards{POSIX.1, unistd.h}
 559 Inquire about the parameter corresponding to@*
 560 @code{_POSIX_THREAD_ATTR_STACKSIZE}.
 561
 562 @item _SC_THREAD_PRIORITY_SCHEDULING
 563 @standards{POSIX.1, unistd.h}
 564 Inquire about the parameter corresponding to
 565 @code{_POSIX_THREAD_PRIORITY_SCHEDULING}.
 566
 567 @item _SC_THREAD_PRIO_INHERIT
 568 @standards{POSIX.1, unistd.h}
 569 Inquire about the parameter corresponding to @code{_POSIX_THREAD_PRIO_INHERIT}.
 570
 571 @item _SC_THREAD_PRIO_PROTECT
 572 @standards{POSIX.1, unistd.h}
 573 Inquire about the parameter corresponding to @code{_POSIX_THREAD_PRIO_PROTECT}.
 574
 575 @item _SC_THREAD_PROCESS_SHARED
 576 @standards{POSIX.1, unistd.h}
 577 Inquire about the parameter corresponding to
 578 @code{_POSIX_THREAD_PROCESS_SHARED}.
 579
 580 @item _SC_2_C_DEV
 581 @standards{POSIX.2, unistd.h}
 582 Inquire about whether the system has the POSIX.2 C compiler command,
 583 @code{c89}.
 584
 585 @item _SC_2_FORT_DEV
 586 @standards{POSIX.2, unistd.h}
 587 Inquire about whether the system has the POSIX.2 Fortran compiler
 588 command, @code{fort77}.
 589
 590 @item _SC_2_FORT_RUN
 591 @standards{POSIX.2, unistd.h}
 592 Inquire about whether the system has the POSIX.2 @code{asa} command to
 593 interpret Fortran carriage control.
 594
 595 @item _SC_2_LOCALEDEF
 596 @standards{POSIX.2, unistd.h}
 597 Inquire about whether the system has the POSIX.2 @code{localedef}
 598 command.
 599
 600 @item _SC_2_SW_DEV
 601 @standards{POSIX.2, unistd.h}
 602 Inquire about whether the system has the POSIX.2 commands @code{ar},
 603 @code{make}, and @code{strip}.
 604
 605 @item _SC_BC_BASE_MAX
 606 @standards{POSIX.2, unistd.h}
 607 Inquire about the maximum value of @code{obase} in the @code{bc}
 608 utility.
 609
 610 @item _SC_BC_DIM_MAX
 611 @standards{POSIX.2, unistd.h}
 612 Inquire about the maximum size of an array in the @code{bc}
 613 utility.
 614
 615 @item _SC_BC_SCALE_MAX
 616 @standards{POSIX.2, unistd.h}
 617 Inquire about the maximum value of @code{scale} in the @code{bc}
 618 utility.
 619
 620 @item _SC_BC_STRING_MAX
 621 @standards{POSIX.2, unistd.h}
 622 Inquire about the maximum size of a string constant in the
 623 @code{bc} utility.
 624
 625 @item _SC_COLL_WEIGHTS_MAX
 626 @standards{POSIX.2, unistd.h}
 627 Inquire about the maximum number of weights that can necessarily
 628 be used in defining the collating sequence for a locale.
 629
 630 @item _SC_EXPR_NEST_MAX
 631 @standards{POSIX.2, unistd.h}
 632 Inquire about the maximum number of expressions nested within
 633 parentheses when using the @code{expr} utility.
 634
 635 @item _SC_LINE_MAX
 636 @standards{POSIX.2, unistd.h}
 637 Inquire about the maximum size of a text line that the POSIX.2 text
 638 utilities can handle.
 639
 640 @item _SC_EQUIV_CLASS_MAX
 641 @standards{POSIX.2, unistd.h}
 642 Inquire about the maximum number of weights that can be assigned to an
 643 entry of the @code{LC_COLLATE} category @samp{order} keyword in a locale
 644 definition.  @Theglibc{} does not presently support locale
 645 definitions.
 646
 647 @item _SC_VERSION
 648 @standards{POSIX.2, unistd.h}
 649 Inquire about the version number of POSIX.1 that the library and kernel
 650 support.
 651
 652 @item _SC_2_VERSION
 653 @standards{POSIX.2, unistd.h}
 654 Inquire about the version number of POSIX.2 that the system utilities
 655 support.
 656
 657 @item _SC_PAGESIZE
 658 @standards{GNU, unistd.h}
 659 Inquire about the virtual memory page size of the machine.
 660 @code{getpagesize} returns the same value (@pxref{Query Memory Parameters}).
 661
 662 @item _SC_NPROCESSORS_CONF
 663 @standards{GNU, unistd.h}
 664 Inquire about the number of configured processors.
 665
 666 @item _SC_NPROCESSORS_ONLN
 667 @standards{GNU, unistd.h}
 668 Inquire about the number of processors online.
 669
 670 @item _SC_PHYS_PAGES
 671 @standards{GNU, unistd.h}
 672 Inquire about the number of physical pages in the system.
 673
 674 @item _SC_AVPHYS_PAGES
 675 @standards{GNU, unistd.h}
 676 Inquire about the number of available physical pages in the system.
 677
 678 @item _SC_ATEXIT_MAX
 679 @standards{GNU, unistd.h}
 680 Inquire about the number of functions which can be registered as termination
 681 functions for @code{atexit}; @pxref{Cleanups on Exit}.
