1SYSCONF(3P) POSIX Programmer's Manual SYSCONF(3P)
2
6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
10
13 sysconf — get configurable system variables
14
16 #include <unistd.h>
17 long sysconf(int name);
19
21 The sysconf() function provides a method for the application to deter‐
22 mine the current value of a configurable system limit or option (vari‐
23 able). The implementation shall support all of the variables listed in
24 the following table and may support others.
25 The name argument represents the system variable to be queried. The
27 following table lists the minimal set of system variables from <lim‐
28 its.h> or <unistd.h> that can be returned by sysconf(), and the sym‐
29 bolic constants defined in <unistd.h> that are the corresponding values
30 used for name.
31 ┌─────────────────────────────────┬──────────────────────────────────┐
33 │ Variable │ Value of Name │
34 ├─────────────────────────────────┼──────────────────────────────────┤
35 │{AIO_LISTIO_MAX} │_SC_AIO_LISTIO_MAX │
36 │{AIO_MAX} │_SC_AIO_MAX │
37 │{AIO_PRIO_DELTA_MAX} │_SC_AIO_PRIO_DELTA_MAX │
38 │{ARG_MAX} │_SC_ARG_MAX │
39 │{ATEXIT_MAX} │_SC_ATEXIT_MAX │
40 │{BC_BASE_MAX} │_SC_BC_BASE_MAX │
41 │{BC_DIM_MAX} │_SC_BC_DIM_MAX │
42 │{BC_SCALE_MAX} │_SC_BC_SCALE_MAX │
43 │{BC_STRING_MAX} │_SC_BC_STRING_MAX │
44 │{CHILD_MAX} │_SC_CHILD_MAX │
45 │Clock ticks/second │_SC_CLK_TCK │
46 │{COLL_WEIGHTS_MAX} │_SC_COLL_WEIGHTS_MAX │
47 │{DELAYTIMER_MAX} │_SC_DELAYTIMER_MAX │
48 │{EXPR_NEST_MAX} │_SC_EXPR_NEST_MAX │
49 │{HOST_NAME_MAX} │_SC_HOST_NAME_MAX │
50 │{IOV_MAX} │_SC_IOV_MAX │
51 │{LINE_MAX} │_SC_LINE_MAX │
52 │{LOGIN_NAME_MAX} │_SC_LOGIN_NAME_MAX │
53 │{NGROUPS_MAX} │_SC_NGROUPS_MAX │
54 │Initial size of getgrgid_r() and │_SC_GETGR_R_SIZE_MAX │
55 │getgrnam_r() data buffers │ │
56 │Initial size of getpwuid_r() and │_SC_GETPW_R_SIZE_MAX │
57 │getpwnam_r() data buffers │ │
58 │{MQ_OPEN_MAX} │_SC_MQ_OPEN_MAX │
59 │{MQ_PRIO_MAX} │_SC_MQ_PRIO_MAX │
60 │{OPEN_MAX} │_SC_OPEN_MAX │
61 │_POSIX_ADVISORY_INFO │_SC_ADVISORY_INFO │
62 │_POSIX_BARRIERS │_SC_BARRIERS │
63 │_POSIX_ASYNCHRONOUS_IO │_SC_ASYNCHRONOUS_IO │
64 │_POSIX_CLOCK_SELECTION │_SC_CLOCK_SELECTION │
65 │_POSIX_CPUTIME │_SC_CPUTIME │
66 │_POSIX_FSYNC │_SC_FSYNC │
67 │_POSIX_IPV6 │_SC_IPV6 │
68 │_POSIX_JOB_CONTROL │_SC_JOB_CONTROL │
69 │_POSIX_MAPPED_FILES │_SC_MAPPED_FILES │
70 │_POSIX_MEMLOCK │_SC_MEMLOCK │
71 │_POSIX_MEMLOCK_RANGE │_SC_MEMLOCK_RANGE │
72 │_POSIX_MEMORY_PROTECTION │_SC_MEMORY_PROTECTION │
73 │_POSIX_MESSAGE_PASSING │_SC_MESSAGE_PASSING │
74 │_POSIX_MONOTONIC_CLOCK │_SC_MONOTONIC_CLOCK │
75 │_POSIX_PRIORITIZED_IO │_SC_PRIORITIZED_IO │
