.\" Copyright (c) 2003 Andries Brouwer (aeb@cwi.nl)

.\"

.\" %%%LICENSE_START(GPLv2+_DOC_FULL)

.\" This is free documentation; you can redistribute it and/or

.\" modify it under the terms of the GNU General Public License as

.\" published by the Free Software Foundation; either version 2 of

.\" the License, or (at your option) any later version.

.\"

.\" The GNU General Public License's references to "object code"

.\" and "executables" are to be interpreted as the output of any

.\" document formatting or typesetting system, including

.\" intermediate and printed output.

.\"

.\" This manual is distributed in the hope that it will be useful,

.\" but WITHOUT ANY WARRANTY; without even the implied warranty of

.\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the

.\" GNU General Public License for more details.

.\"

.\" You should have received a copy of the GNU General Public

.\" License along with this manual; if not, see

.\" <http://www.gnu.org/licenses/>.

.\" %%%LICENSE_END

.\"

.TH POSIXOPTIONS 7 2007-12-21 "" "Linux Programmer's Manual"

.SH NAME

posixoptions \- optional parts of the POSIX standard

.SH DESCRIPTION

The POSIX standard (the information below is from POSIX.1-2001)

describes a set of behaviors and interfaces for a compliant system.

However, many interfaces are optional and there are feature test macros

to test the availability of interfaces at compile time, and functions

.BR sysconf (3),

.BR fpathconf (3),

.BR pathconf (3),

.BR confstr (3)

to do this at run time.

From shell scripts one can use

.BR getconf (1).

For more detail, see

.BR sysconf (3).

.PP

We give the name of the POSIX abbreviation, the option, the name of the

.BR sysconf (3)

parameter used to inquire about the option, and possibly

a very short description.

Much more precise detail can be found in the POSIX standard itself,

versions of which can nowadays be accessed freely on the web.

.SS ADV - _POSIX_ADVISORY_INFO - _SC_ADVISORY_INFO

The following advisory functions are present:

.PP

.nf

.in +4n

.IR posix_fadvise (),

.IR posix_fallocate (),

.IR posix_memalign (),

.IR posix_madvise ().

.in

.fi

.SS AIO - _POSIX_ASYNCHRONOUS_IO - _SC_ASYNCHRONOUS_IO

The header

.I <aio.h>

is present.

The following functions are present:

.PP

.nf

.in +4n

.IR aio_cancel (),

.IR aio_error (),

.IR aio_fsync (),

.IR aio_read (),

.IR aio_return (),

.IR aio_suspend (),

.IR aio_write (),

.IR lio_listio ().

.in

.fi

.SS BAR - _POSIX_BARRIERS - _SC_BARRIERS

This option implies the

.B _POSIX_THREADS

and

.B _POSIX_THREAD_SAFE_FUNCTIONS

options.

The following functions are present:

.PP

.nf

.in +4n

.IR pthread_barrier_destroy (),

.IR pthread_barrier_init (),

.IR pthread_barrier_wait (),

.IR pthread_barrierattr_destroy (),

.IR pthread_barrierattr_init ().

.in

.fi

.\" .SS BE

.\" Batch environment.

.\" .SS CD

.\" C development.

.SS --- - POSIX_CHOWN_RESTRICTED

If this option is in effect (as it always is under POSIX.1-2001),

then only root may change the owner of a file, and nonroot can

set the group of a file only to one of the groups it belongs to.

This affects the functions

.IR chown (),

.IR fchown ().

.\" What about lchown() ?

.SS CS - _POSIX_CLOCK_SELECTION - _SC_CLOCK_SELECTION

This option implies the

.B _POSIX_TIMERS

option.

The following functions are present:

.PP

.nf

.in +4n

.IR pthread_condattr_getclock (),

.IR pthread_condattr_setclock (),

.IR clock_nanosleep ().

.in

.fi

.PP

If

.B CLOCK_REALTIME

is changed by the function

.IR clock_settime (),

then this affects all timers set for an absolute time.

.SS CPT - _POSIX_CPUTIME - _SC_CPUTIME

The clockID CLOCK_PROCESS_CPUTIME_ID is supported.

The initial value of this clock is 0 for each process.

This option implies the

.B _POSIX_TIMERS

option.

The function

.IR clock_getcpuclockid ()

is present.