 682
 683 @item _SC_LEVEL1_ICACHE_SIZE
 684 @standards{GNU, unistd.h}
 685 Inquire about the size of the Level 1 instruction cache.
 686
 687 @item _SC_LEVEL1_ICACHE_ASSOC
 688 @standards{GNU, unistd.h}
 689 Inquire about the associativity of the Level 1 instruction cache.
 690
 691 @item _SC_LEVEL1_ICACHE_LINESIZE
 692 @standards{GNU, unistd.h}
 693 Inquire about the line length of the Level 1 instruction cache.
 694
 695 On aarch64, the cache line size returned is the minimum instruction cache line
 696 size observable by userspace.  This is typically the same as the L1 icache
 697 size but on some cores it may not be so.  However, it is specified in the
 698 architecture that operations such as cache line invalidation are consistent
 699 with the size reported with this variable.
 700
 701 @item _SC_LEVEL1_DCACHE_SIZE
 702 @standards{GNU, unistd.h}
 703 Inquire about the size of the Level 1 data cache.
 704
 705 @item _SC_LEVEL1_DCACHE_ASSOC
 706 @standards{GNU, unistd.h}
 707 Inquire about the associativity of the Level 1 data cache.
 708
 709 @item _SC_LEVEL1_DCACHE_LINESIZE
 710 @standards{GNU, unistd.h}
 711 Inquire about the line length of the Level 1 data cache.
 712
 713 On aarch64, the cache line size returned is the minimum data cache line size
 714 observable by userspace.  This is typically the same as the L1 dcache size but
 715 on some cores it may not be so.  However, it is specified in the architecture
 716 that operations such as cache line invalidation are consistent with the size
 717 reported with this variable.
 718
 719 @item _SC_LEVEL2_CACHE_SIZE
 720 @standards{GNU, unistd.h}
 721 Inquire about the size of the Level 2 cache.
 722
 723 @item _SC_LEVEL2_CACHE_ASSOC
 724 @standards{GNU, unistd.h}
 725 Inquire about the associativity of the Level 2 cache.
 726
 727 @item _SC_LEVEL2_CACHE_LINESIZE
 728 @standards{GNU, unistd.h}
 729 Inquire about the line length of the Level 2 cache.
 730
 731 @item _SC_LEVEL3_CACHE_SIZE
 732 @standards{GNU, unistd.h}
 733 Inquire about the size of the Level 3 cache.
 734
 735 @item _SC_LEVEL3_CACHE_ASSOC
 736 @standards{GNU, unistd.h}
 737 Inquire about the associativity of the Level 3 cache.
 738
 739 @item _SC_LEVEL3_CACHE_LINESIZE
 740 @standards{GNU, unistd.h}
 741 Inquire about the line length of the Level 3 cache.
 742
 743 @item _SC_LEVEL4_CACHE_SIZE
 744 @standards{GNU, unistd.h}
 745 Inquire about the size of the Level 4 cache.
 746
 747 @item _SC_LEVEL4_CACHE_ASSOC
 748 @standards{GNU, unistd.h}
 749 Inquire about the associativity of the Level 4 cache.
 750
 751 @item _SC_LEVEL4_CACHE_LINESIZE
 752 @standards{GNU, unistd.h}
 753 Inquire about the line length of the Level 4 cache.
 754
 755
 756 @item _SC_XOPEN_VERSION
 757 @standards{X/Open, unistd.h}
 758 Inquire about the parameter corresponding to @code{_XOPEN_VERSION}.
 759
 760 @item _SC_XOPEN_XCU_VERSION
 761 @standards{X/Open, unistd.h}
 762 Inquire about the parameter corresponding to @code{_XOPEN_XCU_VERSION}.
 763
 764 @item _SC_XOPEN_UNIX
 765 @standards{X/Open, unistd.h}
 766 Inquire about the parameter corresponding to @code{_XOPEN_UNIX}.
 767
 768 @item _SC_XOPEN_REALTIME
 769 @standards{X/Open, unistd.h}
 770 Inquire about the parameter corresponding to @code{_XOPEN_REALTIME}.
 771
 772 @item _SC_XOPEN_REALTIME_THREADS
 773 @standards{X/Open, unistd.h}
 774 Inquire about the parameter corresponding to @code{_XOPEN_REALTIME_THREADS}.
 775
 776 @item _SC_XOPEN_LEGACY
 777 @standards{X/Open, unistd.h}
 778 Inquire about the parameter corresponding to @code{_XOPEN_LEGACY}.
 779
 780 @item _SC_XOPEN_CRYPT
 781 @standards{X/Open, unistd.h}
 782 Inquire about the parameter corresponding to @code{_XOPEN_CRYPT}.
 783 @Theglibc no longer implements the @code{_XOPEN_CRYPT} extensions,
 784 so @samp{sysconf (_SC_XOPEN_CRYPT)} always returns @code{-1}.
 785
 786 @item _SC_XOPEN_ENH_I18N
 787 @standards{X/Open, unistd.h}
 788 Inquire about the parameter corresponding to @code{_XOPEN_ENH_I18N}.
 789
 790 @item _SC_XOPEN_SHM
 791 @standards{X/Open, unistd.h}
 792 Inquire about the parameter corresponding to @code{_XOPEN_SHM}.
 793
 794 @item _SC_XOPEN_XPG2
 795 @standards{X/Open, unistd.h}
 796 Inquire about the parameter corresponding to @code{_XOPEN_XPG2}.
 797
 798 @item _SC_XOPEN_XPG3
 799 @standards{X/Open, unistd.h}
 800 Inquire about the parameter corresponding to @code{_XOPEN_XPG3}.
 801
 802 @item _SC_XOPEN_XPG4
 803 @standards{X/Open, unistd.h}
 804 Inquire about the parameter corresponding to @code{_XOPEN_XPG4}.
 805
 806 @item _SC_CHAR_BIT
 807 @standards{X/Open, unistd.h}
 808 Inquire about the number of bits in a variable of type @code{char}.
 809
 810 @item _SC_CHAR_MAX
 811 @standards{X/Open, unistd.h}
 812 Inquire about the maximum value which can be stored in a variable of type
 813 @code{char}.
