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PROLOG
This 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
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NAME
unistd.h — standard symbolic constants and types
SYNOPSIS
#include <unistd.h>
DESCRIPTION
The <unistd.h> header defines miscellaneous symbolic constants and types, and declares miscellaneous
functions. The actual values of the constants are unspecified except as shown. The contents of this
header are shown below.
Version Test Macros
The <unistd.h> header shall define the following symbolic constants. The values shall be suitable for use
in #if preprocessing directives.
_POSIX_VERSION
Integer value indicating version of this standard (C-language binding) to which the implementation
conforms. For implementations conforming to POSIX.1‐2008, the value shall be 200809L.
_POSIX2_VERSION
Integer value indicating version of the Shell and Utilities volume of POSIX.1 to which the
implementation conforms. For implementations conforming to POSIX.1‐2008, the value shall be
200809L. For profile implementations that define _POSIX_SUBPROFILE (see Section 2.1.5.1,
Subprofiling Considerations) in <unistd.h>, _POSIX2_VERSION may be left undefined or be defined
with the value -1 to indicate that the Shell and Utilities volume of POSIX.1 is not supported. In
this case, a call to sysconf(_SC_2_VERSION) shall return either 200809L or -1 indicating that the
Shell and Utilities volume of POSIX.1 is or is not, respectively, supported at runtime.
The <unistd.h> header shall define the following symbolic constant only if the implementation supports
the XSI option; see Section 2.1.4, XSI Conformance. If defined, its value shall be suitable for use in
#if preprocessing directives.
_XOPEN_VERSION
Integer value indicating version of the X/Open Portability Guide to which the implementation
conforms. The value shall be 700.
Constants for Options and Option Groups
The following symbolic constants, if defined in <unistd.h>, shall have a value of -1, 0, or greater,
unless otherwise specified below. For profile implementations that define _POSIX_SUBPROFILE (see Section
2.1.5.1, Subprofiling Considerations) in <unistd.h>, constants described below as always having a value
greater than zero need not be defined and, if defined, may have a value of -1, 0, or greater. The values
shall be suitable for use in #if preprocessing directives.
If a symbolic constant is not defined or is defined with the value -1, the option is not supported for
compilation. If it is defined with a value greater than zero, the option shall always be supported when
the application is executed. If it is defined with the value zero, the option shall be supported for
compilation and might or might not be supported at runtime. See Section 2.1.6, Options for further
information about the conformance requirements of these three categories of support.
_POSIX_ADVISORY_INFO
The implementation supports the Advisory Information option. If this symbol is defined in
<unistd.h>, it shall be defined to be -1, 0, or 200809L. The value of this symbol reported by
sysconf() shall either be -1 or 200809L.
_POSIX_ASYNCHRONOUS_IO
The implementation supports asynchronous input and output. This symbol shall always be set to the
value 200809L.
_POSIX_BARRIERS
The implementation supports barriers. This symbol shall always be set to the value 200809L.
_POSIX_CHOWN_RESTRICTED
The use of chown() and fchown() is restricted to a process with appropriate privileges, and to
changing the group ID of a file only to the effective group ID of the process or to one of its
supplementary group IDs. This symbol shall be defined with a value other than -1.
_POSIX_CLOCK_SELECTION
The implementation supports clock selection. This symbol shall always be set to the value 200809L.
_POSIX_CPUTIME
The implementation supports the Process CPU-Time Clocks option. If this symbol is defined in
<unistd.h>, it shall be defined to be -1, 0, or 200809L. The value of this symbol reported by
sysconf() shall either be -1 or 200809L.
_POSIX_FSYNC
The implementation supports the File Synchronization option. If this symbol is defined in
<unistd.h>, it shall be defined to be -1, 0, or 200809L. The value of this symbol reported by
sysconf() shall either be -1 or 200809L.
_POSIX_IPV6
The implementation supports the IPv6 option. If this symbol is defined in <unistd.h>, it shall be
defined to be -1, 0, or 200809L. The value of this symbol reported by sysconf() shall either be -1
or 200809L.
_POSIX_JOB_CONTROL
The implementation supports job control. This symbol shall always be set to a value greater than
zero.
_POSIX_MAPPED_FILES
The implementation supports memory mapped Files. This symbol shall always be set to the value
200809L.
_POSIX_MEMLOCK
The implementation supports the Process Memory Locking option. If this symbol is defined in
<unistd.h>, it shall be defined to be -1, 0, or 200809L. The value of this symbol reported by
sysconf() shall either be -1 or 200809L.
_POSIX_MEMLOCK_RANGE
The implementation supports the Range Memory Locking option. If this symbol is defined in
<unistd.h>, it shall be defined to be -1, 0, or 200809L. The value of this symbol reported by
sysconf() shall either be -1 or 200809L.
_POSIX_MEMORY_PROTECTION
The implementation supports memory protection. This symbol shall always be set to the value
200809L.
_POSIX_MESSAGE_PASSING
The implementation supports the Message Passing option. If this symbol is defined in <unistd.h>,
it shall be defined to be -1, 0, or 200809L. The value of this symbol reported by sysconf() shall
either be -1 or 200809L.
