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. Usually the value (such as 200112L) indicates the year and month
  70 of the POSIX revision describing the option. Glibc uses the value 1
  71 to indicate support as long as the POSIX revision has not been published yet.
  72 .\" and 999 to indicate support for options no longer present in the latest
  73 .\" standard. (?)
  74 The
  75 .B sysconf()
  76 argument will be
  77 .BR _SC_FOO .
  78 For a list of options, see
  79 .BR posixoptions (7).
  80 .LP
  81 For variables or limits, typically, there is a constant
  82 .BR _FOO ,
  83 maybe defined in
  84 .IR <limits.h> ,
  85 or
  86 .BR _POSIX_FOO ,
  87 maybe defined in
  88 .IR <unistd.h> .
  89 The constant will not be defined if the limit is unspecified.
  90 If the constant is defined, it gives a guaranteed value, and
  91 more might actually be supported.
  92 If an application wants to take advantage of values which may change
  93 between systems, a call to
  94 .B sysconf()
  95 can be made.
  96 The
  97 .B
  98 sysconf()
  99 argument will be
 100 .BR _SC_FOO .
 101 .SH "POSIX.1 VARIABLES"
 102 We give the name of the variable, the name of the
 103 .B sysconf()
 104 parameter used to inquire about its value,
 105 and a short description.
 106 .LP
 107 First, the POSIX.1 compatible values.
 108 .\" [for the moment: only the things that are unconditionally present]
 109 .\" .TP
 110 .\" .BR AIO_LISTIO_MAX " - " _SC_AIO_LISTIO_MAX
 111 .\" (if _POSIX_ASYNCHRONOUS_IO)
 112 .\" Maximum number of I/O operations in a single list I/O call.
 113 .\" Must not be less than _POSIX_AIO_LISTIO_MAX.
 114 .\" .TP
 115 .\" .BR AIO_MAX " - " _SC_AIO_MAX
 116 .\" (if _POSIX_ASYNCHRONOUS_IO)
 117 .\" Maximum number of outstanding asynchronous I/O operations.
 118 .\" Must not be less than _POSIX_AIO_MAX.
 119 .\" .TP
 120 .\" .BR AIO_PRIO_DELTA_MAX " - " _SC_AIO_PRIO_DELTA_MAX
 121 .\" (if _POSIX_ASYNCHRONOUS_IO)
 122 .\" The maximum amount by which a process can decrease its
 123 .\" asynchronous I/O priority level from its own scheduling priority.
 124 .\" Must be nonnegative.
 125 .TP
 126 .BR ARG_MAX " - " _SC_ARG_MAX
 127 The maximum length of the arguments to the
 128 .B exec()
 129 family of functions.
 130 Must not be less than _POSIX_ARG_MAX (4096).
 131 .TP
 132 .BR CHILD_MAX " - " _SC_CHILD_MAX
 133 The max number of simultaneous processes per user id.
 134 Must not be less than _POSIX_CHILD_MAX (25).
 135 .TP
 136 .BR HOST_NAME_MAX " - " _SC_HOST_NAME_MAX
 137 Max length of a hostname, not including the final NUL, as returned by
 138 .BR gethostname (2).
 139 Must not be less than _POSIX_HOST_NAME_MAX (255).
 140 .TP
 141 .BR LOGIN_NAME_MAX " - " _SC_LOGIN_NAME_MAX
 142 Maximum length of a login name, including the final NUL.
 143 Must not be less than _POSIX_LOGIN_NAME_MAX (9).
 144 .TP
 145 .BR "" "clock ticks - " _SC_CLK_TCK
 146 The number of clock ticks per second.
 147 The corresponding variable is obsolete. It was of course called
 148 .BR CLK_TCK .
 149 (Note: the macro
 150 .B CLOCKS_PER_SEC
 151 does not give information: it must equal 1000000.)
 152 .TP
 153 .BR OPEN_MAX " - " _SC_OPEN_MAX
 154 The maximum number of files that a process can have open at any time.
 155 Must not be less than _POSIX_OPEN_MAX (20).
 156 .TP
 157 .BR PAGESIZE " - " _SC_PAGESIZE
 158 Size of a page in bytes. Must not be less than 1.
 159 (Some systems use PAGE_SIZE instead.)
 160 .TP
 161 .BR RE_DUP_MAX " - " _SC_RE_DUP_MAX
 162 The number of repeated occurrences of a BRE permitted by
 163 .BR regexec (3)
 164 and
 165 .BR regcomp (3).
 166 Must not be less than _POSIX2_RE_DUP_MAX (255).
 167 .TP
 168 .BR STREAM_MAX " - " _SC_STREAM_MAX
 169 The maximum number of streams that a process can have open at any
 170 time. If defined, it has the same value as the standard C macro
 171 .BR FOPEN_MAX .
 172 Must not be less than _POSIX_STREAM_MAX (8).
 173 .TP
 174 .BR SYMLOOP_MAX
 175 The maximum number of symbolic links seen in a pathname before resolution
 176 returns ELOOP. Must not be less than _POSIX_SYMLOOP_MAX (8).