76 │_POSIX_PRIORITY_SCHEDULING │_SC_PRIORITY_SCHEDULING │
77 │_POSIX_RAW_SOCKETS │_SC_RAW_SOCKETS │
78 │_POSIX_READER_WRITER_LOCKS │_SC_READER_WRITER_LOCKS │
79 │_POSIX_REALTIME_SIGNALS │_SC_REALTIME_SIGNALS │
80 │_POSIX_REGEXP │_SC_REGEXP │
81 │_POSIX_SAVED_IDS │_SC_SAVED_IDS │
82 │_POSIX_SEMAPHORES │_SC_SEMAPHORES │
83 │_POSIX_SHARED_MEMORY_OBJECTS │_SC_SHARED_MEMORY_OBJECTS │
84 │_POSIX_SHELL │_SC_SHELL │
85 │_POSIX_SPAWN │_SC_SPAWN │
86 │_POSIX_SPIN_LOCKS │_SC_SPIN_LOCKS │
87 │_POSIX_SPORADIC_SERVER │_SC_SPORADIC_SERVER │
88 │_POSIX_SS_REPL_MAX │_SC_SS_REPL_MAX │
89 │_POSIX_SYNCHRONIZED_IO │_SC_SYNCHRONIZED_IO │
90 │_POSIX_THREAD_ATTR_STACKADDR │_SC_THREAD_ATTR_STACKADDR │
91 │_POSIX_THREAD_ATTR_STACKSIZE │_SC_THREAD_ATTR_STACKSIZE │
92 │_POSIX_THREAD_CPUTIME │_SC_THREAD_CPUTIME │
93 │_POSIX_THREAD_PRIO_INHERIT │_SC_THREAD_PRIO_INHERIT │
94 │_POSIX_THREAD_PRIO_PROTECT │_SC_THREAD_PRIO_PROTECT │
95 │_POSIX_THREAD_PRIORITY_SCHEDULING│_SC_THREAD_PRIORITY_SCHEDULING │
96 │_POSIX_THREAD_PROCESS_SHARED │_SC_THREAD_PROCESS_SHARED │
97 │_POSIX_THREAD_ROBUST_PRIO_INHERIT│_SC_THREAD_ROBUST_PRIO_INHERIT │
98 │_POSIX_THREAD_ROBUST_PRIO_PROTECT│_SC_THREAD_ROBUST_PRIO_PROTECT │
99 │_POSIX_THREAD_SAFE_FUNCTIONS │_SC_THREAD_SAFE_FUNCTIONS │
100 │_POSIX_THREAD_SPORADIC_SERVER │_SC_THREAD_SPORADIC_SERVER │
101 │_POSIX_THREADS │_SC_THREADS │
102 │_POSIX_TIMEOUTS │_SC_TIMEOUTS │
103 │_POSIX_TIMERS │_SC_TIMERS │
104 │_POSIX_TRACE │_SC_TRACE │
105 │_POSIX_TRACE_EVENT_FILTER │_SC_TRACE_EVENT_FILTER │
106 │_POSIX_TRACE_EVENT_NAME_MAX │_SC_TRACE_EVENT_NAME_MAX │
107 │_POSIX_TRACE_INHERIT │_SC_TRACE_INHERIT │
108 │_POSIX_TRACE_LOG │_SC_TRACE_LOG │
109 │_POSIX_TRACE_NAME_MAX │_SC_TRACE_NAME_MAX │
110 │_POSIX_TRACE_SYS_MAX │_SC_TRACE_SYS_MAX │
111 │_POSIX_TRACE_USER_EVENT_MAX │_SC_TRACE_USER_EVENT_MAX │
112 │_POSIX_TYPED_MEMORY_OBJECTS │_SC_TYPED_MEMORY_OBJECTS │
113 │_POSIX_VERSION │_SC_VERSION │
114 │_POSIX_V7_ILP32_OFF32 │_SC_V7_ILP32_OFF32 │
115 │_POSIX_V7_ILP32_OFFBIG │_SC_V7_ILP32_OFFBIG │
116 │_POSIX_V7_LP64_OFF64 │_SC_V7_LP64_OFF64 │
117 │_POSIX_V7_LPBIG_OFFBIG │_SC_V7_LPBIG_OFFBIG │
118 └─────────────────────────────────┴──────────────────────────────────┘
119 ┌────────────────────────────────┬─────────────────────────────────┐
120 │ Variable │ Value of Name │
121 ├────────────────────────────────┼─────────────────────────────────┤
122 │_POSIX_V6_ILP32_OFF32 │_SC_V6_ILP32_OFF32 │
123 │_POSIX_V6_ILP32_OFFBIG │_SC_V6_ILP32_OFFBIG │
124 │_POSIX_V6_LP64_OFF64 │_SC_V6_LP64_OFF64 │
125 │_POSIX_V6_LPBIG_OFFBIG │_SC_V6_LPBIG_OFFBIG │
126 │_POSIX2_C_BIND │_SC_2_C_BIND │
127 │_POSIX2_C_DEV │_SC_2_C_DEV │