 814
 815 @item _SC_CHAR_MIN
 816 @standards{X/Open, unistd.h}
 817 Inquire about the minimum value which can be stored in a variable of type
 818 @code{char}.
 819
 820 @item _SC_INT_MAX
 821 @standards{X/Open, unistd.h}
 822 Inquire about the maximum value which can be stored in a variable of type
 823 @code{int}.
 824
 825 @item _SC_INT_MIN
 826 @standards{X/Open, unistd.h}
 827 Inquire about the minimum value which can be stored in a variable of type
 828 @code{int}.
 829
 830 @item _SC_LONG_BIT
 831 @standards{X/Open, unistd.h}
 832 Inquire about the number of bits in a variable of type @code{long int}.
 833
 834 @item _SC_WORD_BIT
 835 @standards{X/Open, unistd.h}
 836 Inquire about the number of bits in a variable of a register word.
 837
 838 @item _SC_MB_LEN_MAX
 839 @standards{X/Open, unistd.h}
 840 Inquire about the maximum length of a multi-byte representation of a wide
 841 character value.
 842
 843 @item _SC_NZERO
 844 @standards{X/Open, unistd.h}
 845 Inquire about the value used to internally represent the zero priority level for
 846 the process execution.
 847
 848 @item _SC_SSIZE_MAX
 849 @standards{X/Open, unistd.h}
 850 Inquire about the maximum value which can be stored in a variable of type
 851 @code{ssize_t}.
 852
 853 @item _SC_SCHAR_MAX
 854 @standards{X/Open, unistd.h}
 855 Inquire about the maximum value which can be stored in a variable of type
 856 @code{signed char}.
 857
 858 @item _SC_SCHAR_MIN
 859 @standards{X/Open, unistd.h}
 860 Inquire about the minimum value which can be stored in a variable of type
 861 @code{signed char}.
 862
 863 @item _SC_SHRT_MAX
 864 @standards{X/Open, unistd.h}
 865 Inquire about the maximum value which can be stored in a variable of type
 866 @code{short int}.
 867
 868 @item _SC_SHRT_MIN
 869 @standards{X/Open, unistd.h}
 870 Inquire about the minimum value which can be stored in a variable of type
 871 @code{short int}.
 872
 873 @item _SC_UCHAR_MAX
 874 @standards{X/Open, unistd.h}
 875 Inquire about the maximum value which can be stored in a variable of type
 876 @code{unsigned char}.
 877
 878 @item _SC_UINT_MAX
 879 @standards{X/Open, unistd.h}
 880 Inquire about the maximum value which can be stored in a variable of type
 881 @code{unsigned int}.
 882
 883 @item _SC_ULONG_MAX
 884 @standards{X/Open, unistd.h}
 885 Inquire about the maximum value which can be stored in a variable of type
 886 @code{unsigned long int}.
 887
 888 @item _SC_USHRT_MAX
 889 @standards{X/Open, unistd.h}
 890 Inquire about the maximum value which can be stored in a variable of type
 891 @code{unsigned short int}.
 892
 893 @item _SC_NL_ARGMAX
 894 @standards{X/Open, unistd.h}
 895 Inquire about the parameter corresponding to @code{NL_ARGMAX}.
 896
 897 @item _SC_NL_LANGMAX
 898 @standards{X/Open, unistd.h}
 899 Inquire about the parameter corresponding to @code{NL_LANGMAX}.
 900
 901 @item _SC_NL_MSGMAX
 902 @standards{X/Open, unistd.h}
 903 Inquire about the parameter corresponding to @code{NL_MSGMAX}.
 904
 905 @item _SC_NL_NMAX
 906 @standards{X/Open, unistd.h}
 907 Inquire about  the parameter corresponding to @code{NL_NMAX}.
 908
 909 @item _SC_NL_SETMAX
 910 @standards{X/Open, unistd.h}
 911 Inquire about the parameter corresponding to @code{NL_SETMAX}.
 912
 913 @item _SC_NL_TEXTMAX
 914 @standards{X/Open, unistd.h}
 915 Inquire about the parameter corresponding to @code{NL_TEXTMAX}.
 916 @end vtable
 917
 918 @node Examples of Sysconf
 919 @subsection Examples of @code{sysconf}
 920
 921 We recommend that you first test for a macro definition for the
 922 parameter you are interested in, and call @code{sysconf} only if the
 923 macro is not defined.  For example, here is how to test whether job
 924 control is supported:
 925
 926 @smallexample
 927 @group
 928 int
 929 have_job_control (void)
 930 @{
 931 #ifdef _POSIX_JOB_CONTROL
 932   return 1;
 933 #else
 934   int value = sysconf (_SC_JOB_CONTROL);
 935   if (value < 0)
 936     /* @r{If the system is that badly wedged,}
 937        @r{there's no use trying to go on.}  */
 938     fatal (strerror (errno));
 939   return value;
 940 #endif
 941 @}
 942 @end group
 943 @end smallexample
 944
 945 Here is how to get the value of a numeric limit:
 946
 947 @smallexample
 948 int
 949 get_child_max ()
 950 @{
 951 #ifdef CHILD_MAX
 952   return CHILD_MAX;
 953 #else
 954   int value = sysconf (_SC_CHILD_MAX);
 955   if (value < 0)
 956     fatal (strerror (errno));
 957   return value;
 958 #endif
 959 @}
 960 @end smallexample
 961
 962 @node Minimums
 963 @section Minimum Values for General Capacity Limits
 964
 965 Here are the names for the POSIX minimum upper bounds for the system
 966 limit parameters.  The significance of these values is that you can
 967 safely push to these limits without checking whether the particular
 968 system you are using can go that far.