 177 .TP
 178 .BR TTY_NAME_MAX " - " _SC_TTY_NAME_MAX
 179 The maximum length of terminal device name, including final NUL.
 180 Must not be less than _POSIX_TTY_NAME_MAX (9).
 181 .TP
 182 .BR TZNAME_MAX " - " _SC_TZNAME_MAX
 183 The maximum number of bytes in a timezone name.
 184 Must not be less than _POSIX_TZNAME_MAX (6).
 185 .TP
 186 .BR _POSIX_VERSION " - " _SC_VERSION
 187 indicates the year and month the POSIX.1 standard was approved in the
 188 format
 189 .BR YYYYMML ; the
 190 value
 191 .B 199009L
 192 indicates the Sept. 1990 revision.
 193 .SH "POSIX.2 VARIABLES"
 194 Next, the POSIX.2 values, giving limits for utilities.
 195 .TP
 196 .BR BC_BASE_MAX " - " _SC_BC_BASE_MAX
 197 indicates the maximum
 198 .I obase
 199 value accepted by the
 200 .BR bc (1)
 201 utility.
 202 .TP
 203 .BR BC_DIM_MAX " - " _SC_BC_DIM_MAX
 204 indicates the maximum value of elements permitted in an array by
 205 .BR bc (1).
 206 .TP
 207 .BR BC_SCALE_MAX " - " _SC_BC_SCALE_MAX
 208 indicates the maximum
 209 .I scale
 210 value allowed by
 211 .BR bc (1).
 212 .TP
 213 .BR BC_STRING_MAX " - " _SC_BC_STRING_MAX
 214 indicates the maximum length of a string accepted by
 215 .BR bc (1).
 216 .TP
 217 .BR COLL_WEIGHTS_MAX " - " _SC_COLL_WEIGHTS_MAX
 218 indicates the maximum numbers of weights that can be assigned to an
 219 entry of the
 220 .B LC_COLLATE order
 221 keyword in the locale definition file,
 222 .TP
 223 .BR EXPR_NEST_MAX " - " _SC_EXPR_NEST_MAX
 224 is the maximum number of expressions which can be nested within
 225 parentheses by
 226 .BR expr (1).
 227 .TP
 228 .BR LINE_MAX " - " _SC_LINE_MAX
 229 The maximum length of a utility's input line length, either from
 230 standard input or from a file. This includes length for a trailing
 231 newline.
 232 .TP
 233 .BR RE_DUP_MAX " - " _SC_RE_DUP_MAX
 234 The maximum number of repeated occurrences of a regular expression when
 235 the interval notation
 236 .B \e{m,n\e}
 237 is used.
 238 .TP
 239 .BR POSIX2_VERSION " - " _SC_2_VERSION
 240 indicates the version of the POSIX.2 standard in the format of
 241 YYYYMML.
 242 .TP
 243 .BR POSIX2_C_DEV " - " _SC_2_DEV
 244 indicates whether the POSIX.2 C language development facilities are
 245 supported.
 246 .TP
 247 .BR POSIX2_FORT_RUN " - " _SC_2_FORT_DEV
 248 indicates whether the POSIX.2 FORTRAN development utilities are
 249 supported.
 250 .TP
 251 .BR POSIX2_FORT_RUN " - " _SC_2_FORT_RUN
 252 indicates whether the POSIX.2 FORTRAN runtime utilities are supported.
 253 .TP
 254 .BR _POSIX2_LOCALEDEF " - " _SC_2_LOCALEDEF
 255 indicates whether the POSIX.2 creation of locates via
 256 .BR localedef (1)
 257 is supported.
 258 .TP
 259 .BR POSIX2_SW_DEV " - " _SC_2_SW_DEV
 260 indicates whether the POSIX.2 software development utilities option is
 261 supported.
 262 .PP
 263 These values also exist, but may not be standard.
 264 .TP
 265 .BR "" " - " _SC_PHYS_PAGES
 266 The number of pages of physical memory.  Note that it is possible
 267 for the product of this value and the value of
 268 .B _SC_PAGE_SIZE
 269 to overflow.
 270 .TP
 271 .BR "" " - " _SC_AVPHYS_PAGES
 272 The number of currently available pages of physical memory.
 273 .SH "RETURN VALUE"
 274 If
 275 .I name
 276 is invalid, \-1 is returned, and
 277 .I errno
 278 is set to
 279 .BR EINVAL .
 280 Otherwise, the value returned is the value of the system resource and
 281 .I errno
 282 is not changed.
 283 In the case of options, a positive value is returned if a queried option
 284 is available, and \-1 if it is not.
 285 In the case of limits, \-1 means that there is no definite limit.
 286 .SH "CONFORMING TO"
 287 POSIX.1.
 288 .SH BUGS
 289 It is difficult to use
 290 .B ARG_MAX
 291 because it is not specified how much of the argument space for
 292 .B exec()
 293 is consumed by the user's environment variables.
 294 .PP
 295 Some returned values may be huge; they are not suitable for allocating
 296 memory.
 297 .SH "SEE ALSO"
 298 .BR bc (1),
 299 .BR expr (1),
 300 .BR locale (1),
 301 .BR fpathconf (3),
 302 .BR pathconf (3),
 303 .BR posixoptions (7)