128 │_POSIX2_CHAR_TERM │_SC_2_CHAR_TERM │
129 │_POSIX2_FORT_DEV │_SC_2_FORT_DEV │
130 │_POSIX2_FORT_RUN │_SC_2_FORT_RUN │
131 │_POSIX2_LOCALEDEF │_SC_2_LOCALEDEF │
132 │_POSIX2_PBS │_SC_2_PBS │
133 │_POSIX2_PBS_ACCOUNTING │_SC_2_PBS_ACCOUNTING │
134 │_POSIX2_PBS_CHECKPOINT │_SC_2_PBS_CHECKPOINT │
135 │_POSIX2_PBS_LOCATE │_SC_2_PBS_LOCATE │
136 │_POSIX2_PBS_MESSAGE │_SC_2_PBS_MESSAGE │
137 │_POSIX2_PBS_TRACK │_SC_2_PBS_TRACK │
138 │_POSIX2_SW_DEV │_SC_2_SW_DEV │
139 │_POSIX2_UPE │_SC_2_UPE │
140 │_POSIX2_VERSION │_SC_2_VERSION │
141 │{PAGE_SIZE} │_SC_PAGE_SIZE │
142 │{PAGESIZE} │_SC_PAGESIZE │
143 │{PTHREAD_DESTRUCTOR_ITERATIONS} │_SC_THREAD_DESTRUCTOR_ITERATIONS │
144 │{PTHREAD_KEYS_MAX} │_SC_THREAD_KEYS_MAX │
145 │{PTHREAD_STACK_MIN} │_SC_THREAD_STACK_MIN │
146 │{PTHREAD_THREADS_MAX} │_SC_THREAD_THREADS_MAX │
147 │{RE_DUP_MAX} │_SC_RE_DUP_MAX │
148 │{RTSIG_MAX} │_SC_RTSIG_MAX │
149 │{SEM_NSEMS_MAX} │_SC_SEM_NSEMS_MAX │
150 │{SEM_VALUE_MAX} │_SC_SEM_VALUE_MAX │
151 │{SIGQUEUE_MAX} │_SC_SIGQUEUE_MAX │
152 │{STREAM_MAX} │_SC_STREAM_MAX │
153 │{SYMLOOP_MAX} │_SC_SYMLOOP_MAX │
154 │{TIMER_MAX} │_SC_TIMER_MAX │
155 │{TTY_NAME_MAX} │_SC_TTY_NAME_MAX │
156 │{TZNAME_MAX} │_SC_TZNAME_MAX │
157 │_XOPEN_CRYPT │_SC_XOPEN_CRYPT │
158 │_XOPEN_ENH_I18N │_SC_XOPEN_ENH_I18N │
159 │_XOPEN_REALTIME │_SC_XOPEN_REALTIME │
160 │_XOPEN_REALTIME_THREADS │_SC_XOPEN_REALTIME_THREADS │
161 │_XOPEN_SHM │_SC_XOPEN_SHM │
162 │_XOPEN_STREAMS │_SC_XOPEN_STREAMS │
163 │_XOPEN_UNIX │_SC_XOPEN_UNIX │
164 │_XOPEN_UUCP │_SC_XOPEN_UUCP │
165 │_XOPEN_VERSION │_SC_XOPEN_VERSION │
166 └────────────────────────────────┴─────────────────────────────────┘
168 If name is an invalid value, sysconf() shall return −1 and set errno to
169 indicate the error. If the variable corresponding to name is described
170 in <limits.h> as a maximum or minimum value and the variable has no
171 limit, sysconf() shall return −1 without changing the value of errno.
172 Note that indefinite limits do not imply infinite limits; see <lim‐
173 its.h>.
174 Otherwise, sysconf() shall return the current variable value on the
176 system. The value returned shall not be more restrictive than the cor‐
177 responding value described to the application when it was compiled with
178 the implementation's <limits.h> or <unistd.h>. The value shall not
179 change during the lifetime of the calling process, except that
180 sysconf(_SC_OPEN_MAX) may return different values before and after a
181 call to setrlimit() which changes the RLIMIT_NOFILE soft limit.
182 If the variable corresponding to name is dependent on an unsupported
184 option, the results are unspecified.