 969
 970 @vtable @code
 971 @item _POSIX_AIO_LISTIO_MAX
 972 @standards{POSIX.1, limits.h}
 973 The most restrictive limit permitted by POSIX for the maximum number of
 974 I/O operations that can be specified in a list I/O call.  The value of
 975 this constant is @code{2}; thus you can add up to two new entries
 976 of the list of outstanding operations.
 977
 978 @item _POSIX_AIO_MAX
 979 @standards{POSIX.1, limits.h}
 980 The most restrictive limit permitted by POSIX for the maximum number of
 981 outstanding asynchronous I/O operations.  The value of this constant is
 982 @code{1}.  So you cannot expect that you can issue more than one
 983 operation and immediately continue with the normal work, receiving the
 984 notifications asynchronously.
 985
 986 @item _POSIX_ARG_MAX
 987 @standards{POSIX.1, limits.h}
 988 The value of this macro is the most restrictive limit permitted by POSIX
 989 for the maximum combined length of the @var{argv} and @var{environ}
 990 arguments that can be passed to the @code{exec} functions.
 991 Its value is @code{4096}.
 992
 993 @item _POSIX_CHILD_MAX
 994 @standards{POSIX.1, limits.h}
 995 The value of this macro is the most restrictive limit permitted by POSIX
 996 for the maximum number of simultaneous processes per real user ID.  Its
 997 value is @code{6}.
 998
 999 @item _POSIX_NGROUPS_MAX
1000 @standards{POSIX.1, limits.h}
1001 The value of this macro is the most restrictive limit permitted by POSIX
1002 for the maximum number of supplementary group IDs per process.  Its
1003 value is @code{0}.
1004
1005 @item _POSIX_OPEN_MAX
1006 @standards{POSIX.1, limits.h}
1007 The value of this macro is the most restrictive limit permitted by POSIX
1008 for the maximum number of files that a single process can have open
1009 simultaneously.  Its value is @code{16}.
1010
1011 @item _POSIX_SSIZE_MAX
1012 @standards{POSIX.1, limits.h}
1013 The value of this macro is the most restrictive limit permitted by POSIX
1014 for the maximum value that can be stored in an object of type
1015 @code{ssize_t}.  Its value is @code{32767}.
1016
1017 @item _POSIX_STREAM_MAX
1018 @standards{POSIX.1, limits.h}
1019 The value of this macro is the most restrictive limit permitted by POSIX
1020 for the maximum number of streams that a single process can have open
1021 simultaneously.  Its value is @code{8}.
1022
1023 @item _POSIX_TZNAME_MAX
1024 @standards{POSIX.1, limits.h}
1025 The value of this macro is the most restrictive limit permitted by POSIX
1026 for the maximum length of a time zone name.  Its value is @code{3}.
1027
1028 @item _POSIX2_RE_DUP_MAX
1029 @standards{POSIX.2, limits.h}
1030 The value of this macro is the most restrictive limit permitted by POSIX
1031 for the numbers used in the @samp{\@{@var{min},@var{max}\@}} construct
1032 in a regular expression.  Its value is @code{255}.
1033 @end vtable
1034
1035 @node Limits for Files
1036 @section Limits on File System Capacity
1037
1038 The POSIX.1 standard specifies a number of parameters that describe the
1039 limitations of the file system.  It's possible for the system to have a
1040 fixed, uniform limit for a parameter, but this isn't the usual case.  On
1041 most systems, it's possible for different file systems (and, for some
1042 parameters, even different files) to have different maximum limits.  For
1043 example, this is very likely if you use NFS to mount some of the file
1044 systems from other machines.
1045
1046 @pindex limits.h
1047 Each of the following macros is defined in @file{limits.h} only if the
1048 system has a fixed, uniform limit for the parameter in question.  If the
1049 system allows different file systems or files to have different limits,
1050 then the macro is undefined; use @code{pathconf} or @code{fpathconf} to
1051 find out the limit that applies to a particular file.  @xref{Pathconf}.
1052
1053 Each parameter also has another macro, with a name starting with
1054 @samp{_POSIX}, which gives the lowest value that the limit is allowed to
1055 have on @emph{any} POSIX system.  @xref{File Minimums}.
1056
1057 @cindex limits, link count of files
1058 @deftypevr Macro int LINK_MAX
1059 @standards{POSIX.1, limits.h (optional)}
1060 The uniform system limit (if any) for the number of names for a given
1061 file.  @xref{Hard Links}.
1062 @end deftypevr
1063
1064 @cindex limits, terminal input queue
1065 @deftypevr Macro int MAX_CANON
1066 @standards{POSIX.1, limits.h}
1067 The uniform system limit (if any) for the amount of text in a line of
1068 input when input editing is enabled.  @xref{Canonical or Not}.
1069 @end deftypevr
1070
1071 @deftypevr Macro int MAX_INPUT
1072 @standards{POSIX.1, limits.h}
1073 The uniform system limit (if any) for the total number of characters
1074 typed ahead as input.  @xref{I/O Queues}.
1075 @end deftypevr
1076
1077 @cindex limits, file name length
1078 @deftypevr Macro int NAME_MAX
1079 @standards{POSIX.1, limits.h}
1080 The uniform system limit (if any) for the length of a file name component, not
1081 including the terminating null character.
1082
1083 @strong{Portability Note:} On some systems, @theglibc{} defines
1084 @code{NAME_MAX}, but does not actually enforce this limit.
1085 @end deftypevr
1086
1087 @deftypevr Macro int PATH_MAX
1088 @standards{POSIX.1, limits.h}
1089 The uniform system limit (if any) for the length of an entire file name (that
1090 is, the argument given to system calls such as @code{open}), including the
1091 terminating null character.
1092
1093 @strong{Portability Note:} @Theglibc{} does not enforce this limit
1094 even if @code{PATH_MAX} is defined.