.\" .SS FD

.\" Fortran development

.\" .SS FR

.\" Fortran runtime

.SS --- - _POSIX_FILE_LOCKING - _SC_FILE_LOCKING

This option has been deleted.

Not in final XPG6.

.SS FSC - _POSIX_FSYNC - _SC_FSYNC

The function

.IR fsync ()

is present.

.SS IP6 - _POSIX_IPV6 - _SC_IPV6

Internet Protocol Version 6 is supported.

.SS --- - _POSIX_JOB_CONTROL - _SC_JOB_CONTROL

If this option is in effect (as it always is under POSIX.1-2001),

then the system implements POSIX-style job control,

and the following functions are present:

.PP

.nf

.in +4n

.IR setpgid (),

.IR tcdrain (),

.IR tcflush (),

.IR tcgetpgrp (),

.IR tcsendbreak (),

.IR tcsetattr (),

.IR tcsetpgrp ().

.in

.fi

.SS MF - _POSIX_MAPPED_FILES - _SC_MAPPED_FILES

Shared memory is supported.

The include file

.I <sys/mman.h>

is present.

The following functions are present:

.IR mmap (),

.IR msync (),

.IR munmap ().

.SS ML - _POSIX_MEMLOCK - _SC_MEMLOCK

Shared memory can be locked into core.

The functions

.IR mlockall (),

.IR munlockall ()

are present.

.SS MR/MLR - _POSIX_MEMLOCK_RANGE - _SC_MEMLOCK_RANGE

More precisely, ranges can be locked into core.

The functions

.IR mlock (),

.IR munlock ()

are present.

.SS MPR - _POSIX_MEMORY_PROTECTION - _SC_MEMORY_PROTECTION

The function

.IR mprotect ()

is present.

.SS MSG - _POSIX_MESSAGE_PASSING - _SC_MESSAGE_PASSING

The include file

.I <mqueue.h>

is present.

The following functions are present:

.PP

.nf

.in +4n

.IR mq_close (),

.IR mq_getattr (),

.IR mq_notify (),

.IR mq_open (),

.IR mq_receive (),

.IR mq_send (),

.IR mq_setattr (),

.IR mq_unlink ().

.in

.fi

.SS MON - _POSIX_MONOTONIC_CLOCK - _SC_MONOTONIC_CLOCK

.B CLOCK_MONOTONIC

is supported.

This option implies the

.B _POSIX_TIMERS

option.

Affected functions are

.PP

.nf

.in +4n

.IR aio_suspend (),

.IR clock_getres (),

.IR clock_gettime (),

.IR clock_settime (),

.IR timer_create ().

.in

.fi

.SS --- - _POSIX_MULTI_PROCESS - _SC_MULTI_PROCESS

This option has been deleted.

Not in final XPG6.

.\" .SS MX

.\" IEC 60559 Floating-Point Option.

.SS --- - _POSIX_NO_TRUNC

If this option is in effect (as it always is under POSIX.1-2001)

then pathname components longer than

.B NAME_MAX

are not truncated,

but give an error.

This property may be dependent on the path prefix of the component.

.SS PIO - _POSIX_PRIORITIZED_IO - _SC_PRIORITIZED_IO

This option says that one can specify priorities for asynchronous I/O.

This affects the functions

.PP

.nf

.in +4n

.IR aio_read (),

.IR aio_write ().

.in

.fi

.SS PS - _POSIX_PRIORITY_SCHEDULING - _SC_PRIORITY_SCHEDULING

The include file

.I <sched.h>

is present.

The following functions are present:

.PP

.nf

.in +4n

.IR sched_get_priority_max (),

.IR sched_get_priority_min (),

.IR sched_getparam (),

.IR sched_getscheduler (),

.IR sched_rr_get_interval (),

.IR sched_setparam (),

.IR sched_setscheduler (),

.IR sched_yield ().

.in

.fi

If also

.B _POSIX_SPAWN

is in effect, then the following functions are present:

.PP

.nf

.in +4n

.IR posix_spawnattr_getschedparam (),

.IR posix_spawnattr_getschedpolicy (),

.IR posix_spawnattr_setschedparam (),

.IR posix_spawnattr_setschedpolicy ().

.in

.fi

.SS RS - _POSIX_RAW_SOCKETS

Raw sockets are supported.