185
187 The sysconf() function shall fail if:
188 EINVAL The value of the name argument is invalid.
190 The following sections are informative.
192
194 None.
195
197 As −1 is a permissible return value in a successful situation, an
198 application wishing to check for error situations should set errno to
199 0, then call sysconf(), and, if it returns −1, check to see if errno is
200 non-zero.
201 Application developers should check whether an option, such as
203 _POSIX_TRACE, is supported prior to obtaining and using values for
204 related variables, such as _POSIX_TRACE_NAME_MAX.
205
207 This functionality was added in response to requirements of application
208 developers and of system vendors who deal with many international sys‐
209 tem configurations. It is closely related to pathconf() and fpath‐
210 conf().
211 Although a conforming application can run on all systems by never
213 demanding more resources than the minimum values published in this vol‐
214 ume of POSIX.1‐2008, it is useful for that application to be able to
215 use the actual value for the quantity of a resource available on any
216 given system. To do this, the application makes use of the value of a
217 symbolic constant in <limits.h> or <unistd.h>.
218 However, once compiled, the application must still be able to cope if
220 the amount of resource available is increased. To that end, an applica‐
221 tion may need a means of determining the quantity of a resource, or the
222 presence of an option, at execution time.
223 Two examples are offered:
225 1. Applications may wish to act differently on systems with or without
227 job control. Applications vendors who wish to distribute only a
228 single binary package to all instances of a computer architecture
229 would be forced to assume job control is never available if it were
230 to rely solely on the <unistd.h> value published in this volume of
231 POSIX.1‐2008.
232 2. International applications vendors occasionally require knowledge
234 of the number of clock ticks per second. Without these facilities,
235 they would be required to either distribute their applications par‐
236 tially in source form or to have 50 Hz and 60 Hz versions for the
237 various countries in which they operate.
238 It is the knowledge that many applications are actually distributed
240 widely in executable form that leads to this facility. If limited to
241 the most restrictive values in the headers, such applications would
242 have to be prepared to accept the most limited environments offered by
243 the smallest microcomputers. Although this is entirely portable, there
244 was a consensus that they should be able to take advantage of the
245 facilities offered by large systems, without the restrictions associ‐
246 ated with source and object distributions.
247 During the discussions of this feature, it was pointed out that it is
249 almost always possible for an application to discern what a value might
250 be at runtime by suitably testing the various functions themselves.
251 And, in any event, it could always be written to adequately deal with
252 error returns from the various functions. In the end, it was felt that
253 this imposed an unreasonable level of complication and sophistication
254 on the application developer.
255 This runtime facility is not meant to provide ever-changing values that
257 applications have to check multiple times. The values are seen as
258 changing no more frequently than once per system initialization, such
259 as by a system administrator or operator with an automatic configura‐
260 tion program. This volume of POSIX.1‐2008 specifies that they shall not
261 change within the lifetime of the process.
262 Some values apply to the system overall and others vary at the file
264 system or directory level. The latter are described in fpathconf().
265 Note that all values returned must be expressible as integers. String
267 values were considered, but the additional flexibility of this approach
268 was rejected due to its added complexity of implementation and use.
269 Some values, such as {PATH_MAX}, are sometimes so large that they must
271 not be used to, say, allocate arrays. The sysconf() function returns a
272 negative value to show that this symbolic constant is not even defined
273 in this case.
274 Similar to pathconf(), this permits the implementation not to have a
276 limit. When one resource is infinite, returning an error indicating
277 that some other resource limit has been reached is conforming behavior.
278
280 None.
281
283 confstr(), fpathconf()
284 The Base Definitions volume of POSIX.1‐2008, <limits.h>, <unistd.h>
286 The Shell and Utilities volume of POSIX.1‐2008, getconf
288
290 Portions of this text are reprinted and reproduced in electronic form
291 from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
292 -- Portable Operating System Interface (POSIX), The Open Group Base
293 Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
294 cal and Electronics Engineers, Inc and The Open Group. (This is
295 POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
296 event of any discrepancy between this version and the original IEEE and
297 The Open Group Standard, the original IEEE and The Open Group Standard
298 is the referee document. The original Standard can be obtained online
299 at http://www.unix.org/online.html .
300 Any typographical or formatting errors that appear in this page are
302 most likely to have been introduced during the conversion of the source
303 files to man page format. To report such errors, see https://www.ker‐
304 nel.org/doc/man-pages/reporting_bugs.html .
305IEEE/The Open Group 2013 SYSCONF(3P)