man3/sysconf.3

   1 .\" (c) 1993 by Thomas Koenig (ig25@rz.uni-karlsruhe.de)
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  23 .\" Modified Sat Jul 24 17:51:42 1993 by Rik Faith (faith@cs.unc.edu)
  24 .\" Modified Tue Aug 17 11:42:20 1999 by Ariel Scolnicov (ariels@compugen.co.il)
  25 .TH SYSCONF 3  1993-04-18 "GNU" "Linux Programmer's Manual"
  26 .SH NAME
  27 sysconf \- Get configuration information at runtime
  28 .SH SYNOPSIS
  29 .nf
  30 .B #include <unistd.h>
  31 .sp
  32 .BI "long sysconf(int " "name" );
  33 .fi
  34 .SH DESCRIPTION
  35 POSIX allows an application to test at compile- or run-time
  36 whether certain options are supported, or what the value is
  37 of certain configurable constants or limits.
  38 .LP
  39 At compile time this is done by including
  40 .I <unistd.h>
  41 and/or
  42 .I <limits.h>
  43 and testing the value of certain macros.
  44 .LP
  45 At run time, one can ask for numerical values using the present function
  46 .BR sysconf ().
  47 On can ask for numerical values that may depend
  48 on the filesystem a file is in using the calls
  49 .BR fpathconf (3)
  50 and
  51 .BR pathconf (3).
  52 One can ask for string values using
  53 .BR confstr (3).
  54 .LP
  55 The values obtained from these functions are system configuration constants.
  56 They do not change during the lifetime of a process.
  57 .\" except that sysconf(_SC_OPEN_MAX) may change answer after a call
  58 .\" to  setrlimit( ) which changes the RLIMIT_NOFILE soft limit
  59 .LP
  60 For options, typically, there is a constant
  61 .B _POSIX_FOO
  62 that may be defined in
  63 .IR <unistd.h> .
  64 If it is undefined, one should ask at run-time.
  65 If it is defined to \-1, then the option is not supported.
  66 If it is defined to 0, then relevant functions and headers exist,
  67 but one has to ask at runtime what degree of support is available.
  68 If it is defined to a value other than \-1 or 0, then the option is
  69 supported.
  70 Usually the value (such as 200112L) indicates the year and month
  71 of the POSIX revision describing the option.
  72 Glibc uses the value 1
  73 to indicate support as long as the POSIX revision has not been published yet.
  74 .\" and 999 to indicate support for options no longer present in the latest
  75 .\" standard. (?)
  76 The
  77 .BR sysconf ()
  78 argument will be
  79 .BR _SC_FOO .
  80 For a list of options, see
  81 .BR posixoptions (7).
  82 .LP
  83 For variables or limits, typically, there is a constant
  84 .BR _FOO ,
  85 maybe defined in
  86 .IR <limits.h> ,
  87 or
  88 .BR _POSIX_FOO ,
  89 maybe defined in
  90 .IR <unistd.h> .
  91 The constant will not be defined if the limit is unspecified.
  92 If the constant is defined, it gives a guaranteed value, and
  93 more might actually be supported.
  94 If an application wants to take advantage of values which may change
  95 between systems, a call to
  96 .BR sysconf ()
  97 can be made.
  98 The
  99 .BR sysconf ()
 100 argument will be
 101 .BR _SC_FOO .
 102 .SH "POSIX.1 VARIABLES"
 103 We give the name of the variable, the name of the
 104 .BR sysconf ()
 105 parameter used to inquire about its value,
 106 and a short description.
 107 .LP
 108 First, the POSIX.1 compatible values.
 109 .\" [for the moment: only the things that are unconditionally present]
 110 .\" .TP
 111 .\" .BR AIO_LISTIO_MAX " - " _SC_AIO_LISTIO_MAX
 112 .\" (if _POSIX_ASYNCHRONOUS_IO)
 113 .\" Maximum number of I/O operations in a single list I/O call.
 114 .\" Must not be less than _POSIX_AIO_LISTIO_MAX.
 115 .\" .TP
 116 .\" .BR AIO_MAX " - " _SC_AIO_MAX
 117 .\" (if _POSIX_ASYNCHRONOUS_IO)
 118 .\" Maximum number of outstanding asynchronous I/O operations.