Affected functions are

.IR getsockopt (),

.IR setsockopt ().

.SS --- - _POSIX_READER_WRITER_LOCKS - _SC_READER_WRITER_LOCKS

This option implies the

.B _POSIX_THREADS

option.

Conversely,

under POSIX.1-2001 the

.B _POSIX_THREADS

option implies this option.

.PP

The following functions are present:

.in +4n

.nf

.IR pthread_rwlock_destroy (),

.IR pthread_rwlock_init (),

.IR pthread_rwlock_rdlock (),

.IR pthread_rwlock_tryrdlock (),

.IR pthread_rwlock_trywrlock (),

.IR pthread_rwlock_unlock (),

.IR pthread_rwlock_wrlock (),

.IR pthread_rwlockattr_destroy (),

.IR pthread_rwlockattr_init ().

.in

.fi

.SS RTS - _POSIX_REALTIME_SIGNALS - _SC_REALTIME_SIGNALS

Realtime signals are supported.

The following functions are present:

.PP

.nf

.in +4n

.IR sigqueue (),

.IR sigtimedwait (),

.IR sigwaitinfo ().

.in

.fi

.SS --- - _POSIX_REGEXP - _SC_REGEXP

If this option is in effect (as it always is under POSIX.1-2001)

then POSIX regular expressions are supported

and the following functions are present:

.PP

.nf

.in +4n

.IR regcomp (),

.IR regerror (),

.IR regexec (),

.IR regfree ().

.in

.fi

.SS --- - _POSIX_SAVED_IDS - _SC_SAVED_IDS

If this option is in effect (as it always is under POSIX.1-2001),

then a process has a saved set-user-ID and a saved set-group-ID.

Affected functions are

.PP

.nf

.in +4n

.IR exec (),

.IR kill (),

.IR seteuid (),

.IR setegid (),

.IR setgid (),

.IR setuid ().

.in

.fi

.\" .SS SD

.\" Software development

.SS SEM - _POSIX_SEMAPHORES - _SC_SEMAPHORES

The include file

.I <semaphore.h>

is present.

The following functions are present:

.PP

.nf

.in +4n

.IR sem_close (),

.IR sem_destroy (),

.IR sem_getvalue (),

.IR sem_init (),

.IR sem_open (),

.IR sem_post (),

.IR sem_trywait (),

.IR sem_unlink (),

.IR sem_wait ().

.in

.fi

.SS SHM - _POSIX_SHARED_MEMORY_OBJECTS - _SC_SHARED_MEMORY_OBJECTS

The following functions are present:

.PP

.nf

.in +4n

.IR mmap (),

.IR munmap (),

.IR shm_open (),

.IR shm_unlink ().

.in

.fi

.SS --- - _POSIX_SHELL - _SC_SHELL

If this option is in effect (as it always is under POSIX.1-2001),

the function

.IR system ()

is present.

.SS SPN - _POSIX_SPAWN - _SC_SPAWN

This option describes support for process creation in a context where

it is difficult or impossible to use

.IR fork (),

for example, because no MMU is present.

If

.B _POSIX_SPAWN

is in effect, then the include file

.I <spawn.h>

and the following functions are present:

.PP

.nf

.in +4n

.IR posix_spawn (),

.IR posix_spawn_file_actions_addclose (),

.IR posix_spawn_file_actions_adddup2 (),

.IR posix_spawn_file_actions_addopen (),

.IR posix_spawn_file_actions_destroy (),

.IR posix_spawn_file_actions_init (),

.IR posix_spawnattr_destroy (),

.IR posix_spawnattr_getsigdefault (),

.IR posix_spawnattr_getflags (),

.IR posix_spawnattr_getpgroup (),

.IR posix_spawnattr_getsigmask (),

.IR posix_spawnattr_init (),

.IR posix_spawnattr_setsigdefault (),

.IR posix_spawnattr_setflags (),

.IR posix_spawnattr_setpgroup (),

.IR posix_spawnattr_setsigmask (),

.IR posix_spawnp ().

.in

.fi

If also

.B _POSIX_PRIORITY_SCHEDULING

is in effect, then

the following functions are present:

.PP

.nf

.in +4n

.IR posix_spawnattr_getschedparam (),

.IR posix_spawnattr_getschedpolicy (),

.IR posix_spawnattr_setschedparam (),

.IR posix_spawnattr_setschedpolicy ().

