SYSCONF(3P)

📦 version 2017a, in alpine 3.22, alpine 3.23, alpine 3.24, arch rolling, nixpkgs unstable, opensuse-leap 16.0

SYSCONF(3P) 💖 POSIX Programmer's Manual 🎀

okay bestie, buckle up, because this one's a whole POSIX Programmer's Manual page and it's lowkey long, but we're gonna get through it together, no cap ✨

🌸 PROLOG 🌸

This manual page is part of the POSIX Programmer's Manual. Heads up though, because the vibes get ✨complicated✨:

  • The Linux implementation of this interface may differ — so if you're on Linux, go consult the corresponding Linux manual page for the actual deets on Linux behavior, fr fr.
  • Or the interface may not be implemented on Linux at all. Not every queen gets invited to every party, and that's just how it is 💅
Not me warning you about platform differences in the prolog. We're literally two paragraphs in. This is giving "read the fine print" energy and honestly? Respect it.

✨ NAME ✨

sysconf — get configurable system variables 🦋

💅 SYNOPSIS 💅

#include <unistd.h>

long sysconf(int name);

short, sweet, iconic, main character energy 👑

🧚 DESCRIPTION 🧚

So here's the tea, bestie ☕

The sysconf() function is the way your application goes and asks the system what's up. Specifically, it provides a method for the application to determine the current value of a configurable system limit or option (a variable, basically). Like, "hey system, what's your number for this thing?" and the system tells you 🫶

And here's the important part, so listen up:

  • The implementation shall support all of the variables listed in the following table
  • It may support others — so if you see extra variables out there, that's allowed, she's just being generous 💜
the way that a function can just. ask. and the system answers. obsessed.

aaaand that's the setup, bestie — the actual table of variables is coming in the next part, so don't go anywhere 💖

okay so bestie, the name argument is literally the system variable you're trying to query. the following list is the minimal set of system variables from <limits.h> or <unistd.h> that can be returned by sysconf(), plus the symbolic constants defined in <unistd.h> that are the corresponding values used for name. periodt 💖

here's the whole lineup, variable on the left, the constant you pass as name on the right:

  • {AIO_LISTIO_MAX} — _SC_AIO_LISTIO_MAX
  • {AIO_MAX} — _SC_AIO_MAX
  • {AIO_PRIO_DELTA_MAX} — _SC_AIO_PRIO_DELTA_MAX
  • {ARG_MAX} — _SC_ARG_MAX
  • {ATEXIT_MAX} — _SC_ATEXIT_MAX
  • {BC_BASE_MAX} — _SC_BC_BASE_MAX
  • {BC_DIM_MAX} — _SC_BC_DIM_MAX
  • {BC_SCALE_MAX} — _SC_BC_SCALE_MAX
  • {BC_STRING_MAX} — _SC_BC_STRING_MAX
  • {CHILD_MAX} — _SC_CHILD_MAX
  • Clock ticks/second — _SC_CLK_TCK
  • {COLL_WEIGHTS_MAX} — _SC_COLL_WEIGHTS_MAX
  • {DELAYTIMER_MAX} — _SC_DELAYTIMER_MAX
  • {EXPR_NEST_MAX} — _SC_EXPR_NEST_MAX
  • {HOST_NAME_MAX} — _SC_HOST_NAME_MAX
  • {IOV_MAX} — _SC_IOV_MAX
  • {LINE_MAX} — _SC_LINE_MAX
  • {LOGIN_NAME_MAX} — _SC_LOGIN_NAME_MAX
  • {NGROUPS_MAX} — _SC_NGROUPS_MAX
  • Initial size of getgrgid_r() and getgrnam_r() data buffers — _SC_GETGR_R_SIZE_MAX
  • Initial size of getpwuid_r() and getpwnam_r() data buffers — _SC_GETPW_R_SIZE_MAX
  • {MQ_OPEN_MAX} — _SC_MQ_OPEN_MAX
  • {MQ_PRIO_MAX} — _SC_MQ_PRIO_MAX
  • {OPEN_MAX} — _SC_OPEN_MAX
  • {PAGE_SIZE} — _SC_PAGE_SIZE
  • {PAGESIZE} — _SC_PAGESIZE
  • {PTHREAD_DESTRUCTOR_ITERATIONS} — _SC_THREAD_DESTRUCTOR_ITERATIONS
  • {PTHREAD_KEYS_MAX} — _SC_THREAD_KEYS_MAX
  • {PTHREAD_STACK_MIN} — _SC_THREAD_STACK_MIN
  • {PTHREAD_THREADS_MAX} — _SC_THREAD_THREADS_MAX
  • {RE_DUP_MAX} — _SC_RE_DUP_MAX
  • {RTSIG_MAX} — _SC_RTSIG_MAX
  • {SEM_NSEMS_MAX} — _SC_SEM_NSEMS_MAX
  • {SEM_VALUE_MAX} — _SC_SEM_VALUE_MAX
  • {SIGQUEUE_MAX} — _SC_SIGQUEUE_MAX
  • {STREAM_MAX} — _SC_STREAM_MAX
  • {SYMLOOP_MAX} — _SC_SYMLOOP_MAX
  • {TIMER_MAX} — _SC_TIMER_MAX
  • {TTY_NAME_MAX} — _SC_TTY_NAME_MAX
  • {TZNAME_MAX} — _SC_TZNAME_MAX
  • _POSIX_ADVISORY_INFO — _SC_ADVISORY_INFO
  • _POSIX_BARRIERS — _SC_BARRIERS
  • _POSIX_ASYNCHRONOUS_IO — _SC_ASYNCHRONOUS_IO
  • _POSIX_CLOCK_SELECTION — _SC_CLOCK_SELECTION
  • _POSIX_CPUTIME — _SC_CPUTIME
  • _POSIX_FSYNC — _SC_FSYNC
  • _POSIX_IPV6 — _SC_IPV6
  • _POSIX_JOB_CONTROL — _SC_JOB_CONTROL
  • _POSIX_MAPPED_FILES — _SC_MAPPED_FILES
  • _POSIX_MEMLOCK — _SC_MEMLOCK
  • _POSIX_MEMLOCK_RANGE — _SC_MEMLOCK_RANGE
  • _POSIX_MEMORY_PROTECTION — _SC_MEMORY_PROTECTION
  • _POSIX_MESSAGE_PASSING — _SC_MESSAGE_PASSING
  • _POSIX_MONOTONIC_CLOCK — _SC_MONOTONIC_CLOCK
  • _POSIX_PRIORITIZED_IO — _SC_PRIORITIZED_IO
  • _POSIX_PRIORITY_SCHEDULING — _SC_PRIORITY_SCHEDULING
  • _POSIX_RAW_SOCKETS — _SC_RAW_SOCKETS
  • _POSIX_READER_WRITER_LOCKS — _SC_READER_WRITER_LOCKS
  • _POSIX_REALTIME_SIGNALS — _SC_REALTIME_SIGNALS
  • _POSIX_REGEXP — _SC_REGEXP
  • _POSIX_SAVED_IDS — _SC_SAVED_IDS
  • _POSIX_SEMAPHORES — _SC_SEMAPHORES
  • _POSIX_SHARED_MEMORY_OBJECTS — _SC_SHARED_MEMORY_OBJECTS
  • _POSIX_SHELL — _SC_SHELL
  • _POSIX_SPAWN — _SC_SPAWN
