1SYSCONF(3) Linux Programmer's Manual SYSCONF(3)
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6 sysconf - get configuration information at run time
7
9 #include <unistd.h>
10 long sysconf(int name);
12
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 At compile time this is done by including <unistd.h> and/or <limits.h>
19 and testing the value of certain macros.
20 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 The values obtained from these functions are system configuration con‐
27 stants. They do not change during the lifetime of a process.
28 For options, typically, there is a constant _POSIX_FOO that may be de‐
30 fined 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 de‐
32 fined 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 For variables or limits, typically, there is a constant _FOO, maybe de‐
42 fined 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 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 First, the POSIX.1 compatible values.
54 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 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 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 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 NGROUPS_MAX - _SC_NGROUPS_MAX
73 Maximum number of supplementary group IDs.
74 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 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 PAGESIZE - _SC_PAGESIZE
86 Size of a page in bytes. Must not be less than 1.
87 PAGE_SIZE - _SC_PAGE_SIZE
89 A synonym for PAGESIZE/_SC_PAGESIZE. (Both PAGESIZE and
90 PAGE_SIZE are specified in POSIX.)
91 RE_DUP_MAX - _SC_RE_DUP_MAX
93 The number of repeated occurrences of a BRE permitted by
94 regexec(3) and regcomp(3). Must not be less than
95 _POSIX2_RE_DUP_MAX (255).
96 STREAM_MAX - _SC_STREAM_MAX
98 The maximum number of streams that a process can have open at
99 any time. If defined, it has the same value as the standard C
100 macro FOPEN_MAX. Must not be less than _POSIX_STREAM_MAX (8).
101 SYMLOOP_MAX - _SC_SYMLOOP_MAX
103 The maximum number of symbolic links seen in a pathname before
104 resolution returns ELOOP. Must not be less than _POSIX_SYM‐
105 LOOP_MAX (8).
106 TTY_NAME_MAX - _SC_TTY_NAME_MAX
108 The maximum length of terminal device name, including the termi‐
109 nating null byte. Must not be less than _POSIX_TTY_NAME_MAX
110 (9).
111 TZNAME_MAX - _SC_TZNAME_MAX
113 The maximum number of bytes in a timezone name. Must not be
114 less than _POSIX_TZNAME_MAX (6).
115 _POSIX_VERSION - _SC_VERSION
117 indicates the year and month the POSIX.1 standard was approved
118 in the format YYYYMML; the value 199009L indicates the Sept.
119 1990 revision.
120 POSIX.2 variables
122 Next, the POSIX.2 values, giving limits for utilities.
123 BC_BASE_MAX - _SC_BC_BASE_MAX
125 indicates the maximum obase value accepted by the bc(1) utility.
126 BC_DIM_MAX - _SC_BC_DIM_MAX
128 indicates the maximum value of elements permitted in an array by
129 bc(1).
130 BC_SCALE_MAX - _SC_BC_SCALE_MAX
132 indicates the maximum scale value allowed by bc(1).
133 BC_STRING_MAX - _SC_BC_STRING_MAX
135 indicates the maximum length of a string accepted by bc(1).
136 COLL_WEIGHTS_MAX - _SC_COLL_WEIGHTS_MAX
138 indicates the maximum numbers of weights that can be assigned to
139 an entry of the LC_COLLATE order keyword in the locale defini‐
140 tion file.
141 EXPR_NEST_MAX - _SC_EXPR_NEST_MAX
143 is the maximum number of expressions which can be nested within
144 parentheses by expr(1).
145 LINE_MAX - _SC_LINE_MAX
147 The maximum length of a utility's input line, either from stan‐
148 dard input or from a file. This includes space for a trailing
149 newline.
150 RE_DUP_MAX - _SC_RE_DUP_MAX
152 The maximum number of repeated occurrences of a regular expres‐
153 sion when the interval notation \{m,n\} is used.
154 POSIX2_VERSION - _SC_2_VERSION
156 indicates the version of the POSIX.2 standard in the format of
157 YYYYMML.
158 POSIX2_C_DEV - _SC_2_C_DEV
160 indicates whether the POSIX.2 C language development facilities
161 are supported.
162 POSIX2_FORT_DEV - _SC_2_FORT_DEV
164 indicates whether the POSIX.2 FORTRAN development utilities are
165 supported.
166 POSIX2_FORT_RUN - _SC_2_FORT_RUN
168 indicates whether the POSIX.2 FORTRAN run-time utilities are
169 supported.
170 _POSIX2_LOCALEDEF - _SC_2_LOCALEDEF
172 indicates whether the POSIX.2 creation of locales via lo‐
173 caledef(1) is supported.
174 POSIX2_SW_DEV - _SC_2_SW_DEV
176 indicates whether the POSIX.2 software development utilities op‐
177 tion is supported.
178 These values also exist, but may not be standard.
180 - _SC_PHYS_PAGES
182 The number of pages of physical memory. Note that it is possi‐
183 ble for the product of this value and the value of _SC_PAGESIZE
184 to overflow.
185 - _SC_AVPHYS_PAGES
187 The number of currently available pages of physical memory.
188 - _SC_NPROCESSORS_CONF
190 The number of processors configured. See also
191 get_nprocs_conf(3).
192 - _SC_NPROCESSORS_ONLN
194 The number of processors currently online (available). See also
195 get_nprocs_conf(3).
196
198 The return value of sysconf() is one of the following:
199 * On error, -1 is returned and errno is set to indicate the error (for
201 example, EINVAL, indicating that name is invalid).
202 * If name corresponds to a maximum or minimum limit, and that limit is
204 indeterminate, -1 is returned and errno is not changed. (To distin‐
205 guish an indeterminate limit from an error, set errno to zero before
206 the call, and then check whether errno is nonzero when -1 is re‐
207 turned.)
208 * If name corresponds to an option, a positive value is returned if
210 the option is supported, and -1 is returned if the option is not
211 supported.
212 * Otherwise, the current value of the option or limit is returned.
214 This value will not be more restrictive than the corresponding value
215 that was described to the application in <unistd.h> or <limits.h>
216 when the application was compiled.
217
219 EINVAL name is invalid.
220
222 For an explanation of the terms used in this section, see at‐
223 tributes(7).
224 ┌────────────────────────────────────────┬───────────────┬─────────────┐
226 │Interface │ Attribute │ Value │
227 ├────────────────────────────────────────┼───────────────┼─────────────┤
228 │sysconf() │ Thread safety │ MT-Safe env │
229 └────────────────────────────────────────┴───────────────┴─────────────┘
230
232 POSIX.1-2001, POSIX.1-2008.
233
235 It is difficult to use ARG_MAX because it is not specified how much of
236 the argument space for exec(3) is consumed by the user's environment
237 variables.
238 Some returned values may be huge; they are not suitable for allocating
240 memory.
241
243 bc(1), expr(1), getconf(1), locale(1), confstr(3), fpathconf(3), path‐
244 conf(3), posixoptions(7)
245
247 This page is part of release 5.13 of the Linux man-pages project. A
248 description of the project, information about reporting bugs, and the
249 latest version of this page, can be found at
250 https://www.kernel.org/doc/man-pages/.
251GNU 2021-03-22 SYSCONF(3)