.in

.fi

.SS SPI - _POSIX_SPIN_LOCKS - _SC_SPIN_LOCKS

This option implies the

.B _POSIX_THREADS

and

.B _POSIX_THREAD_SAFE_FUNCTIONS

options.

The following functions are present:

.PP

.nf

.in +4n

.IR pthread_spin_destroy (),

.IR pthread_spin_init (),

.IR pthread_spin_lock (),

.IR pthread_spin_trylock (),

.IR pthread_spin_unlock ().

.in -4n

.fi

.SS SS - _POSIX_SPORADIC_SERVER - _SC_SPORADIC_SERVER

The scheduling policy

.B SCHED_SPORADIC

is supported.

This option implies the

.B _POSIX_PRIORITY_SCHEDULING

option.

Affected functions are

.PP

.nf

.in +4n

.IR sched_setparam (),

.IR sched_setscheduler ().

.in

.fi

.SS SIO - _POSIX_SYNCHRONIZED_IO - _SC_SYNCHRONIZED_IO

Affected functions are

.IR open (),

.IR msync (),

.IR fsync (),

.IR fdatasync ().

.SS TSA - _POSIX_THREAD_ATTR_STACKADDR - _SC_THREAD_ATTR_STACKADDR

Affected functions are

.PP

.nf

.in +4n

.IR pthread_attr_getstack (),

.IR pthread_attr_getstackaddr (),

.IR pthread_attr_setstack (),

.IR pthread_attr_setstackaddr ().

.in

.fi

.SS TSS - _POSIX_THREAD_ATTR_STACKSIZE - _SC_THREAD_ATTR_STACKSIZE

Affected functions are

.PP

.nf

.in +4n

.IR pthread_attr_getstack (),

.IR pthread_attr_getstacksize (),

.IR pthread_attr_setstack (),

.IR pthread_attr_setstacksize ().

.in

.fi

.SS TCT - _POSIX_THREAD_CPUTIME - _SC_THREAD_CPUTIME

The clockID CLOCK_THREAD_CPUTIME_ID is supported.

This option implies the

.B _POSIX_TIMERS

option.

Affected functions are

.PP

.nf

.in +4n

.IR pthread_getcpuclockid (),

.IR clock_getres (),

.IR clock_gettime (),

.IR clock_settime (),

.IR timer_create ().

.in

.fi

.SS TPI - _POSIX_THREAD_PRIO_INHERIT - _SC_THREAD_PRIO_INHERIT

Affected functions are

.PP

.nf

.in +4n

.IR pthread_mutexattr_getprotocol (),

.IR pthread_mutexattr_setprotocol ().

.in

.fi

.SS TPP - _POSIX_THREAD_PRIO_PROTECT - _SC_THREAD_PRIO_PROTECT

Affected functions are

.PP

.nf

.in +4n

.IR pthread_mutex_getprioceiling (),

.IR pthread_mutex_setprioceiling (),

.IR pthread_mutexattr_getprioceiling (),

.IR pthread_mutexattr_getprotocol (),

.IR pthread_mutexattr_setprioceiling (),

.IR pthread_mutexattr_setprotocol ().

.in

.fi

.SS TPS - _POSIX_THREAD_PRIORITY_SCHEDULING - _SC_THREAD_PRIORITY_SCHEDULING

If this option is in effect, the different threads inside a process

can run with different priorities and/or different schedulers.

Affected functions are

.PP

.nf

.in +4n

.IR pthread_attr_getinheritsched (),

.IR pthread_attr_getschedpolicy (),

.IR pthread_attr_getscope (),

.IR pthread_attr_setinheritsched (),

.IR pthread_attr_setschedpolicy (),

.IR pthread_attr_setscope (),

.IR pthread_getschedparam (),

.IR pthread_setschedparam (),

.IR pthread_setschedprio ().

.in

.fi

.SS TSH - _POSIX_THREAD_PROCESS_SHARED - _SC_THREAD_PROCESS_SHARED

Affected functions are

.PP

.nf

.in +4n

.IR pthread_barrierattr_getpshared (),

.IR pthread_barrierattr_setpshared (),

.IR pthread_condattr_getpshared (),

.IR pthread_condattr_setpshared (),

.IR pthread_mutexattr_getpshared (),

.IR pthread_mutexattr_setpshared (),

.IR pthread_rwlockattr_getpshared (),

.IR pthread_rwlockattr_setpshared ().