_POSIX_MONOTONIC_CLOCK
The implementation supports the Monotonic Clock option. If this symbol is defined in <unistd.h>,
it shall be defined to be -1, 0, or 200809L. The value of this symbol reported by sysconf() shall
either be -1 or 200809L.
_POSIX_NO_TRUNC
Pathname components longer than {NAME_MAX} generate an error. This symbol shall be defined with a
value other than -1.
_POSIX_PRIORITIZED_IO
The implementation supports the Prioritized Input and Output option. If this symbol is defined in
<unistd.h>, it shall be defined to be -1, 0, or 200809L. The value of this symbol reported by
sysconf() shall either be -1 or 200809L.
_POSIX_PRIORITY_SCHEDULING
The implementation supports the Process Scheduling option. If this symbol is defined in
<unistd.h>, it shall be defined to be -1, 0, or 200809L. The value of this symbol reported by
sysconf() shall either be -1 or 200809L.
_POSIX_RAW_SOCKETS
The implementation supports the Raw Sockets option. If this symbol is defined in <unistd.h>, it
shall be defined to be -1, 0, or 200809L. The value of this symbol reported by sysconf() shall
either be -1 or 200809L.
_POSIX_READER_WRITER_LOCKS
The implementation supports read-write locks. This symbol shall always be set to the value
200809L.
_POSIX_REALTIME_SIGNALS
The implementation supports realtime signals. This symbol shall always be set to the value
200809L.
_POSIX_REGEXP
The implementation supports the Regular Expression Handling option. This symbol shall always be
set to a value greater than zero.
_POSIX_SAVED_IDS
Each process has a saved set-user-ID and a saved set-group-ID. This symbol shall always be set to
a value greater than zero.
_POSIX_SEMAPHORES
The implementation supports semaphores. This symbol shall always be set to the value 200809L.
_POSIX_SHARED_MEMORY_OBJECTS
The implementation supports the Shared Memory Objects option. If this symbol is defined in
<unistd.h>, it shall be defined to be -1, 0, or 200809L. The value of this symbol reported by
sysconf() shall either be -1 or 200809L.
_POSIX_SHELL
The implementation supports the POSIX shell. This symbol shall always be set to a value greater
than zero.
_POSIX_SPAWN
The implementation supports the Spawn option. If this symbol is defined in <unistd.h>, it shall be
defined to be -1, 0, or 200809L. The value of this symbol reported by sysconf() shall either be -1
or 200809L.
_POSIX_SPIN_LOCKS
The implementation supports spin locks. This symbol shall always be set to the value 200809L.
_POSIX_SPORADIC_SERVER
The implementation supports the Process Sporadic Server option. If this symbol is defined in
<unistd.h>, it shall be defined to be -1, 0, or 200809L. The value of this symbol reported by
sysconf() shall either be -1 or 200809L.
_POSIX_SYNCHRONIZED_IO
The implementation supports the Synchronized Input and Output option. If this symbol is defined in
<unistd.h>, it shall be defined to be -1, 0, or 200809L. The value of this symbol reported by
sysconf() shall either be -1 or 200809L.
_POSIX_THREAD_ATTR_STACKADDR
The implementation supports the Thread Stack Address Attribute option. If this symbol is defined
in <unistd.h>, it shall be defined to be -1, 0, or 200809L. The value of this symbol reported by
sysconf() shall either be -1 or 200809L.
_POSIX_THREAD_ATTR_STACKSIZE
The implementation supports the Thread Stack Size Attribute option. If this symbol is defined in
<unistd.h>, it shall be defined to be -1, 0, or 200809L. The value of this symbol reported by
sysconf() shall either be -1 or 200809L.
_POSIX_THREAD_CPUTIME
The implementation supports the Thread CPU-Time Clocks option. If this symbol is defined in
<unistd.h>, it shall be defined to be -1, 0, or 200809L. The value of this symbol reported by
sysconf() shall either be -1 or 200809L.
_POSIX_THREAD_PRIO_INHERIT
The implementation supports the Non-Robust Mutex Priority Inheritance option. If this symbol is
defined in <unistd.h>, it shall be defined to be -1, 0, or 200809L. The value of this symbol
reported by sysconf() shall either be -1 or 200809L.
_POSIX_THREAD_PRIO_PROTECT
The implementation supports the Non-Robust Mutex Priority Protection option. If this symbol is
defined in <unistd.h>, it shall be defined to be -1, 0, or 200809L. The value of this symbol
reported by sysconf() shall either be -1 or 200809L.
_POSIX_THREAD_PRIORITY_SCHEDULING
The implementation supports the Thread Execution Scheduling option. If this symbol is defined in
<unistd.h>, it shall be defined to be -1, 0, or 200809L. The value of this symbol reported by
sysconf() shall either be -1 or 200809L.
_POSIX_THREAD_PROCESS_SHARED
The implementation supports the Thread Process-Shared Synchronization option. If this symbol is
defined in <unistd.h>, it shall be defined to be -1, 0, or 200809L. The value of this symbol
reported by sysconf() shall either be -1 or 200809L.
