sysconf.3p - Online Linux Manual Page

SYSCONF - Online Linux Manual PageSection : 3P
Updated : 2017
Source : IEEE/The Open Group
Note : POSIX Programmer's Manual

PROLOGThis manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

NAMEsysconf — get configurable system variables

SYNOPSIS#include <unistd.h> long sysconf(int name);

DESCRIPTIONThe sysconf() function provides a method for the application to determine the current value of a configurable system limit or option ( variable). The implementation shall support all of the variables listed in the following table and may support others. The name argument represents the system variable to be queried. The following table lists the minimal set of system variables from <limits.h> or <unistd.h> that can be returned by sysconf(), and the symbolic constants defined in <unistd.h> that are the corresponding values used for name. VariableValue of Name {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_SC_GETGR_R_SIZE_MAX getgrnam_r ( ) data buffers Initial size of getpwuid_r ( ) and_SC_GETPW_R_SIZE_MAX getpwnam_r ( ) data buffers {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 VariableValue of Name _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 VALUEIf 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. Note that indefinite limits do not imply infinite limits; see <limits.h>. 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.

ERRORSThe sysconf() function shall fail if: EINVAL The value of the name argument is invalid. The following sections are informative.

EXAMPLESNone.

APPLICATION USAGEAs −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.

RATIONALEThis 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 DIRECTIONSNone.

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

COPYRIGHTPortions 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 . 0

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