.in

.fi

.SS TSF - _POSIX_THREAD_SAFE_FUNCTIONS - _SC_THREAD_SAFE_FUNCTIONS

Affected functions are

.PP

.nf

.in +4n

.IR readdir_r (),

.IR getgrgid_r (),

.IR getgrnam_r (),

.IR getpwnam_r (),

.IR getpwuid_r (),

.IR flockfile (),

.IR ftrylockfile (),

.IR funlockfile (),

.IR getc_unlocked (),

.IR getchar_unlocked (),

.IR putc_unlocked (),

.IR putchar_unlocked (),

.IR rand_r (),

.IR strerror_r (),

.IR strtok_r (),

.IR asctime_r (),

.IR ctime_r (),

.IR gmtime_r (),

.IR localtime_r ().

.in

.fi

.SS TSP - _POSIX_THREAD_SPORADIC_SERVER - _SC_THREAD_SPORADIC_SERVER

This option implies the

.B _POSIX_THREAD_PRIORITY_SCHEDULING

option.

Affected functions are

.PP

.nf

.in +4n

.IR sched_getparam (),

.IR sched_setparam (),

.IR sched_setscheduler ().

.in

.fi

.SS THR - _POSIX_THREADS - _SC_THREADS

Basic support for POSIX threads is available.

The following functions are present:

.PP

.nf

.in +4n

.IR pthread_atfork (),

.IR pthread_attr_destroy (),

.IR pthread_attr_getdetachstate (),

.IR pthread_attr_getschedparam (),

.IR pthread_attr_init (),

.IR pthread_attr_setdetachstate (),

.IR pthread_attr_setschedparam (),

.IR pthread_cancel (),

.IR pthread_cleanup_push (),

.IR pthread_cleanup_pop (),

.IR pthread_cond_broadcast (),

.IR pthread_cond_destroy (),

.IR pthread_cond_init (),

.IR pthread_cond_signal (),

.IR pthread_cond_timedwait (),

.IR pthread_cond_wait (),

.IR pthread_condattr_destroy (),

.IR pthread_condattr_init (),

.IR pthread_create (),

.IR pthread_detach (),

.IR pthread_equal (),

.IR pthread_exit (),

.IR pthread_getspecific (),

.IR pthread_join (),

.IR pthread_key_create (),

.IR pthread_key_delete (),

.IR pthread_mutex_destroy (),

.IR pthread_mutex_init (),

.IR pthread_mutex_lock (),

.IR pthread_mutex_trylock (),

.IR pthread_mutex_unlock (),

.IR pthread_mutexattr_destroy (),

.IR pthread_mutexattr_init (),

.IR pthread_once (),

.IR pthread_rwlock_destroy (),

.IR pthread_rwlock_init (),

.IR pthread_rwlock_rdlock (),

.IR pthread_rwlock_tryrdlock (),

.IR pthread_rwlock_trywrlock (),

.IR pthread_rwlock_unlock (),

.IR pthread_rwlock_wrlock (),

.IR pthread_rwlockattr_destroy (),

.IR pthread_rwlockattr_init (),

.IR pthread_self (),

.IR pthread_setcancelstate (),

.IR pthread_setcanceltype (),

.IR pthread_setspecific (),

.IR pthread_testcancel ().

.in

.fi

.SS TMO - _POSIX_TIMEOUTS - _SC_TIMEOUTS

The following functions are present:

.PP

.nf

.in +4n

.IR mq_timedreceive (),

.IR mq_timedsend (),

.IR pthread_mutex_timedlock (),

.IR pthread_rwlock_timedrdlock (),

.IR pthread_rwlock_timedwrlock (),

.IR sem_timedwait (),

.IR posix_trace_timedgetnext_event ().

.in

.fi

.SS TMR - _POSIX_TIMERS - _SC_TIMERS

The following functions are present:

.PP

.nf

.in +4n

.IR clock_getres (),

.IR clock_gettime (),

.IR clock_settime (),

.IR nanosleep (),

.IR timer_create (),

.IR timer_delete (),

.IR timer_gettime (),

.IR timer_getoverrun (),

.IR timer_settime ().