_POSIX_THREAD_ROBUST_PRIO_INHERIT
The implementation supports the Robust Mutex Priority Inheritance option. If this symbol is defined
in <unistd.h>, it shall be defined to be -1, 0, or 200809L. The value of this symbol reported by
sysconf() shall either be -1 or 200809L.
_POSIX_THREAD_ROBUST_PRIO_PROTECT
The implementation supports the Robust Mutex Priority Protection option. If this symbol is defined
in <unistd.h>, it shall be defined to be -1, 0, or 200809L. The value of this symbol reported by
sysconf() shall either be -1 or 200809L.
_POSIX_THREAD_SAFE_FUNCTIONS
The implementation supports thread-safe functions. This symbol shall always be set to the value
200809L.
_POSIX_THREAD_SPORADIC_SERVER
The implementation supports the Thread Sporadic Server option. If this symbol is defined in
<unistd.h>, it shall be defined to be -1, 0, or 200809L. The value of this symbol reported by
sysconf() shall either be -1 or 200809L.
_POSIX_THREADS
The implementation supports threads. This symbol shall always be set to the value 200809L.
_POSIX_TIMEOUTS
The implementation supports timeouts. This symbol shall always be set to the value 200809L.
_POSIX_TIMERS
The implementation supports timers. This symbol shall always be set to the value 200809L.
_POSIX_TRACE
The implementation supports the Trace option. If this symbol is defined in <unistd.h>, it shall be
defined to be -1, 0, or 200809L. The value of this symbol reported by sysconf() shall either be -1
or 200809L.
_POSIX_TRACE_EVENT_FILTER
The implementation supports the Trace Event Filter option. If this symbol is defined in
<unistd.h>, it shall be defined to be -1, 0, or 200809L. The value of this symbol reported by
sysconf() shall either be -1 or 200809L.
_POSIX_TRACE_INHERIT
The implementation supports the Trace Inherit option. If this symbol is defined in <unistd.h>, it
shall be defined to be -1, 0, or 200809L. The value of this symbol reported by sysconf() shall
either be -1 or 200809L.
_POSIX_TRACE_LOG
The implementation supports the Trace Log option. If this symbol is defined in <unistd.h>, it
shall be defined to be -1, 0, or 200809L. The value of this symbol reported by sysconf() shall
either be -1 or 200809L.
_POSIX_TYPED_MEMORY_OBJECTS
The implementation supports the Typed Memory Objects option. If this symbol is defined in
<unistd.h>, it shall be defined to be -1, 0, or 200809L. The value of this symbol reported by
sysconf() shall either be -1 or 200809L.
_POSIX_V6_ILP32_OFF32
The implementation provides a C-language compilation environment with 32-bit int, long, pointer,
and off_t types.
_POSIX_V6_ILP32_OFFBIG
The implementation provides a C-language compilation environment with 32-bit int, long, and pointer
types and an off_t type using at least 64 bits.
_POSIX_V6_LP64_OFF64
The implementation provides a C-language compilation environment with 32-bit int and 64-bit long,
pointer, and off_t types.
_POSIX_V6_LPBIG_OFFBIG
The implementation provides a C-language compilation environment with an int type using at least 32
bits and long, pointer, and off_t types using at least 64 bits.
_POSIX_V7_ILP32_OFF32
The implementation provides a C-language compilation environment with 32-bit int, long, pointer,
and off_t types.
_POSIX_V7_ILP32_OFFBIG
The implementation provides a C-language compilation environment with 32-bit int, long, and pointer
types and an off_t type using at least 64 bits.
_POSIX_V7_LP64_OFF64
The implementation provides a C-language compilation environment with 32-bit int and 64-bit long,
pointer, and off_t types.
_POSIX_V7_LPBIG_OFFBIG
The implementation provides a C-language compilation environment with an int type using at least 32
bits and long, pointer, and off_t types using at least 64 bits.
_POSIX2_C_BIND
The implementation supports the C-Language Binding option. This symbol shall always have the value
200809L.
_POSIX2_C_DEV
The implementation supports the C-Language Development Utilities option. If this symbol is defined
in <unistd.h>, it shall be defined to be -1, 0, or 200809L. The value of this symbol reported by
sysconf() shall either be -1 or 200809L.
_POSIX2_CHAR_TERM
The implementation supports the Terminal Characteristics option. The value of this symbol reported
by sysconf() shall either be -1 or a value greater than zero.
_POSIX2_FORT_DEV
The implementation supports the FORTRAN Development Utilities option. If this symbol is defined in
<unistd.h>, it shall be defined to be -1, 0, or 200809L. The value of this symbol reported by
sysconf() shall either be -1 or 200809L.
_POSIX2_FORT_RUN
The implementation supports the FORTRAN Runtime Utilities option. If this symbol is defined in
<unistd.h>, it shall be defined to be -1, 0, or 200809L. The value of this symbol reported by
sysconf() shall either be -1 or 200809L.
_POSIX2_LOCALEDEF
The implementation supports the creation of locales by the localedef utility. If this symbol is
defined in <unistd.h>, it shall be defined to be -1, 0, or 200809L. The value of this symbol
reported by sysconf() shall either be -1 or 200809L.
