1SYSCONF(3)                 Linux Programmer's Manual                SYSCONF(3)
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NAME

6       sysconf - get configuration information at run time
7

SYNOPSIS

9       #include <unistd.h>
10
11       long sysconf(int name);
12

DESCRIPTION

14       POSIX allows an application to test at compile or run time whether cer‐
15       tain options are supported, or what the value  is  of  certain  config‐
16       urable constants or limits.
17
18       At  compile time this is done by including <unistd.h> and/or <limits.h>
19       and testing the value of certain macros.
20
21       At run time, one can ask for numerical values using the  present  func‐
22       tion  sysconf().   One  can ask for numerical values that may depend on
23       the filesystem in which a file resides  using  fpathconf(3)  and  path‐
24       conf(3).  One can ask for string values using confstr(3).
25
26       The  values obtained from these functions are system configuration con‐
27       stants.  They do not change during the lifetime of a process.
28
29       For options, typically, there is a  constant  _POSIX_FOO  that  may  be
30       defined in <unistd.h>.  If it is undefined, one should ask at run time.
31       If it is defined to -1, then the option is not  supported.   If  it  is
32       defined to 0, then relevant functions and headers exist, but one has to
33       ask at run time what degree of support is available.  If it is  defined
34       to  a  value other than -1 or 0, then the option is supported.  Usually
35       the value (such as 200112L) indicates the year and month of  the  POSIX
36       revision  describing  the  option.   Glibc uses the value 1 to indicate
37       support as long as the POSIX revision has not been published yet.   The
38       sysconf()  argument  will  be  _SC_FOO.   For  a  list  of options, see
39       posixoptions(7).
40
41       For variables or limits, typically, there is  a  constant  _FOO,  maybe
42       defined in <limits.h>, or _POSIX_FOO, maybe defined in <unistd.h>.  The
43       constant will not be defined if the limit is unspecified.  If the  con‐
44       stant  is  defined,  it  gives  a guaranteed value, and a greater value
45       might actually be supported.  If an application wants to take advantage
46       of  values which may change between systems, a call to sysconf() can be
47       made.  The sysconf() argument will be _SC_FOO.
48
49   POSIX.1 variables
50       We give the name of the variable, the name of  the  sysconf()  argument
51       used to inquire about its value, and a short description.
52
53       First, the POSIX.1 compatible values.
54
55       ARG_MAX - _SC_ARG_MAX
56              The  maximum  length  of  the arguments to the exec(3) family of
57              functions.  Must not be less than _POSIX_ARG_MAX (4096).
58
59       CHILD_MAX - _SC_CHILD_MAX
60              The maximum number of simultaneous processes per user ID.   Must
61              not be less than _POSIX_CHILD_MAX (25).
62
63       HOST_NAME_MAX - _SC_HOST_NAME_MAX
64              Maximum length of a hostname, not including the terminating null
65              byte, as returned by gethostname(2).   Must  not  be  less  than
66              _POSIX_HOST_NAME_MAX (255).
67
68       LOGIN_NAME_MAX - _SC_LOGIN_NAME_MAX
69              Maximum  length  of a login name, including the terminating null
70              byte.  Must not be less than _POSIX_LOGIN_NAME_MAX (9).
71
72       NGROUPS_MAX - _SC_NGROUPS_MAX
73              Maximum number of supplementary group IDs.
74
75       clock ticks - _SC_CLK_TCK
76              The number of clock ticks per second.  The  corresponding  vari‐
77              able  is obsolete.  It was of course called CLK_TCK.  (Note: the
78              macro CLOCKS_PER_SEC does not give information:  it  must  equal
79              1000000.)
80
81       OPEN_MAX - _SC_OPEN_MAX
82              The  maximum number of files that a process can have open at any
83              time.  Must not be less than _POSIX_OPEN_MAX (20).
84
85       PAGESIZE - _SC_PAGESIZE
86              Size of a page in bytes.  Must not be less than 1.   (Some  sys‐
87              tems use PAGE_SIZE instead.)
88
89       RE_DUP_MAX - _SC_RE_DUP_MAX
90              The  number  of  repeated  occurrences  of  a  BRE  permitted by
91              regexec(3)   and   regcomp(3).    Must   not   be   less    than
92              _POSIX2_RE_DUP_MAX (255).
93
94       STREAM_MAX - _SC_STREAM_MAX
95              The  maximum  number  of streams that a process can have open at
96              any time.  If defined, it has the same value as the  standard  C
97              macro FOPEN_MAX.  Must not be less than _POSIX_STREAM_MAX (8).
98
99       SYMLOOP_MAX - _SC_SYMLOOP_MAX
100              The  maximum  number of symbolic links seen in a pathname before
101              resolution returns ELOOP.  Must not  be  less  than  _POSIX_SYM‐
102              LOOP_MAX (8).
103
104       TTY_NAME_MAX - _SC_TTY_NAME_MAX
105              The maximum length of terminal device name, including the termi‐
106              nating null byte.  Must not  be  less  than  _POSIX_TTY_NAME_MAX
107              (9).
108
109       TZNAME_MAX - _SC_TZNAME_MAX
110              The  maximum  number  of  bytes in a timezone name.  Must not be
111              less than _POSIX_TZNAME_MAX (6).
112
113       _POSIX_VERSION - _SC_VERSION
114              indicates the year and month the POSIX.1 standard  was  approved
115              in  the  format  YYYYMML;  the value 199009L indicates the Sept.
116              1990 revision.
117
118   POSIX.2 variables
119       Next, the POSIX.2 values, giving limits for utilities.
120
121       BC_BASE_MAX - _SC_BC_BASE_MAX
122              indicates the maximum obase value accepted by the bc(1) utility.
123
124       BC_DIM_MAX - _SC_BC_DIM_MAX
125              indicates the maximum value of elements permitted in an array by
126              bc(1).
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128       BC_SCALE_MAX - _SC_BC_SCALE_MAX
129              indicates the maximum scale value allowed by bc(1).
130
131       BC_STRING_MAX - _SC_BC_STRING_MAX
132              indicates the maximum length of a string accepted by bc(1).
133
134       COLL_WEIGHTS_MAX - _SC_COLL_WEIGHTS_MAX
135              indicates the maximum numbers of weights that can be assigned to
136              an entry of the LC_COLLATE order keyword in the  locale  defini‐
137              tion file,
138
139       EXPR_NEST_MAX - _SC_EXPR_NEST_MAX
140              is  the maximum number of expressions which can be nested within
141              parentheses by expr(1).
142
143       LINE_MAX - _SC_LINE_MAX
144              The maximum length of a utility's input line, either from  stan‐
145              dard  input  or from a file.  This includes space for a trailing
146              newline.
147
148       RE_DUP_MAX - _SC_RE_DUP_MAX
149              The maximum number of repeated occurrences of a regular  expres‐
150              sion when the interval notation \{m,n\} is used.
151
152       POSIX2_VERSION - _SC_2_VERSION
153              indicates  the  version of the POSIX.2 standard in the format of
154              YYYYMML.
155
156       POSIX2_C_DEV - _SC_2_C_DEV
157              indicates whether the POSIX.2 C language development  facilities
158              are supported.
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160       POSIX2_FORT_DEV - _SC_2_FORT_DEV
161              indicates  whether the POSIX.2 FORTRAN development utilities are
162              supported.
163
164       POSIX2_FORT_RUN - _SC_2_FORT_RUN
165              indicates whether the POSIX.2  FORTRAN  run-time  utilities  are
166              supported.
167
168       _POSIX2_LOCALEDEF - _SC_2_LOCALEDEF
169              indicates   whether   the   POSIX.2   creation  of  locates  via
170              localedef(1) is supported.
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172       POSIX2_SW_DEV - _SC_2_SW_DEV
173              indicates whether the  POSIX.2  software  development  utilities
174              option is supported.
175
176       These values also exist, but may not be standard.
177
178        - _SC_PHYS_PAGES
179              The  number of pages of physical memory.  Note that it is possi‐
180              ble for the product of this value and the value of  _SC_PAGESIZE
181              to overflow.
182
183        - _SC_AVPHYS_PAGES
184              The number of currently available pages of physical memory.
185
186        - _SC_NPROCESSORS_CONF
187              The    number    of    processors    configured.     See    also
188              get_nprocs_conf(3).
189
190        - _SC_NPROCESSORS_ONLN
191              The number of processors currently online (available).  See also
192              get_nprocs_conf(3).
193