 119 .\" Must not be less than _POSIX_AIO_MAX.
 120 .\" .TP
 121 .\" .BR AIO_PRIO_DELTA_MAX " - " _SC_AIO_PRIO_DELTA_MAX
 122 .\" (if _POSIX_ASYNCHRONOUS_IO)
 123 .\" The maximum amount by which a process can decrease its
 124 .\" asynchronous I/O priority level from its own scheduling priority.
 125 .\" Must be nonnegative.
 126 .TP
 127 .BR ARG_MAX " - " _SC_ARG_MAX
 128 The maximum length of the arguments to the
 129 .BR exec ()
 130 family of functions.
 131 Must not be less than _POSIX_ARG_MAX (4096).
 132 .TP
 133 .BR CHILD_MAX " - " _SC_CHILD_MAX
 134 The max number of simultaneous processes per user ID.
 135 Must not be less than _POSIX_CHILD_MAX (25).
 136 .TP
 137 .BR HOST_NAME_MAX " - " _SC_HOST_NAME_MAX
 138 Max length of a hostname, not including the terminating null byte,
 139 as returned by
 140 .BR gethostname (2).
 141 Must not be less than _POSIX_HOST_NAME_MAX (255).
 142 .TP
 143 .BR LOGIN_NAME_MAX " - " _SC_LOGIN_NAME_MAX
 144 Maximum length of a login name, including the terminating null byte.
 145 Must not be less than _POSIX_LOGIN_NAME_MAX (9).
 146 .TP
 147 .BR "" "clock ticks - " _SC_CLK_TCK
 148 The number of clock ticks per second.
 149 The corresponding variable is obsolete.
 150 It was of course called
 151 .BR CLK_TCK .
 152 (Note: the macro
 153 .B CLOCKS_PER_SEC
 154 does not give information: it must equal 1000000.)
 155 .TP
 156 .BR OPEN_MAX " - " _SC_OPEN_MAX
 157 The maximum number of files that a process can have open at any time.
 158 Must not be less than _POSIX_OPEN_MAX (20).
 159 .TP
 160 .BR PAGESIZE " - " _SC_PAGESIZE
 161 Size of a page in bytes.
 162 Must not be less than 1.
 163 (Some systems use PAGE_SIZE instead.)
 164 .TP
 165 .BR RE_DUP_MAX " - " _SC_RE_DUP_MAX
 166 The number of repeated occurrences of a BRE permitted by
 167 .BR regexec (3)
 168 and
 169 .BR regcomp (3).
 170 Must not be less than _POSIX2_RE_DUP_MAX (255).
 171 .TP
 172 .BR STREAM_MAX " - " _SC_STREAM_MAX
 173 The maximum number of streams that a process can have open at any
 174 time.
 175 If defined, it has the same value as the standard C macro
 176 .BR FOPEN_MAX .
 177 Must not be less than _POSIX_STREAM_MAX (8).
 178 .TP
 179 .BR SYMLOOP_MAX
 180 The maximum number of symbolic links seen in a pathname before resolution
 181 returns ELOOP.
 182 Must not be less than _POSIX_SYMLOOP_MAX (8).
 183 .TP
 184 .BR TTY_NAME_MAX " - " _SC_TTY_NAME_MAX
 185 The maximum length of terminal device name,
 186 including the terminating null byte.
 187 Must not be less than _POSIX_TTY_NAME_MAX (9).
 188 .TP
 189 .BR TZNAME_MAX " - " _SC_TZNAME_MAX
 190 The maximum number of bytes in a timezone name.
 191 Must not be less than _POSIX_TZNAME_MAX (6).
 192 .TP
 193 .BR _POSIX_VERSION " - " _SC_VERSION
 194 indicates the year and month the POSIX.1 standard was approved in the
 195 format
 196 .BR YYYYMML ; the
 197 value
 198 .B 199009L
 199 indicates the Sept. 1990 revision.
 200 .SH "POSIX.2 VARIABLES"
 201 Next, the POSIX.2 values, giving limits for utilities.
 202 .TP
 203 .BR BC_BASE_MAX " - " _SC_BC_BASE_MAX
 204 indicates the maximum
 205 .I obase
 206 value accepted by the
 207 .BR bc (1)
 208 utility.