_POSIX2_PBS
The implementation supports the Batch Environment Services and Utilities option. If this symbol is
defined in <unistd.h>, it shall be defined to be -1, 0, or 200809L. The value of this symbol
reported by sysconf() shall either be -1 or 200809L.
_POSIX2_PBS_ACCOUNTING
The implementation supports the Batch Accounting option. If this symbol is defined in <unistd.h>,
it shall be defined to be -1, 0, or 200809L. The value of this symbol reported by sysconf() shall
either be -1 or 200809L.
_POSIX2_PBS_CHECKPOINT
The implementation supports the Batch Checkpoint/Restart option. If this symbol is defined in
<unistd.h>, it shall be defined to be -1, 0, or 200809L. The value of this symbol reported by
sysconf() shall either be -1 or 200809L.
_POSIX2_PBS_LOCATE
The implementation supports the Locate Batch Job Request option. If this symbol is defined in
<unistd.h>, it shall be defined to be -1, 0, or 200809L. The value of this symbol reported by
sysconf() shall either be -1 or 200809L.
_POSIX2_PBS_MESSAGE
The implementation supports the Batch Job Message Request option. If this symbol is defined in
<unistd.h>, it shall be defined to be -1, 0, or 200809L. The value of this symbol reported by
sysconf() shall either be -1 or 200809L.
_POSIX2_PBS_TRACK
The implementation supports the Track Batch Job Request option. If this symbol is defined in
<unistd.h>, it shall be defined to be -1, 0, or 200809L. The value of this symbol reported by
sysconf() shall either be -1 or 200809L.
_POSIX2_SW_DEV
The implementation supports the Software Development Utilities option. If this symbol is defined
in <unistd.h>, it shall be defined to be -1, 0, or 200809L. The value of this symbol reported by
sysconf() shall either be -1 or 200809L.
_POSIX2_UPE
The implementation supports the User Portability Utilities option. If this symbol is defined in
<unistd.h>, it shall be defined to be -1, 0, or 200809L. The value of this symbol reported by
sysconf() shall either be -1 or 200809L.
_XOPEN_CRYPT
The implementation supports the X/Open Encryption Option Group.
_XOPEN_ENH_I18N
The implementation supports the Issue 4, Version 2 Enhanced Internationalization Option Group. This
symbol shall always be set to a value other than -1.
_XOPEN_REALTIME
The implementation supports the X/Open Realtime Option Group.
_XOPEN_REALTIME_THREADS
The implementation supports the X/Open Realtime Threads Option Group.
_XOPEN_SHM
The implementation supports the Issue 4, Version 2 Shared Memory Option Group. This symbol shall
always be set to a value other than -1.
_XOPEN_STREAMS
The implementation supports the XSI STREAMS Option Group.
_XOPEN_UNIX
The implementation supports the XSI option.
_XOPEN_UUCP
The implementation supports the UUCP Utilities option. If this symbol is defined in <unistd.h>, it
shall be defined to be -1, 0, or 200809L. The value of this symbol reported by sysconf() shall be
either -1 or 200809L.
Execution-Time Symbolic Constants
If any of the following symbolic constants are not defined in the <unistd.h> header, the value shall vary
depending on the file to which it is applied. If defined, they shall have values suitable for use in #if
preprocessing directives.
If any of the following symbolic constants are defined to have value -1 in the <unistd.h> header, the
implementation shall not provide the option on any file; if any are defined to have a value other than -1
in the <unistd.h> header, the implementation shall provide the option on all applicable files.
All of the following values, whether defined as symbolic constants in <unistd.h> or not, may be queried
with respect to a specific file using the pathconf() or fpathconf() functions:
_POSIX_ASYNC_IO
Asynchronous input or output operations may be performed for the associated file.
_POSIX_PRIO_IO
Prioritized input or output operations may be performed for the associated file.
_POSIX_SYNC_IO
Synchronized input or output operations may be performed for the associated file.
If the following symbolic constants are defined in the <unistd.h> header, they apply to files and all
paths in all file systems on the implementation:
_POSIX_TIMESTAMP_RESOLUTION
The resolution in nanoseconds for all file timestamps.
_POSIX2_SYMLINKS
Symbolic links can be created.
Constants for Functions
The <unistd.h> header shall define NULL as described in <stddef.h>.
The <unistd.h> header shall define the following symbolic constants for use with the access() function.
The values shall be suitable for use in #if preprocessing directives.
F_OK Test for existence of file.
R_OK Test for read permission.
W_OK Test for write permission.
X_OK Test for execute (search) permission.
The constants F_OK, R_OK, W_OK, and X_OK and the expressions R_OK|W_OK, R_OK|X_OK, and R_OK|W_OK|X_OK
shall all have distinct values.
The <unistd.h> header shall define the following symbolic constants for the confstr() function:
_CS_PATH
This is the value for the PATH environment variable that finds all of the standard utilities that
are provided in a manner accessible via the exec family of functions.
_CS_POSIX_V7_ILP32_OFF32_CFLAGS
If sysconf(_SC_V7_ILP32_OFF32) returns -1, the meaning of this value is unspecified. Otherwise,
this value is the set of initial options to be given to the c99 utility to build an application
using a programming model with 32-bit int, long, pointer, and off_t types.