1095 @end deftypevr
1096
1097 @cindex limits, pipe buffer size
1098 @deftypevr Macro int PIPE_BUF
1099 @standards{POSIX.1, limits.h}
1100 The uniform system limit (if any) for the number of bytes that can be
1101 written atomically to a pipe.  If multiple processes are writing to the
1102 same pipe simultaneously, output from different processes might be
1103 interleaved in chunks of this size.  @xref{Pipes and FIFOs}.
1104 @end deftypevr
1105
1106 These are alternative macro names for some of the same information.
1107
1108 @deftypevr Macro int MAXNAMLEN
1109 @standards{BSD, dirent.h}
1110 This is the BSD name for @code{NAME_MAX}.  It is defined in
1111 @file{dirent.h}.
1112 @end deftypevr
1113
1114 @deftypevr Macro int FILENAME_MAX
1115 @standards{ISO, stdio.h}
1116 The value of this macro is an integer constant expression that
1117 represents the maximum length of a file name string.  It is defined in
1118 @file{stdio.h}.
1119
1120 Unlike @code{PATH_MAX}, this macro is defined even if there is no actual
1121 limit imposed.  In such a case, its value is typically a very large
1122 number.  @strong{This is always the case on @gnuhurdsystems{}.}
1123
1124 @strong{Usage Note:} Don't use @code{FILENAME_MAX} as the size of an
1125 array in which to store a file name!  You can't possibly make an array
1126 that big!  Use dynamic allocation (@pxref{Memory Allocation}) instead.
1127 @end deftypevr
1128
1129 @node Options for Files
1130 @section Optional Features in File Support
1131
1132 POSIX defines certain system-specific options in the system calls for
1133 operating on files.  Some systems support these options and others do
1134 not.  Since these options are provided in the kernel, not in the
1135 library, simply using @theglibc{} does not guarantee that any of these
1136 features is supported; it depends on the system you are using.  They can
1137 also vary between file systems on a single machine.
1138
1139 @pindex unistd.h
1140 This section describes the macros you can test to determine whether a
1141 particular option is supported on your machine.  If a given macro is
1142 defined in @file{unistd.h}, then its value says whether the
1143 corresponding feature is supported.  (A value of @code{-1} indicates no;
1144 any other value indicates yes.)  If the macro is undefined, it means
1145 particular files may or may not support the feature.
1146
1147 Since all the machines that support @theglibc{} also support NFS,
1148 one can never make a general statement about whether all file systems
1149 support the @code{_POSIX_CHOWN_RESTRICTED} and @code{_POSIX_NO_TRUNC}
1150 features.  So these names are never defined as macros in @theglibc{}.
1151
1152 @deftypevr Macro int _POSIX_CHOWN_RESTRICTED
1153 @standards{POSIX.1, unistd.h}
1154 If this option is in effect, the @code{chown} function is restricted so
1155 that the only changes permitted to nonprivileged processes is to change
1156 the group owner of a file to either be the effective group ID of the
1157 process, or one of its supplementary group IDs.  @xref{File Owner}.
1158 @end deftypevr
1159
1160 @deftypevr Macro int _POSIX_NO_TRUNC
1161 @standards{POSIX.1, unistd.h}
1162 If this option is in effect, file name components longer than
1163 @code{NAME_MAX} generate an @code{ENAMETOOLONG} error.  Otherwise, file
1164 name components that are too long are silently truncated.
1165 @end deftypevr
1166
1167 @deftypevr Macro {unsigned char} _POSIX_VDISABLE
1168 @standards{POSIX.1, unistd.h}
1169 This option is only meaningful for files that are terminal devices.
1170 If it is enabled, then handling for special control characters can
1171 be disabled individually.  @xref{Special Characters}.
1172 @end deftypevr
1173
1174 @pindex unistd.h
1175 If one of these macros is undefined, that means that the option might be
1176 in effect for some files and not for others.  To inquire about a
1177 particular file, call @code{pathconf} or @code{fpathconf}.
1178 @xref{Pathconf}.
1179
1180 @node File Minimums
1181 @section Minimum Values for File System Limits
1182
1183 Here are the names for the POSIX minimum upper bounds for some of the
1184 above parameters.  The significance of these values is that you can
1185 safely push to these limits without checking whether the particular
1186 system you are using can go that far.  In most cases @gnusystems{} do not
1187 have these strict limitations.  The actual limit should be requested if
1188 necessary.
1189
1190 @vtable @code
1191 @item _POSIX_LINK_MAX
1192 @standards{POSIX.1, limits.h}
1193 The most restrictive limit permitted by POSIX for the maximum value of a
1194 file's link count.  The value of this constant is @code{8}; thus, you
1195 can always make up to eight names for a file without running into a
1196 system limit.
1197
1198 @item _POSIX_MAX_CANON
1199 @standards{POSIX.1, limits.h}
1200 The most restrictive limit permitted by POSIX for the maximum number of
1201 bytes in a canonical input line from a terminal device.  The value of
1202 this constant is @code{255}.
1203
1204 @item _POSIX_MAX_INPUT
1205 @standards{POSIX.1, limits.h}
1206 The most restrictive limit permitted by POSIX for the maximum number of
1207 bytes in a terminal device input queue (or typeahead buffer).
1208 @xref{Input Modes}.  The value of this constant is @code{255}.
1209
1210 @item _POSIX_NAME_MAX
1211 @standards{POSIX.1, limits.h}
1212 The most restrictive limit permitted by POSIX for the maximum number of
1213 bytes in a file name component.  The value of this constant is
1214 @code{14}.
1215
1216 @item _POSIX_PATH_MAX
1217 @standards{POSIX.1, limits.h}
1218 The most restrictive limit permitted by POSIX for the maximum number of
1219 bytes in a file name.  The value of this constant is @code{256}.
1220
1221 @item _POSIX_PIPE_BUF
1222 @standards{POSIX.1, limits.h}
1223 The most restrictive limit permitted by POSIX for the maximum number of
1224 bytes that can be written atomically to a pipe.  The value of this
1225 constant is @code{512}.