 209 .TP
 210 .BR BC_DIM_MAX " - " _SC_BC_DIM_MAX
 211 indicates the maximum value of elements permitted in an array by
 212 .BR bc (1).
 213 .TP
 214 .BR BC_SCALE_MAX " - " _SC_BC_SCALE_MAX
 215 indicates the maximum
 216 .I scale
 217 value allowed by
 218 .BR bc (1).
 219 .TP
 220 .BR BC_STRING_MAX " - " _SC_BC_STRING_MAX
 221 indicates the maximum length of a string accepted by
 222 .BR bc (1).
 223 .TP
 224 .BR COLL_WEIGHTS_MAX " - " _SC_COLL_WEIGHTS_MAX
 225 indicates the maximum numbers of weights that can be assigned to an
 226 entry of the
 227 .B LC_COLLATE order
 228 keyword in the locale definition file,
 229 .TP
 230 .BR EXPR_NEST_MAX " - " _SC_EXPR_NEST_MAX
 231 is the maximum number of expressions which can be nested within
 232 parentheses by
 233 .BR expr (1).
 234 .TP
 235 .BR LINE_MAX " - " _SC_LINE_MAX
 236 The maximum length of a utility's input line length, either from
 237 standard input or from a file.
 238 This includes length for a trailing
 239 newline.
 240 .TP
 241 .BR RE_DUP_MAX " - " _SC_RE_DUP_MAX
 242 The maximum number of repeated occurrences of a regular expression when
 243 the interval notation
 244 .B \e{m,n\e}
 245 is used.
 246 .TP
 247 .BR POSIX2_VERSION " - " _SC_2_VERSION
 248 indicates the version of the POSIX.2 standard in the format of
 249 YYYYMML.
 250 .TP
 251 .BR POSIX2_C_DEV " - " _SC_2_C_DEV
 252 indicates whether the POSIX.2 C language development facilities are
 253 supported.
 254 .TP
 255 .BR POSIX2_FORT_DEV " - " _SC_2_FORT_DEV
 256 indicates whether the POSIX.2 FORTRAN development utilities are
 257 supported.
 258 .TP
 259 .BR POSIX2_FORT_RUN " - " _SC_2_FORT_RUN
 260 indicates whether the POSIX.2 FORTRAN runtime utilities are supported.
 261 .TP
 262 .BR _POSIX2_LOCALEDEF " - " _SC_2_LOCALEDEF
 263 indicates whether the POSIX.2 creation of locates via
 264 .BR localedef (1)
 265 is supported.
 266 .TP
 267 .BR POSIX2_SW_DEV " - " _SC_2_SW_DEV
 268 indicates whether the POSIX.2 software development utilities option is
 269 supported.
 270 .PP
 271 These values also exist, but may not be standard.
 272 .TP
 273 .BR "" " - " _SC_PHYS_PAGES
 274 The number of pages of physical memory.
 275 Note that it is possible
 276 for the product of this value and the value of
 277 .B _SC_PAGE_SIZE
 278 to overflow.
 279 .TP
 280 .BR "" " - " _SC_AVPHYS_PAGES
 281 The number of currently available pages of physical memory.
 282 .SH "RETURN VALUE"
 283 If
 284 .I name
 285 is invalid, \-1 is returned, and
 286 .I errno
 287 is set to
 288 .BR EINVAL .
 289 Otherwise, the value returned is the value of the system resource and
 290 .I errno
 291 is not changed.
 292 In the case of options, a positive value is returned if a queried option
 293 is available, and \-1 if it is not.
 294 In the case of limits, \-1 means that there is no definite limit.
 295 .SH "CONFORMING TO"
 296 POSIX.1-2001.
 297 .SH BUGS
 298 It is difficult to use
 299 .B ARG_MAX
 300 because it is not specified how much of the argument space for
 301 .BR exec ()
 302 is consumed by the user's environment variables.
 303 .PP
 304 Some returned values may be huge; they are not suitable for allocating
 305 memory.
 306 .SH "SEE ALSO"
 307 .BR bc (1),
 308 .BR expr (1),
 309 .BR locale (1),
 310 .BR fpathconf (3),
 311 .BR pathconf (3),
 312 .BR posixoptions (7)