man3/sysconf.3

   1 .\" Copyright (c) 1993 by Thomas Koenig (ig25@rz.uni-karlsruhe.de)
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  25 .\" Modified Sat Jul 24 17:51:42 1993 by Rik Faith (faith@cs.unc.edu)
  26 .\" Modified Tue Aug 17 11:42:20 1999 by Ariel Scolnicov (ariels@compugen.co.il)
  27 .TH SYSCONF 3  2019-05-09 "GNU" "Linux Programmer's Manual"
  28 .SH NAME
  29 sysconf \- get configuration information at run time
  30 .SH SYNOPSIS
  31 .nf
  32 .B #include <unistd.h>
  33 .PP
  34 .BI "long sysconf(int " "name" );
  35 .fi
  36 .SH DESCRIPTION
  37 POSIX allows an application to test at compile or run time
  38 whether certain options are supported, or what the value is
  39 of certain configurable constants or limits.
  40 .PP
  41 At compile time this is done by including
  42 .I <unistd.h>
  43 and/or
  44 .I <limits.h>
  45 and testing the value of certain macros.
  46 .PP
  47 At run time, one can ask for numerical values using the present function
  48 .BR sysconf ().
  49 One can ask for numerical values that may depend
  50 on the filesystem in which a file resides using
  51 .BR fpathconf (3)
  52 and
  53 .BR pathconf (3).
  54 One can ask for string values using
  55 .BR confstr (3).
  56 .PP
  57 The values obtained from these functions are system configuration constants.
  58 They do not change during the lifetime of a process.
  59 .\" except that sysconf(_SC_OPEN_MAX) may change answer after a call
  60 .\" to setrlimit( ) which changes the RLIMIT_NOFILE soft limit
  61 .PP
  62 For options, typically, there is a constant
  63 .B _POSIX_FOO
  64 that may be defined in
  65 .IR <unistd.h> .
  66 If it is undefined, one should ask at run time.
  67 If it is defined to \-1, then the option is not supported.
  68 If it is defined to 0, then relevant functions and headers exist,
  69 but one has to ask at run time what degree of support is available.
  70 If it is defined to a value other than \-1 or 0, then the option is
  71 supported.
  72 Usually the value (such as 200112L) indicates the year and month
  73 of the POSIX revision describing the option.
  74 Glibc uses the value 1
  75 to indicate support as long as the POSIX revision has not been published yet.
  76 .\" and 999 to indicate support for options no longer present in the latest
  77 .\" standard. (?)
  78 The
  79 .BR sysconf ()
  80 argument will be
  81 .BR _SC_FOO .
  82 For a list of options, see
  83 .BR posixoptions (7).
  84 .PP
  85 For variables or limits, typically, there is a constant
  86 .BR _FOO ,
  87 maybe defined in
  88 .IR <limits.h> ,
  89 or
  90 .BR _POSIX_FOO ,
  91 maybe defined in
  92 .IR <unistd.h> .
  93 The constant will not be defined if the limit is unspecified.
  94 If the constant is defined, it gives a guaranteed value, and
  95 a greater value might actually be supported.
  96 If an application wants to take advantage of values which may change
  97 between systems, a call to
  98 .BR sysconf ()
  99 can be made.
 100 The
 101 .BR sysconf ()
 102 argument will be
 103 .BR _SC_FOO .
 104 .SS POSIX.1 variables
 105 We give the name of the variable, the name of the
 106 .BR sysconf ()
 107 argument used to inquire about its value,
 108 and a short description.
 109 .PP
 110 First, the POSIX.1 compatible values.
 111 .\" [for the moment: only the things that are unconditionally present]
 112 .\" .TP
 113 .\" .BR AIO_LISTIO_MAX " - " _SC_AIO_LISTIO_MAX
 114 .\" (if _POSIX_ASYNCHRONOUS_IO)
 115 .\" Maximum number of I/O operations in a single list I/O call.
 116 .\" Must not be less than _POSIX_AIO_LISTIO_MAX.
 117 .\" .TP
 118 .\" .BR AIO_MAX " - " _SC_AIO_MAX
 119 .\" (if _POSIX_ASYNCHRONOUS_IO)
 120 .\" Maximum number of outstanding asynchronous I/O operations.
 121 .\" Must not be less than _POSIX_AIO_MAX.
 122 .\" .TP
 123 .\" .BR AIO_PRIO_DELTA_MAX " - " _SC_AIO_PRIO_DELTA_MAX
 124 .\" (if _POSIX_ASYNCHRONOUS_IO)
 125 .\" The maximum amount by which a process can decrease its
 126 .\" asynchronous I/O priority level from its own scheduling priority.
 127 .\" Must be nonnegative.
 128 .TP
 129 .BR ARG_MAX " - " _SC_ARG_MAX
 130 The maximum length of the arguments to the
 131 .BR exec (3)
 132 family of functions.
 133 Must not be less than
 134 .B _POSIX_ARG_MAX
 135 (4096).