.in

.fi

.SS TRC - _POSIX_TRACE - _SC_TRACE

POSIX tracing is available.

The following functions are present:

.PP

.nf

.in +4n

.IR posix_trace_attr_destroy (),

.IR posix_trace_attr_getclockres (),

.IR posix_trace_attr_getcreatetime (),

.IR posix_trace_attr_getgenversion (),

.IR posix_trace_attr_getmaxdatasize (),

.IR posix_trace_attr_getmaxsystemeventsize (),

.IR posix_trace_attr_getmaxusereventsize (),

.IR posix_trace_attr_getname (),

.IR posix_trace_attr_getstreamfullpolicy (),

.IR posix_trace_attr_getstreamsize (),

.IR posix_trace_attr_init (),

.IR posix_trace_attr_setmaxdatasize (),

.IR posix_trace_attr_setname (),

.IR posix_trace_attr_setstreamsize (),

.IR posix_trace_attr_setstreamfullpolicy (),

.IR posix_trace_clear (),

.IR posix_trace_create (),

.IR posix_trace_event (),

.IR posix_trace_eventid_equal (),

.IR posix_trace_eventid_get_name (),

.IR posix_trace_eventid_open (),

.IR posix_trace_eventtypelist_getnext_id (),

.IR posix_trace_eventtypelist_rewind (),

.IR posix_trace_flush (),

.IR posix_trace_get_attr (),

.IR posix_trace_get_status (),

.IR posix_trace_getnext_event (),

.IR posix_trace_shutdown (),

.IR posix_trace_start (),

.IR posix_trace_stop (),

.IR posix_trace_trygetnext_event ().

.in

.fi

.SS TEF - _POSIX_TRACE_EVENT_FILTER - _SC_TRACE_EVENT_FILTER

This option implies the

.B _POSIX_TRACE

option.

The following functions are present:

.PP

.nf

.in +4n

.IR posix_trace_eventset_add (),

.IR posix_trace_eventset_del (),

.IR posix_trace_eventset_empty (),

.IR posix_trace_eventset_fill (),

.IR posix_trace_eventset_ismember (),

.IR posix_trace_get_filter (),

.IR posix_trace_set_filter (),

.IR posix_trace_trid_eventid_open ().

.in

.fi

.SS TRI - _POSIX_TRACE_INHERIT - _SC_TRACE_INHERIT

Tracing children of the traced process is supported.

This option implies the

.B _POSIX_TRACE

option.

The following functions are present:

.PP

.nf

.in +4n

.IR posix_trace_attr_getinherited (),

.IR posix_trace_attr_setinherited ().

.in

.fi

.SS TRL - _POSIX_TRACE_LOG - _SC_TRACE_LOG

This option implies the

.B _POSIX_TRACE

option.

The following functions are present:

.PP

.nf

.in +4n

.IR posix_trace_attr_getlogfullpolicy (),

.IR posix_trace_attr_getlogsize (),

.IR posix_trace_attr_setlogfullpolicy (),

.IR posix_trace_attr_setlogsize (),

.IR posix_trace_close (),

.IR posix_trace_create_withlog (),

.IR posix_trace_open (),

.IR posix_trace_rewind ().

.in

.fi

.SS TYM - _POSIX_TYPED_MEMORY_OBJECTS - _SC_TYPED_MEMORY_OBJECT

The following functions are present:

.PP

.nf

.in +4n

.IR posix_mem_offset (),

.IR posix_typed_mem_get_info (),

.IR posix_typed_mem_open ().

.in

.fi

.SS --- - _POSIX_VDISABLE

Always present (probably 0).

Value to set a changeable special control

character to indicate that it is disabled.

.SH XOPEN EXTENSIONS

.BR _XOPEN_CRYPT ,

.BR _XOPEN_LEGACY ,

.BR _XOPEN_REALTIME ,

.BR _XOPEN_REALTIME_THREADS ,

.BR _XOPEN_UNIX .

.\" To be described.

.SH SEE ALSO

.BR sysconf (3),

.BR standards (7)

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I man-pages

project.

A description of the project,

information about reporting bugs,

and the latest version of this page,

can be found at

\%https://www.kernel.org/doc/man\-pages/.