1226
1227 @item SYMLINK_MAX
1228 @standards{POSIX.1, limits.h}
1229 Maximum number of bytes in a symbolic link.
1230
1231 @item POSIX_REC_INCR_XFER_SIZE
1232 @standards{POSIX.1, limits.h}
1233 Recommended increment for file transfer sizes between the
1234 @code{POSIX_REC_MIN_XFER_SIZE} and @code{POSIX_REC_MAX_XFER_SIZE}
1235 values.
1236
1237 @item POSIX_REC_MAX_XFER_SIZE
1238 @standards{POSIX.1, limits.h}
1239 Maximum recommended file transfer size.
1240
1241 @item POSIX_REC_MIN_XFER_SIZE
1242 @standards{POSIX.1, limits.h}
1243 Minimum recommended file transfer size.
1244
1245 @item POSIX_REC_XFER_ALIGN
1246 @standards{POSIX.1, limits.h}
1247 Recommended file transfer buffer alignment.
1248 @end vtable
1249
1250 @node Pathconf
1251 @section Using @code{pathconf}
1252
1253 When your machine allows different files to have different values for a
1254 file system parameter, you can use the functions in this section to find
1255 out the value that applies to any particular file.
1256
1257 These functions and the associated constants for the @var{parameter}
1258 argument are declared in the header file @file{unistd.h}.
1259
1260 @deftypefun {long int} pathconf (const char *@var{filename}, int @var{parameter})
1261 @standards{POSIX.1, unistd.h}
1262 @safety{@prelim{}@mtsafe{}@asunsafe{@asulock{} @ascuheap{}}@acunsafe{@aculock{} @acsfd{} @acsmem{}}}
1263 @c When __statfs_link_max finds an ext* filesystem, it may read
1264 @c /proc/mounts or similar as a mntent stream.
1265 @c __statfs_chown_restricted may read from
1266 @c /proc/sys/fs/xfs/restrict_chown as a file descriptor.
1267 This function is used to inquire about the limits that apply to
1268 the file named @var{filename}.
1269
1270 The @var{parameter} argument should be one of the @samp{_PC_} constants
1271 listed below.
1272
1273 The normal return value from @code{pathconf} is the value you requested.
1274 A value of @code{-1} is returned both if the implementation does not
1275 impose a limit, and in case of an error.  In the former case,
1276 @code{errno} is not set, while in the latter case, @code{errno} is set
1277 to indicate the cause of the problem.  So the only way to use this
1278 function robustly is to store @code{0} into @code{errno} just before
1279 calling it.
1280
1281 Besides the usual file name errors (@pxref{File Name Errors}),
1282 the following error condition is defined for this function:
1283
1284 @table @code
1285 @item EINVAL
1286 The value of @var{parameter} is invalid, or the implementation doesn't
1287 support the @var{parameter} for the specific file.
1288 @end table
1289 @end deftypefun
1290
1291 @deftypefun {long int} fpathconf (int @var{filedes}, int @var{parameter})
1292 @standards{POSIX.1, unistd.h}
1293 @safety{@prelim{}@mtsafe{}@asunsafe{@asulock{} @ascuheap{}}@acunsafe{@aculock{} @acsfd{} @acsmem{}}}
1294 @c Same caveats as pathconf.
1295 This is just like @code{pathconf} except that an open file descriptor
1296 is used to specify the file for which information is requested, instead
1297 of a file name.
1298
1299 The following @code{errno} error conditions are defined for this function:
1300
1301 @table @code
1302 @item EBADF
1303 The @var{filedes} argument is not a valid file descriptor.
1304
1305 @item EINVAL
1306 The value of @var{parameter} is invalid, or the implementation doesn't
1307 support the @var{parameter} for the specific file.
1308 @end table
1309 @end deftypefun
1310
1311 Here are the symbolic constants that you can use as the @var{parameter}
1312 argument to @code{pathconf} and @code{fpathconf}.  The values are all
1313 integer constants.