 136 .TP
 137 .BR CHILD_MAX " - " _SC_CHILD_MAX
 138 The maximum number of simultaneous processes per user ID.
 139 Must not be less than
 140 .B _POSIX_CHILD_MAX
 141 (25).
 142 .TP
 143 .BR HOST_NAME_MAX " - " _SC_HOST_NAME_MAX
 144 Maximum length of a hostname, not including the terminating null byte,
 145 as returned by
 146 .BR gethostname (2).
 147 Must not be less than
 148 .B _POSIX_HOST_NAME_MAX
 149 (255).
 150 .TP
 151 .BR LOGIN_NAME_MAX " - " _SC_LOGIN_NAME_MAX
 152 Maximum length of a login name, including the terminating null byte.
 153 Must not be less than
 154 .B _POSIX_LOGIN_NAME_MAX
 155 (9).
 156 .TP
 157 .BR NGROUPS_MAX " - " _SC_NGROUPS_MAX
 158 Maximum number of supplementary group IDs.
 159 .TP
 160 .BR "" "clock ticks - " _SC_CLK_TCK
 161 The number of clock ticks per second.
 162 The corresponding variable is obsolete.
 163 It was of course called
 164 .BR CLK_TCK .
 165 (Note: the macro
 166 .B CLOCKS_PER_SEC
 167 does not give information: it must equal 1000000.)
 168 .TP
 169 .BR OPEN_MAX " - " _SC_OPEN_MAX
 170 The maximum number of files that a process can have open at any time.
 171 Must not be less than
 172 .B _POSIX_OPEN_MAX
 173 (20).
 174 .TP
 175 .BR PAGESIZE " - " _SC_PAGESIZE
 176 Size of a page in bytes.
 177 Must not be less than 1.
 178 .TP
 179 .BR PAGE_SIZE " - " _SC_PAGE_SIZE
 180 A synonym for
 181 .BR PAGESIZE / _SC_PAGESIZE .
 182 (Both
 183 .BR PAGESIZE
 184 and
 185 .BR PAGE_SIZE
 186 are specified in POSIX.)
 187 .TP
 188 .BR RE_DUP_MAX " - " _SC_RE_DUP_MAX
 189 The number of repeated occurrences of a BRE permitted by
 190 .BR regexec (3)
 191 and
 192 .BR regcomp (3).
 193 Must not be less than
 194 .B _POSIX2_RE_DUP_MAX
 195 (255).
 196 .TP
 197 .BR STREAM_MAX " - " _SC_STREAM_MAX
 198 The maximum number of streams that a process can have open at any
 199 time.
 200 If defined, it has the same value as the standard C macro
 201 .BR FOPEN_MAX .
 202 Must not be less than
 203 .B _POSIX_STREAM_MAX
 204 (8).
 205 .TP
 206 .BR SYMLOOP_MAX " - " _SC_SYMLOOP_MAX
 207 The maximum number of symbolic links seen in a pathname before resolution
 208 returns
 209 .BR ELOOP .
 210 Must not be less than
 211 .B _POSIX_SYMLOOP_MAX
 212 (8).
 213 .TP
 214 .BR TTY_NAME_MAX " - " _SC_TTY_NAME_MAX
 215 The maximum length of terminal device name,
 216 including the terminating null byte.
 217 Must not be less than
 218 .B _POSIX_TTY_NAME_MAX
 219 (9).
 220 .TP
 221 .BR TZNAME_MAX " - " _SC_TZNAME_MAX
 222 The maximum number of bytes in a timezone name.
 223 Must not be less than
 224 .B _POSIX_TZNAME_MAX
 225 (6).
 226 .TP
 227 .BR _POSIX_VERSION " - " _SC_VERSION
 228 indicates the year and month the POSIX.1 standard was approved in the
 229 format
 230 .BR YYYYMML ;
 231 the value
 232 .B 199009L
 233 indicates the Sept. 1990 revision.
 234 .SS POSIX.2 variables
 235 Next, the POSIX.2 values, giving limits for utilities.
 236 .TP
 237 .BR BC_BASE_MAX " - " _SC_BC_BASE_MAX
 238 indicates the maximum
 239 .I obase
 240 value accepted by the
 241 .BR bc (1)
 242 utility.
 243 .TP
 244 .BR BC_DIM_MAX " - " _SC_BC_DIM_MAX
 245 indicates the maximum value of elements permitted in an array by
 246 .BR bc (1).
 247 .TP
 248 .BR BC_SCALE_MAX " - " _SC_BC_SCALE_MAX
 249 indicates the maximum
 250 .I scale
 251 value allowed by
 252 .BR bc (1).
 253 .TP
 254 .BR BC_STRING_MAX " - " _SC_BC_STRING_MAX
 255 indicates the maximum length of a string accepted by
 256 .BR bc (1).
 257 .TP
 258 .BR COLL_WEIGHTS_MAX " - " _SC_COLL_WEIGHTS_MAX
 259 indicates the maximum numbers of weights that can be assigned to an
 260 entry of the
 261 .B LC_COLLATE order
 262 keyword in the locale definition file,
 263 .TP
 264 .BR EXPR_NEST_MAX " - " _SC_EXPR_NEST_MAX
 265 is the maximum number of expressions which can be nested within
 266 parentheses by
 267 .BR expr (1).