_CS_POSIX_V7_ILP32_OFF32_LDFLAGS
If sysconf(_SC_V7_ILP32_OFF32) returns -1, the meaning of this value is unspecified. Otherwise,
this value is the set of final options to be given to the c99 utility to build an application using
a programming model with 32-bit int, long, pointer, and off_t types.
_CS_POSIX_V7_ILP32_OFF32_LIBS
If sysconf(_SC_V7_ILP32_OFF32) returns -1, the meaning of this value is unspecified. Otherwise,
this value is the set of libraries to be given to the c99 utility to build an application using a
programming model with 32-bit int, long, pointer, and off_t types.
_CS_POSIX_V7_ILP32_OFFBIG_CFLAGS
If sysconf(_SC_V7_ILP32_OFFBIG) returns -1, the meaning of this value is unspecified. Otherwise,
this value is the set of initial options to be given to the c99 utility to build an application
using a programming model with 32-bit int, long, and pointer types, and an off_t type using at
least 64 bits.
_CS_POSIX_V7_ILP32_OFFBIG_LDFLAGS
If sysconf(_SC_V7_ILP32_OFFBIG) returns -1, the meaning of this value is unspecified. Otherwise,
this value is the set of final options to be given to the c99 utility to build an application using
a programming model with 32-bit int, long, and pointer types, and an off_t type using at least 64
bits.
_CS_POSIX_V7_ILP32_OFFBIG_LIBS
If sysconf(_SC_V7_ILP32_OFFBIG) returns -1, the meaning of this value is unspecified. Otherwise,
this value is the set of libraries to be given to the c99 utility to build an application using a
programming model with 32-bit int, long, and pointer types, and an off_t type using at least 64
bits.
_CS_POSIX_V7_LP64_OFF64_CFLAGS
If sysconf(_SC_V7_LP64_OFF64) returns -1, the meaning of this value is unspecified. Otherwise, this
value is the set of initial options to be given to the c99 utility to build an application using a
programming model with 32-bit int and 64-bit long, pointer, and off_t types.
_CS_POSIX_V7_LP64_OFF64_LDFLAGS
If sysconf(_SC_V7_LP64_OFF64) returns -1, the meaning of this value is unspecified. Otherwise, this
value is the set of final options to be given to the c99 utility to build an application using a
programming model with 32-bit int and 64-bit long, pointer, and off_t types.
_CS_POSIX_V7_LP64_OFF64_LIBS
If sysconf(_SC_V7_LP64_OFF64) returns -1, the meaning of this value is unspecified. Otherwise, this
value is the set of libraries to be given to the c99 utility to build an application using a
programming model with 32-bit int and 64-bit long, pointer, and off_t types.
_CS_POSIX_V7_LPBIG_OFFBIG_CFLAGS
If sysconf(_SC_V7_LPBIG_OFFBIG) returns -1, the meaning of this value is unspecified. Otherwise,
this value is the set of initial options to be given to the c99 utility to build an application
using a programming model with an int type using at least 32 bits and long, pointer, and off_t
types using at least 64 bits.
_CS_POSIX_V7_LPBIG_OFFBIG_LDFLAGS
If sysconf(_SC_V7_LPBIG_OFFBIG) returns -1, the meaning of this value is unspecified. Otherwise,
this value is the set of final options to be given to the c99 utility to build an application using
a programming model with an int type using at least 32 bits and long, pointer, and off_t types
using at least 64 bits.
_CS_POSIX_V7_LPBIG_OFFBIG_LIBS
If sysconf(_SC_V7_LPBIG_OFFBIG) returns -1, the meaning of this value is unspecified. Otherwise,
this value is the set of libraries to be given to the c99 utility to build an application using a
programming model with an int type using at least 32 bits and long, pointer, and off_t types using
at least 64 bits.
_CS_POSIX_V7_THREADS_CFLAGS
If sysconf(_SC_POSIX_THREADS) returns -1, the meaning of this value is unspecified. Otherwise, this
value is the set of initial options to be given to the c99 utility to build a multi-threaded
application. These flags are in addition to those associated with any of the other
_CS_POSIX_V7_*_CFLAGS values used to specify particular type size programing environments.
_CS_POSIX_V7_THREADS_LDFLAGS
If sysconf(_SC_POSIX_THREADS) returns -1, the meaning of this value is unspecified. Otherwise, this
value is the set of final options to be given to the c99 utility to build a multi-threaded
application. These flags are in addition to those associated with any of the other
_CS_POSIX_V7_*_LDFLAGS values used to specify particular type size programing environments.
_CS_POSIX_V7_WIDTH_RESTRICTED_ENVS
This value is a <newline>-separated list of names of programming environments supported by the
implementation in which the widths of the blksize_t, cc_t, mode_t, nfds_t, pid_t, ptrdiff_t,
size_t, speed_t, ssize_t, suseconds_t, tcflag_t, wchar_t, and wint_t types are no greater than the
width of type long. The format of each name shall be suitable for use with the getconf -v option.
_CS_V7_ENV
This is the value that provides the environment variable information (other than that provided by
_CS_PATH) that is required by the implementation to create a conforming environment, as described
in the implementation's conformance documentation.