  • _POSIX_SPIN_LOCKS — _SC_SPIN_LOCKS
  • _POSIX_SPORADIC_SERVER — _SC_SPORADIC_SERVER
  • _POSIX_SS_REPL_MAX — _SC_SS_REPL_MAX
  • _POSIX_SYNCHRONIZED_IO — _SC_SYNCHRONIZED_IO
  • _POSIX_THREAD_ATTR_STACKADDR — _SC_THREAD_ATTR_STACKADDR
  • _POSIX_THREAD_ATTR_STACKSIZE — _SC_THREAD_ATTR_STACKSIZE
  • _POSIX_THREAD_CPUTIME — _SC_THREAD_CPUTIME
  • _POSIX_THREAD_PRIO_INHERIT — _SC_THREAD_PRIO_INHERIT
  • _POSIX_THREAD_PRIO_PROTECT — _SC_THREAD_PRIO_PROTECT
  • _POSIX_THREAD_PRIORITY_SCHEDULING — _SC_THREAD_PRIORITY_SCHEDULING
  • _POSIX_THREAD_PROCESS_SHARED — _SC_THREAD_PROCESS_SHARED
  • _POSIX_THREAD_ROBUST_PRIO_INHERIT — _SC_THREAD_ROBUST_PRIO_INHERIT
  • _POSIX_THREAD_ROBUST_PRIO_PROTECT — _SC_THREAD_ROBUST_PRIO_PROTECT
  • _POSIX_THREAD_SAFE_FUNCTIONS — _SC_THREAD_SAFE_FUNCTIONS
  • _POSIX_THREAD_SPORADIC_SERVER — _SC_THREAD_SPORADIC_SERVER
  • _POSIX_THREADS — _SC_THREADS
  • _POSIX_TIMEOUTS — _SC_TIMEOUTS
  • _POSIX_TIMERS — _SC_TIMERS
  • _POSIX_TRACE — _SC_TRACE
  • _POSIX_TRACE_EVENT_FILTER — _SC_TRACE_EVENT_FILTER
  • _POSIX_TRACE_EVENT_NAME_MAX — _SC_TRACE_EVENT_NAME_MAX
  • _POSIX_TRACE_INHERIT — _SC_TRACE_INHERIT
  • _POSIX_TRACE_LOG — _SC_TRACE_LOG
  • _POSIX_TRACE_NAME_MAX — _SC_TRACE_NAME_MAX
  • _POSIX_TRACE_SYS_MAX — _SC_TRACE_SYS_MAX
  • _POSIX_TRACE_USER_EVENT_MAX — _SC_TRACE_USER_EVENT_MAX
  • _POSIX_TYPED_MEMORY_OBJECTS — _SC_TYPED_MEMORY_OBJECTS
  • _POSIX_VERSION — _SC_VERSION
  • _POSIX_V7_ILP32_OFF32 — _SC_V7_ILP32_OFF32
  • _POSIX_V7_ILP32_OFFBIG — _SC_V7_ILP32_OFFBIG
  • _POSIX_V7_LP64_OFF64 — _SC_V7_LP64_OFF64
  • _POSIX_V7_LPBIG_OFFBIG — _SC_V7_LPBIG_OFFBIG
  • _POSIX_V6_ILP32_OFF32 — _SC_V6_ILP32_OFF32
  • _POSIX_V6_ILP32_OFFBIG — _SC_V6_ILP32_OFFBIG
  • _POSIX_V6_LP64_OFF64 — _SC_V6_LP64_OFF64
  • _POSIX_V6_LPBIG_OFFBIG — _SC_V6_LPBIG_OFFBIG
  • _POSIX2_C_BIND — _SC_2_C_BIND
  • _POSIX2_C_DEV — _SC_2_C_DEV
  • _POSIX2_CHAR_TERM — _SC_2_CHAR_TERM
  • _POSIX2_FORT_DEV — _SC_2_FORT_DEV
  • _POSIX2_FORT_RUN — _SC_2_FORT_RUN
  • _POSIX2_LOCALEDEF — _SC_2_LOCALEDEF
  • _POSIX2_PBS — _SC_2_PBS
  • _POSIX2_PBS_ACCOUNTING — _SC_2_PBS_ACCOUNTING
  • _POSIX2_PBS_CHECKPOINT — _SC_2_PBS_CHECKPOINT
  • _POSIX2_PBS_LOCATE — _SC_2_PBS_LOCATE
  • _POSIX2_PBS_MESSAGE — _SC_2_PBS_MESSAGE
  • _POSIX2_PBS_TRACK — _SC_2_PBS_TRACK
  • _POSIX2_SW_DEV — _SC_2_SW_DEV
  • _POSIX2_UPE — _SC_2_UPE
  • _POSIX2_VERSION — _SC_2_VERSION
  • _XOPEN_CRYPT — _SC_XOPEN_CRYPT
  • _XOPEN_ENH_I18N — _SC_XOPEN_ENH_I18N
  • _XOPEN_REALTIME — _SC_XOPEN_REALTIME
  • _XOPEN_REALTIME_THREADS — _SC_XOPEN_REALTIME_THREADS
  • _XOPEN_SHM — _SC_XOPEN_SHM
  • _XOPEN_STREAMS — _SC_XOPEN_STREAMS
  • _XOPEN_UNIX — _SC_XOPEN_UNIX
  • _XOPEN_UUCP — _SC_XOPEN_UUCP