RETURN VALUE

195       The return value of sysconf() is one of the following:
196
197       *  On  error,  -1 is returned and errno is set to indicate the cause of
198          the error (for example, EINVAL, indicating that name is invalid).
199
200       *  If name corresponds to a maximum or minimum limit, and that limit is
201          indeterminate, -1 is returned and errno is not changed.  (To distin‐
202          guish an indeterminate limit from an error, set errno to zero before
203          the  call,  and  then  check  whether  errno  is  nonzero when -1 is
204          returned.)
205
206       *  If name corresponds to an option, a positive value  is  returned  if
207          the  option  is  supported,  and -1 is returned if the option is not
208          supported.
209
210       *  Otherwise, the current value of the option  or  limit  is  returned.
211          This value will not be more restrictive than the corresponding value
212          that was described to the application in  <unistd.h>  or  <limits.h>
213          when the application was compiled.
214

ERRORS

216       EINVAL name is invalid.
217

ATTRIBUTES

219       For   an   explanation   of   the  terms  used  in  this  section,  see
220       attributes(7).
221
222       ┌──────────┬───────────────┬─────────────┐
223Interface Attribute     Value       
224       ├──────────┼───────────────┼─────────────┤
225sysconf() │ Thread safety │ MT-Safe env │
226       └──────────┴───────────────┴─────────────┘

CONFORMING TO

228       POSIX.1-2001, POSIX.1-2008.
229

BUGS

231       It is difficult to use ARG_MAX because it is not specified how much  of
232       the  argument  space  for exec(3) is consumed by the user's environment
233       variables.
234
235       Some returned values may be huge; they are not suitable for  allocating
236       memory.
237

SEE ALSO

239       bc(1),  expr(1), getconf(1), locale(1), confstr(3), fpathconf(3), path‐
240       conf(3), posixoptions(7)
241

COLOPHON

243       This page is part of release 4.16 of the Linux  man-pages  project.   A
244       description  of  the project, information about reporting bugs, and the
245       latest    version    of    this    page,    can     be     found     at
246       https://www.kernel.org/doc/man-pages/.
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250GNU                               2017-11-26                        SYSCONF(3)
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