The following symbolic constants are reserved for compatibility with Issue 6:
_CS_POSIX_V6_ILP32_OFF32_CFLAGS
_CS_POSIX_V6_ILP32_OFF32_LDFLAGS
_CS_POSIX_V6_ILP32_OFF32_LIBS
_CS_POSIX_V6_ILP32_OFFBIG_CFLAGS
_CS_POSIX_V6_ILP32_OFFBIG_LDFLAGS
_CS_POSIX_V6_ILP32_OFFBIG_LIBS
_CS_POSIX_V6_LP64_OFF64_CFLAGS
_CS_POSIX_V6_LP64_OFF64_LDFLAGS
_CS_POSIX_V6_LP64_OFF64_LIBS
_CS_POSIX_V6_LPBIG_OFFBIG_CFLAGS
_CS_POSIX_V6_LPBIG_OFFBIG_LDFLAGS
_CS_POSIX_V6_LPBIG_OFFBIG_LIBS
_CS_POSIX_V6_WIDTH_RESTRICTED_ENVS
_CS_V6_ENV
The <unistd.h> header shall define SEEK_CUR, SEEK_END, and SEEK_SET as described in <stdio.h>.
The <unistd.h> header shall define the following symbolic constants as possible values for the function
argument to the lockf() function:
F_LOCK Lock a section for exclusive use.
F_TEST Test section for locks by other processes.
F_TLOCK Test and lock a section for exclusive use.
F_ULOCK Unlock locked sections.
The <unistd.h> header shall define the following symbolic constants for pathconf():
_PC_2_SYMLINKS
_PC_ALLOC_SIZE_MIN
_PC_ASYNC_IO
_PC_CHOWN_RESTRICTED
_PC_FILESIZEBITS
_PC_LINK_MAX
_PC_MAX_CANON
_PC_MAX_INPUT
_PC_NAME_MAX
_PC_NO_TRUNC
_PC_PATH_MAX
_PC_PIPE_BUF
_PC_PRIO_IO
_PC_REC_INCR_XFER_SIZE
_PC_REC_MAX_XFER_SIZE
_PC_REC_MIN_XFER_SIZE
_PC_REC_XFER_ALIGN
_PC_SYMLINK_MAX
_PC_SYNC_IO
_PC_TIMESTAMP_RESOLUTION
_PC_VDISABLE
The <unistd.h> header shall define the following symbolic constants for sysconf():
_SC_2_C_BIND
_SC_2_C_DEV
_SC_2_CHAR_TERM
_SC_2_FORT_DEV
_SC_2_FORT_RUN
_SC_2_LOCALEDEF
_SC_2_PBS
_SC_2_PBS_ACCOUNTING
_SC_2_PBS_CHECKPOINT
_SC_2_PBS_LOCATE
_SC_2_PBS_MESSAGE
_SC_2_PBS_TRACK
_SC_2_SW_DEV
_SC_2_UPE
_SC_2_VERSION
_SC_ADVISORY_INFO
_SC_AIO_LISTIO_MAX
_SC_AIO_MAX
_SC_AIO_PRIO_DELTA_MAX
_SC_ARG_MAX
_SC_ASYNCHRONOUS_IO
_SC_ATEXIT_MAX
_SC_BARRIERS
_SC_BC_BASE_MAX
_SC_BC_DIM_MAX
_SC_BC_SCALE_MAX
_SC_BC_STRING_MAX
_SC_CHILD_MAX
_SC_CLK_TCK
_SC_CLOCK_SELECTION
_SC_COLL_WEIGHTS_MAX
_SC_CPUTIME
_SC_DELAYTIMER_MAX
_SC_EXPR_NEST_MAX
_SC_FSYNC
_SC_GETGR_R_SIZE_MAX
_SC_GETPW_R_SIZE_MAX
_SC_HOST_NAME_MAX
_SC_IOV_MAX
_SC_IPV6
_SC_JOB_CONTROL
_SC_LINE_MAX
_SC_LOGIN_NAME_MAX
_SC_MAPPED_FILES
_SC_MEMLOCK
_SC_MEMLOCK_RANGE
_SC_MEMORY_PROTECTION
_SC_MESSAGE_PASSING
_SC_MONOTONIC_CLOCK
_SC_MQ_OPEN_MAX
_SC_MQ_PRIO_MAX
_SC_NGROUPS_MAX
_SC_OPEN_MAX
_SC_PAGE_SIZE
_SC_PAGESIZE
_SC_PRIORITIZED_IO
_SC_PRIORITY_SCHEDULING
_SC_RAW_SOCKETS
_SC_RE_DUP_MAX
_SC_READER_WRITER_LOCKS
_SC_REALTIME_SIGNALS
_SC_REGEXP
_SC_RTSIG_MAX
_SC_SAVED_IDS
_SC_SEM_NSEMS_MAX
_SC_SEM_VALUE_MAX
_SC_SEMAPHORES
_SC_SHARED_MEMORY_OBJECTS
_SC_SHELL
_SC_SIGQUEUE_MAX
_SC_SPAWN
_SC_SPIN_LOCKS
_SC_SPORADIC_SERVER
_SC_SS_REPL_MAX
_SC_STREAM_MAX
_SC_SYMLOOP_MAX
_SC_SYNCHRONIZED_IO
_SC_THREAD_ATTR_STACKADDR
_SC_THREAD_ATTR_STACKSIZE
_SC_THREAD_CPUTIME
_SC_THREAD_DESTRUCTOR_ITERATIONS
_SC_THREAD_KEYS_MAX
_SC_THREAD_PRIO_INHERIT
_SC_THREAD_PRIO_PROTECT
_SC_THREAD_PRIORITY_SCHEDULING
_SC_THREAD_PROCESS_SHARED
_SC_THREAD_ROBUST_PRIO_INHERIT
_SC_THREAD_ROBUST_PRIO_PROTECT
_SC_THREAD_SAFE_FUNCTIONS
_SC_THREAD_SPORADIC_SERVER
_SC_THREAD_STACK_MIN
_SC_THREAD_THREADS_MAX
_SC_THREADS
_SC_TIMEOUTS
_SC_TIMER_MAX
_SC_TIMERS
_SC_TRACE
_SC_TRACE_EVENT_FILTER
_SC_TRACE_EVENT_NAME_MAX
_SC_TRACE_INHERIT
_SC_TRACE_LOG
_SC_TRACE_NAME_MAX
_SC_TRACE_SYS_MAX
_SC_TRACE_USER_EVENT_MAX
_SC_TTY_NAME_MAX
_SC_TYPED_MEMORY_OBJECTS
_SC_TZNAME_MAX
_SC_V7_ILP32_OFF32
_SC_V7_ILP32_OFFBIG
_SC_V7_LP64_OFF64
_SC_V7_LPBIG_OFFBIG
_SC_V6_ILP32_OFF32
_SC_V6_ILP32_OFFBIG
_SC_V6_LP64_OFF64
_SC_V6_LPBIG_OFFBIG
_SC_VERSION
_SC_XOPEN_CRYPT
_SC_XOPEN_ENH_I18N
_SC_XOPEN_REALTIME
_SC_XOPEN_REALTIME_THREADS
_SC_XOPEN_SHM
_SC_XOPEN_STREAMS
_SC_XOPEN_UNIX
_SC_XOPEN_UUCP