 268 .TP
 269 .BR LINE_MAX " - " _SC_LINE_MAX
 270 The maximum length of a utility's input line, either from
 271 standard input or from a file.
 272 This includes space for a trailing
 273 newline.
 274 .TP
 275 .BR RE_DUP_MAX " - " _SC_RE_DUP_MAX
 276 The maximum number of repeated occurrences of a regular expression when
 277 the interval notation
 278 .B \e{m,n\e}
 279 is used.
 280 .TP
 281 .BR POSIX2_VERSION " - " _SC_2_VERSION
 282 indicates the version of the POSIX.2 standard in the format of
 283 YYYYMML.
 284 .TP
 285 .BR POSIX2_C_DEV " - " _SC_2_C_DEV
 286 indicates whether the POSIX.2 C language development facilities are
 287 supported.
 288 .TP
 289 .BR POSIX2_FORT_DEV " - " _SC_2_FORT_DEV
 290 indicates whether the POSIX.2 FORTRAN development utilities are
 291 supported.
 292 .TP
 293 .BR POSIX2_FORT_RUN " - " _SC_2_FORT_RUN
 294 indicates whether the POSIX.2 FORTRAN run-time utilities are supported.
 295 .TP
 296 .BR _POSIX2_LOCALEDEF " - " _SC_2_LOCALEDEF
 297 indicates whether the POSIX.2 creation of locates via
 298 .BR localedef (1)
 299 is supported.
 300 .TP
 301 .BR POSIX2_SW_DEV " - " _SC_2_SW_DEV
 302 indicates whether the POSIX.2 software development utilities option is
 303 supported.
 304 .PP
 305 These values also exist, but may not be standard.
 306 .TP
 307 .BR "" " - " _SC_PHYS_PAGES
 308 The number of pages of physical memory.
 309 Note that it is possible
 310 for the product of this value and the value of
 311 .B _SC_PAGESIZE
 312 to overflow.
 313 .TP
 314 .BR "" " - " _SC_AVPHYS_PAGES
 315 The number of currently available pages of physical memory.
 316 .TP
 317 .BR "" " - " _SC_NPROCESSORS_CONF
 318 The number of processors configured.
 319 See also
 320 .BR get_nprocs_conf (3).
 321 .TP
 322 .BR "" " - " _SC_NPROCESSORS_ONLN
 323 The number of processors currently online (available).
 324 See also
 325 .BR get_nprocs_conf (3).
 326 .SH RETURN VALUE
 327 The return value of
 328 .BR sysconf ()
 329 is one of the following:
 330 .IP * 3
 331 On error, \-1 is returned and
 332 .I errno
 333 is set to indicate the cause of the error
 334 (for example,
 335 .BR EINVAL ,
 336 indicating that
 337 .I name
 338 is invalid).
 339 .IP *
 340 If
 341 .I name
 342 corresponds to a maximum or minimum limit, and that limit is indeterminate,
 343 \-1 is returned and
 344 .I errno
 345 is not changed.
 346 (To distinguish an indeterminate limit from an error, set
 347 .I errno
 348 to zero before the call, and then check whether
 349 .I errno
 350 is nonzero when \-1 is returned.)
 351 .IP *
 352 If
 353 .I name
 354 corresponds to an option,
 355 a positive value is returned if the option is supported,
 356 and \-1 is returned if the option is not supported.
 357 .IP *
 358 Otherwise,
 359 the current value of the option or limit is returned.
 360 This value will not be more restrictive than
 361 the corresponding value that was described to the application in
 362 .I <unistd.h>
 363 or
 364 .I <limits.h>
 365 when the application was compiled.
 366 .SH ERRORS
 367 .TP
 368 .B EINVAL
 369 .I name
 370 is invalid.
 371 .SH ATTRIBUTES
 372 For an explanation of the terms used in this section, see
 373 .BR attributes (7).
 374 .TS
 375 allbox;
 376 lb lb lb
 377 l l l.
 378 Interface       Attribute       Value
 379 T{
 380 .BR sysconf ()
 381 T}      Thread safety   MT-Safe env
 382 .TE
 383 .SH CONFORMING TO
 384 POSIX.1-2001, POSIX.1-2008.
 385 .SH BUGS
 386 It is difficult to use
 387 .B ARG_MAX
 388 because it is not specified how much of the argument space for
 389 .BR exec (3)
 390 is consumed by the user's environment variables.
 391 .PP
 392 Some returned values may be huge; they are not suitable for allocating
 393 memory.
 394 .SH SEE ALSO
 395 .BR bc (1),
 396 .BR expr (1),
 397 .BR getconf (1),
 398 .BR locale (1),
 399 .BR confstr (3),
 400 .BR fpathconf (3),
 401 .BR pathconf (3),
 402 .BR posixoptions (7)