1314
1315 @vtable @code
1316 @item _PC_LINK_MAX
1317 @standards{POSIX.1, unistd.h}
1318 Inquire about the value of @code{LINK_MAX}.
1319
1320 @item _PC_MAX_CANON
1321 @standards{POSIX.1, unistd.h}
1322 Inquire about the value of @code{MAX_CANON}.
1323
1324 @item _PC_MAX_INPUT
1325 @standards{POSIX.1, unistd.h}
1326 Inquire about the value of @code{MAX_INPUT}.
1327
1328 @item _PC_NAME_MAX
1329 @standards{POSIX.1, unistd.h}
1330 Inquire about the value of @code{NAME_MAX}.
1331
1332 @item _PC_PATH_MAX
1333 @standards{POSIX.1, unistd.h}
1334 Inquire about the value of @code{PATH_MAX}.
1335
1336 @item _PC_PIPE_BUF
1337 @standards{POSIX.1, unistd.h}
1338 Inquire about the value of @code{PIPE_BUF}.
1339
1340 @item _PC_CHOWN_RESTRICTED
1341 @standards{POSIX.1, unistd.h}
1342 Inquire about the value of @code{_POSIX_CHOWN_RESTRICTED}.
1343
1344 @item _PC_NO_TRUNC
1345 @standards{POSIX.1, unistd.h}
1346 Inquire about the value of @code{_POSIX_NO_TRUNC}.
1347
1348 @item _PC_VDISABLE
1349 @standards{POSIX.1, unistd.h}
1350 Inquire about the value of @code{_POSIX_VDISABLE}.
1351
1352 @item _PC_SYNC_IO
1353 @standards{POSIX.1, unistd.h}
1354 Inquire about the value of @code{_POSIX_SYNC_IO}.
1355
1356 @item _PC_ASYNC_IO
1357 @standards{POSIX.1, unistd.h}
1358 Inquire about the value of @code{_POSIX_ASYNC_IO}.
1359
1360 @item _PC_PRIO_IO
1361 @standards{POSIX.1, unistd.h}
1362 Inquire about the value of @code{_POSIX_PRIO_IO}.
1363
1364 @item _PC_FILESIZEBITS
1365 @standards{LFS, unistd.h}
1366 Inquire about the availability of large files on the filesystem.
1367
1368 @item _PC_REC_INCR_XFER_SIZE
1369 @standards{POSIX.1, unistd.h}
1370 Inquire about the value of @code{POSIX_REC_INCR_XFER_SIZE}.
1371
1372 @item _PC_REC_MAX_XFER_SIZE
1373 @standards{POSIX.1, unistd.h}
1374 Inquire about the value of @code{POSIX_REC_MAX_XFER_SIZE}.
1375
1376 @item _PC_REC_MIN_XFER_SIZE
1377 @standards{POSIX.1, unistd.h}
1378 Inquire about the value of @code{POSIX_REC_MIN_XFER_SIZE}.
1379
1380 @item _PC_REC_XFER_ALIGN
1381 @standards{POSIX.1, unistd.h}
1382 Inquire about the value of @code{POSIX_REC_XFER_ALIGN}.
1383 @end vtable
1384
1385 @strong{Portability Note:} On some systems, @theglibc{} does not
1386 enforce @code{_PC_NAME_MAX} or @code{_PC_PATH_MAX} limits.
1387
1388 @node Utility Limits
1389 @section Utility Program Capacity Limits
1390
1391 The POSIX.2 standard specifies certain system limits that you can access
1392 through @code{sysconf} that apply to utility behavior rather than the
1393 behavior of the library or the operating system.
1394
1395 @Theglibc{} defines macros for these limits, and @code{sysconf}
1396 returns values for them if you ask; but these values convey no
1397 meaningful information.  They are simply the smallest values that
1398 POSIX.2 permits.
1399
1400 @deftypevr Macro int BC_BASE_MAX
1401 @standards{POSIX.2, limits.h}
1402 The largest value of @code{obase} that the @code{bc} utility is
1403 guaranteed to support.
1404 @end deftypevr
1405
1406 @deftypevr Macro int BC_DIM_MAX
1407 @standards{POSIX.2, limits.h}
1408 The largest number of elements in one array that the @code{bc} utility
1409 is guaranteed to support.
1410 @end deftypevr
1411
1412 @deftypevr Macro int BC_SCALE_MAX
1413 @standards{POSIX.2, limits.h}
1414 The largest value of @code{scale} that the @code{bc} utility is
1415 guaranteed to support.
1416 @end deftypevr
1417
1418 @deftypevr Macro int BC_STRING_MAX
1419 @standards{POSIX.2, limits.h}
1420 The largest number of characters in one string constant that the
1421 @code{bc} utility is guaranteed to support.
1422 @end deftypevr
1423
1424 @deftypevr Macro int COLL_WEIGHTS_MAX
1425 @standards{POSIX.2, limits.h}
1426 The largest number of weights that can necessarily be used in defining
1427 the collating sequence for a locale.
1428 @end deftypevr
1429
1430 @deftypevr Macro int EXPR_NEST_MAX
1431 @standards{POSIX.2, limits.h}
1432 The maximum number of expressions that can be nested within parentheses
1433 by the @code{expr} utility.
1434 @end deftypevr
1435
1436 @deftypevr Macro int LINE_MAX
1437 @standards{POSIX.2, limits.h}
1438 The largest text line that the text-oriented POSIX.2 utilities can
1439 support.  (If you are using the GNU versions of these utilities, then
1440 there is no actual limit except that imposed by the available virtual
1441 memory, but there is no way that the library can tell you this.)
1442 @end deftypevr
1443
1444 @deftypevr Macro int EQUIV_CLASS_MAX
1445 @standards{POSIX.2, limits.h}
1446 The maximum number of weights that can be assigned to an entry of the
1447 @code{LC_COLLATE} category @samp{order} keyword in a locale definition.
1448 @Theglibc{} does not presently support locale definitions.
1449 @end deftypevr
1450
1451 @node Utility Minimums
1452 @section Minimum Values for Utility Limits
1453
1454 @vtable @code
1455 @item _POSIX2_BC_BASE_MAX
1456 @standards{POSIX.2, limits.h}
1457 The most restrictive limit permitted by POSIX.2 for the maximum value of
1458 @code{obase} in the @code{bc} utility.  Its value is @code{99}.
1459
1460 @item _POSIX2_BC_DIM_MAX
1461 @standards{POSIX.2, limits.h}
1462 The most restrictive limit permitted by POSIX.2 for the maximum size of
1463 an array in the @code{bc} utility.  Its value is @code{2048}.
1464
1465 @item _POSIX2_BC_SCALE_MAX
1466 @standards{POSIX.2, limits.h}
1467 The most restrictive limit permitted by POSIX.2 for the maximum value of
1468 @code{scale} in the @code{bc} utility.  Its value is @code{99}.
1469
1470 @item _POSIX2_BC_STRING_MAX
1471 @standards{POSIX.2, limits.h}
1472 The most restrictive limit permitted by POSIX.2 for the maximum size of
1473 a string constant in the @code{bc} utility.  Its value is @code{1000}.
1474
1475 @item _POSIX2_COLL_WEIGHTS_MAX
1476 @standards{POSIX.2, limits.h}
1477 The most restrictive limit permitted by POSIX.2 for the maximum number
1478 of weights that can necessarily be used in defining the collating
1479 sequence for a locale.  Its value is @code{2}.