_XOPEN_VERSION → _SC_XOPEN_VERSION

✨ RETURN VALUE ✨

Okay so bestie, here's the tea on what sysconf() gives back. 🫖

If name is an invalid value, sysconf() shall return -1 and set errno to indicate the error. 💀

If the variable corresponding to name is described in <limits.h> as a maximum or minimum value and the variable has no limit, sysconf() shall return -1 without changing the value of errno. And just so you know: indefinite limits do not imply infinite limits — see <limits.h> for the deets. 👀

Otherwise, sysconf() shall return the current variable value on the system. The value returned shall not be more restrictive than the corresponding value described to the application when it was compiled with the implementation's <limits.h> or <unistd.h>. The value shall not change during the lifetime of the calling process, except that sysconf(_SC_OPEN_MAX) may return different values before and after a call to setrlimit() which changes the RLIMIT_NOFILE soft limit. 🔄

If the variable corresponding to name is dependent on an unsupported option, the results are unspecified. 🤷‍♀️

💀 ERRORS 💀

The sysconf() function shall fail if:

  • EINVAL — The value of the name argument is invalid. 🚫

The following sections are informative. 📝

🌸 EXAMPLES 🌸

None. 💁‍♀️

🫶 APPLICATION USAGE 🫶

As -1 is a permissible return value in a successful situation, an application wishing to check for error situations should set errno to 0, then call sysconf(), and, if it returns -1, check to see if errno is non-zero. 🕵️‍♀️

Application developers should check whether an option, such as _POSIX_TRACE, is supported prior to obtaining and using values for related variables, such as _POSIX_TRACE_NAME_MAX. ✅

🧠 RATIONALE 🧠

This functionality was added in response to requirements of application developers and of system vendors who deal with many international system configurations. It is closely related to pathconf() and fpathconf(). 🌍

Although a conforming application can run on all systems by never demanding more resources than the minimum values published in this volume of POSIX.1‐2017, it is useful for that application to be able to use the actual value for the quantity of a resource available on any given system. To do this, the application makes use of the value of a symbolic constant in <limits.h> or <unistd.h>. 📊

However, once compiled, the application must still be able to cope if the amount of resource available is increased. To that end, an application may need a means of determining the quantity of a resource, or the presence of an option, at execution time. ⏳

Two examples are offered:

  1. Applications may wish to act differently on systems with or without job control. Applications vendors who wish to distribute only a single binary package to all instances of a computer architecture would be forced to assume job control is never available if it were to rely solely on the <unistd.h> value published in this volume of POSIX.1‐2017. 💼
  2. International applications vendors occasionally require knowledge of the number of clock ticks per second. Without these facilities, they would be required to either distribute their applications partially in source form or to have 50 Hz and 60 Hz versions for the various countries in which they operate. ⏰

It is the knowledge that many applications are actually distributed widely in executable form that leads to this facility. If limited to the most restrictive values in the headers, such applications would have to be prepared to accept the most limited environments offered by the smallest microcomputers. Although this is entirely portable, there was a consensus that they should be able to take advantage of the facilities offered by large systems, without the restrictions associated with source and object distributions. 🖥️

During the discussions of this feature, it was pointed out that it is almost always possible for an application to discern what a value might be at runtime by suitably testing the various functions themselves. And, in any event, it could always be written to adequately deal with error returns from the various functions. In the end, it was felt that this imposed an unreasonable level of complication and sophistication on the application developer. 😤

This runtime facility is not meant to provide ever-changing values that applications have to check multiple times. The values are seen as changing no more frequently than once per system initialization, such as by a system administrator or operator with an automatic configuration program. This volume of POSIX.1‐2017 specifies that they shall not change within the lifetime of the process. 🔒

Some values apply to the system overall and others vary at the file system or directory level. The latter are described in fpathconf(). 📁

Note that all values returned must be expressible as integers. String values were considered, but the additional flexibility of this approach was rejected due to its added complexity of implementation and use. 🔢

Some values, such as {PATH_MAX}, are sometimes so large that they must not be used to, say, allocate arrays. The sysconf() function returns a negative value to show that this symbolic constant is not even defined in this case. 🚫

Similar to pathconf(), this permits the implementation not to have a limit. When one resource is infinite, returning an error indicating that some other resource limit has been reached is conforming behavior. ♾️

🔮 FUTURE DIRECTIONS 🔮

None. 🔮

👀 SEE ALSO 👀

  • confstr(), fpathconf()
  • The Base Definitions volume of POSIX.1‐2017, <limits.h>, <unistd.h>
  • The Shell and Utilities volume of POSIX.1‐2017, getconf

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html . 💖

Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html . 🐛

This iconic page is brought to you by IEEE/The Open Group, 2017, and it's SYSCONF(3P) fr fr. Periodt. 💅✨

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