_SC_XOPEN_VERSION
The two constants _SC_PAGESIZE and _SC_PAGE_SIZE may be defined to have the same value.
The <unistd.h> header shall define the following symbolic constants for file streams:
STDERR_FILENO File number of stderr; 2.
STDIN_FILENO File number of stdin; 0.
STDOUT_FILENO File number of stdout; 1.
The <unistd.h> header shall define the following symbolic constant for terminal special character
handling:
_POSIX_VDISABLE
This symbol shall be defined to be the value of a character that shall disable terminal
special character handling as described in Section 11.2.6, Special Control Characters.
This symbol shall always be set to a value other than -1.
Type Definitions
The <unistd.h> header shall define the size_t, ssize_t, uid_t, gid_t, off_t, and pid_t types as described
in <sys/types.h>.
The <unistd.h> header shall define the intptr_t type as described in <stdint.h>.
Declarations
The following shall be declared as functions and may also be defined as macros. Function prototypes shall
be provided.
int access(const char *, int);
unsigned alarm(unsigned);
int chdir(const char *);
int chown(const char *, uid_t, gid_t);
int close(int);
size_t confstr(int, char *, size_t);
char *crypt(const char *, const char *);
int dup(int);
int dup2(int, int);
void _exit(int);
void encrypt(char [64], int);
int execl(const char *, const char *, ...);
int execle(const char *, const char *, ...);
int execlp(const char *, const char *, ...);
int execv(const char *, char *const []);
int execve(const char *, char *const [], char *const []);
int execvp(const char *, char *const []);
int faccessat(int, const char *, int, int);
int fchdir(int);
int fchown(int, uid_t, gid_t);
int fchownat(int, const char *, uid_t, gid_t, int);
int fdatasync(int);
int fexecve(int, char *const [], char *const []);
pid_t fork(void);
long fpathconf(int, int);
int fsync(int);
int ftruncate(int, off_t);
char *getcwd(char *, size_t);
gid_t getegid(void);
uid_t geteuid(void);
gid_t getgid(void);
int getgroups(int, gid_t []);
long gethostid(void);
int gethostname(char *, size_t);
char *getlogin(void);
int getlogin_r(char *, size_t);
int getopt(int, char * const [], const char *);
pid_t getpgid(pid_t);
pid_t getpgrp(void);
pid_t getpid(void);
pid_t getppid(void);
pid_t getsid(pid_t);
uid_t getuid(void);
int isatty(int);
int lchown(const char *, uid_t, gid_t);
int link(const char *, const char *);
int linkat(int, const char *, int, const char *, int);
int lockf(int, int, off_t);
off_t lseek(int, off_t, int);
int nice(int);
long pathconf(const char *, int);
int pause(void);
int pipe(int [2]);
ssize_t pread(int, void *, size_t, off_t);
ssize_t pwrite(int, const void *, size_t, off_t);
ssize_t read(int, void *, size_t);
ssize_t readlink(const char *restrict, char *restrict, size_t);
ssize_t readlinkat(int, const char *restrict, char *restrict, size_t);
int rmdir(const char *);
int setegid(gid_t);
int seteuid(uid_t);
int setgid(gid_t);
int setpgid(pid_t, pid_t);
pid_t setpgrp(void);
int setregid(gid_t, gid_t);
int setreuid(uid_t, uid_t);
pid_t setsid(void);
int setuid(uid_t);
unsigned sleep(unsigned);
void swab(const void *restrict, void *restrict, ssize_t);
int symlink(const char *, const char *);
int symlinkat(const char *, int, const char *);
void sync(void);
long sysconf(int);
pid_t tcgetpgrp(int);
int tcsetpgrp(int, pid_t);
int truncate(const char *, off_t);
char *ttyname(int);
int ttyname_r(int, char *, size_t);
int unlink(const char *);
int unlinkat(int, const char *, int);
ssize_t write(int, const void *, size_t);
Implementations may also include the pthread_atfork() prototype as defined in <pthread.h>.