1480
1481 @item _POSIX2_EXPR_NEST_MAX
1482 @standards{POSIX.2, limits.h}
1483 The most restrictive limit permitted by POSIX.2 for the maximum number
1484 of expressions nested within parenthesis when using the @code{expr} utility.
1485 Its value is @code{32}.
1486
1487 @item _POSIX2_LINE_MAX
1488 @standards{POSIX.2, limits.h}
1489 The most restrictive limit permitted by POSIX.2 for the maximum size of
1490 a text line that the text utilities can handle.  Its value is
1491 @code{2048}.
1492
1493 @item _POSIX2_EQUIV_CLASS_MAX
1494 @standards{POSIX.2, limits.h}
1495 The most restrictive limit permitted by POSIX.2 for the maximum number
1496 of weights that can be assigned to an entry of the @code{LC_COLLATE}
1497 category @samp{order} keyword in a locale definition.  Its value is
1498 @code{2}.  @Theglibc{} does not presently support locale
1499 definitions.
1500 @end vtable
1501
1502 @node String Parameters
1503 @section String-Valued Parameters
1504
1505 POSIX.2 defines a way to get string-valued parameters from the operating
1506 system with the function @code{confstr}:
1507
1508 @deftypefun size_t confstr (int @var{parameter}, char *@var{buf}, size_t @var{len})
1509 @standards{POSIX.2, unistd.h}
1510 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
1511 This function reads the value of a string-valued system parameter,
1512 storing the string into @var{len} bytes of memory space starting at
1513 @var{buf}.  The @var{parameter} argument should be one of the
1514 @samp{_CS_} symbols listed below.
1515
1516 The normal return value from @code{confstr} is the length of the string
1517 value that you asked for.  If you supply a null pointer for @var{buf},
1518 then @code{confstr} does not try to store the string; it just returns
1519 its length.  A value of @code{0} indicates an error.
1520
1521 If the string you asked for is too long for the buffer (that is, longer
1522 than @code{@var{len} - 1}), then @code{confstr} stores just that much
1523 (leaving room for the terminating null character).  You can tell that
1524 this has happened because @code{confstr} returns a value greater than or
1525 equal to @var{len}.
1526
1527 The following @code{errno} error conditions are defined for this function:
1528
1529 @table @code
1530 @item EINVAL
1531 The value of the @var{parameter} is invalid.
1532 @end table
1533 @end deftypefun
1534
1535 Currently there is just one parameter you can read with @code{confstr}:
1536
1537 @vtable @code
1538 @item _CS_PATH
1539 @standards{POSIX.2, unistd.h}
1540 This parameter's value is the recommended default path for searching for
1541 executable files.  This is the path that a user has by default just
1542 after logging in.
1543
1544 @item _CS_LFS_CFLAGS
1545 @standards{Unix98, unistd.h}
1546 The returned string specifies which additional flags must be given to
1547 the C compiler if a source is compiled using the
1548 @code{_LARGEFILE_SOURCE} feature select macro; @pxref{Feature Test Macros}.
1549
1550 @item _CS_LFS_LDFLAGS
1551 @standards{Unix98, unistd.h}
1552 The returned string specifies which additional flags must be given to
1553 the linker if a source is compiled using the
1554 @code{_LARGEFILE_SOURCE} feature select macro; @pxref{Feature Test Macros}.
1555
1556 @item _CS_LFS_LIBS
1557 @standards{Unix98, unistd.h}
1558 The returned string specifies which additional libraries must be linked
1559 to the application if a source is compiled using the
1560 @code{_LARGEFILE_SOURCE} feature select macro; @pxref{Feature Test Macros}.
1561
1562 @item _CS_LFS_LINTFLAGS
1563 @standards{Unix98, unistd.h}
1564 The returned string specifies which additional flags must be given to
1565 the lint tool if a source is compiled using the
1566 @code{_LARGEFILE_SOURCE} feature select macro; @pxref{Feature Test Macros}.
1567
1568 @item _CS_LFS64_CFLAGS
1569 @standards{Unix98, unistd.h}
1570 The returned string specifies which additional flags must be given to
1571 the C compiler if a source is compiled using the
1572 @code{_LARGEFILE64_SOURCE} feature select macro; @pxref{Feature Test Macros}.
1573
1574 @item _CS_LFS64_LDFLAGS
1575 @standards{Unix98, unistd.h}
1576 The returned string specifies which additional flags must be given to
1577 the linker if a source is compiled using the
1578 @code{_LARGEFILE64_SOURCE} feature select macro; @pxref{Feature Test Macros}.
1579
1580 @item _CS_LFS64_LIBS
1581 @standards{Unix98, unistd.h}
1582 The returned string specifies which additional libraries must be linked
1583 to the application if a source is compiled using the
1584 @code{_LARGEFILE64_SOURCE} feature select macro; @pxref{Feature Test Macros}.
1585
1586 @item _CS_LFS64_LINTFLAGS
1587 @standards{Unix98, unistd.h}
1588 The returned string specifies which additional flags must be given to
1589 the lint tool if a source is compiled using the
1590 @code{_LARGEFILE64_SOURCE} feature select macro; @pxref{Feature Test Macros}.
1591 @end vtable
1592
1593 The way to use @code{confstr} without any arbitrary limit on string size
1594 is to call it twice: first call it to get the length, allocate the
1595 buffer accordingly, and then call @code{confstr} again to fill the
1596 buffer, like this:
1597
1598 @smallexample
1599 @group
1600 char *
1601 get_default_path (void)
1602 @{
1603   size_t len = confstr (_CS_PATH, NULL, 0);
1604   char *buffer = (char *) xmalloc (len);
1605
1606   if (confstr (_CS_PATH, buf, len + 1) == 0)
1607     @{
1608       free (buffer);
1609       return NULL;
1610     @}
1611
1612   return buffer;
1613 @}
1614 @end group
1615 @end smallexample