Implementations may also include the ctermid() prototype as defined in <stdio.h>.
The <unistd.h> header shall declare the following external variables:
extern char *optarg;
extern int opterr, optind, optopt;
Inclusion of the <unistd.h> header may make visible all symbols from the headers <stddef.h>, <stdint.h>,
and <stdio.h>.
The following sections are informative.
APPLICATION USAGE
POSIX.1‐2008 only describes the behavior of systems that claim conformance to it. However, application
developers who want to write applications that adapt to other versions of this standard (or to systems
that do not conform to any POSIX standard) may find it useful to code them so as to conditionally compile
different code depending on the value of _POSIX_VERSION, for example:
#if _POSIX_VERSION >= 200112L
/* Use the newer function that copes with large files. */
off_t pos=ftello(fp);
#else
/* Either this is an old version of POSIX, or _POSIX_VERSION is
not even defined, so use the traditional function. */
long pos=ftell(fp);
#endif
Earlier versions of POSIX.1‐2008 and of the Single UNIX Specification can be identified by the following
macros:
POSIX.1‐1988 standard
_POSIX_VERSION==198808L
POSIX.1‐1990 standard
_POSIX_VERSION==199009L
ISO POSIX‐1:1996 standard
_POSIX_VERSION==199506L
Single UNIX Specification, Version 1
_XOPEN_UNIX and _XOPEN_VERSION==4
Single UNIX Specification, Version 2
_XOPEN_UNIX and _XOPEN_VERSION==500
ISO POSIX‐1:2001 and Single UNIX Specification, Version 3
_POSIX_VERSION==200112L, plus (if the XSI option is supported) _XOPEN_UNIX and _XOPEN_VERSION==600
POSIX.1‐2008 does not make any attempt to define application binary interaction with the underlying
operating system. However, application developers may find it useful to query _SC_VERSION at runtime via
sysconf() to determine whether the current version of the operating system supports the necessary
functionality as in the following program fragment:
if (sysconf(_SC_VERSION) < 200809L) {
fprintf(stderr, "POSIX.1-2008 system required, terminating \n");
exit(1);
}
New applications should not use _XOPEN_SHM or _XOPEN_ENH_I18N.
RATIONALE
As POSIX.1‐2008 evolved, certain options became sufficiently standardized that it was concluded that
simply requiring one of the option choices was simpler than retaining the option. However, for backwards-
compatibility, the option flags (with required constant values) are retained.
Version Test Macros
The standard developers considered altering the definition of _POSIX_VERSION and removing _SC_VERSION
from the specification of sysconf() since the utility to an application was deemed by some to be minimal,
and since the implementation of the functionality is potentially problematic. However, they recognized
that support for existing application binaries is a concern to manufacturers, application developers, and
the users of implementations conforming to POSIX.1‐2008.
While the example using _SC_VERSION in the APPLICATION USAGE section does not provide the greatest degree
of imaginable utility to the application developer or user, it is arguably better than a core file or
some other equally obscure result. (It is also possible for implementations to encode and recognize
application binaries compiled in various POSIX.1-conforming environments, and modify the semantics of the
underlying system to conform to the expectations of the application.) For the reasons outlined in the
preceding paragraphs and in the APPLICATION USAGE section, the standard developers elected to retain the
_POSIX_VERSION and _SC_VERSION functionality.
Compile-Time Symbolic Constants for System-Wide Options
POSIX.1‐2008 includes support in certain areas for the newly adopted policy governing options and stubs.
This policy provides flexibility for implementations in how they support options. It also specifies how
conforming applications can adapt to different implementations that support different sets of options. It
allows the following:
1. If an implementation has no interest in supporting an option, it does not have to provide anything
associated with that option beyond the announcement that it does not support it.
2. An implementation can support a partial or incompatible version of an option (as a non-standard
extension) as long as it does not claim to support the option.
3. An application can determine whether the option is supported. A strictly conforming application must
check this announcement mechanism before first using anything associated with the option.
There is an important implication of this policy. POSIX.1‐2008 cannot dictate the behavior of interfaces
associated with an option when the implementation does not claim to support the option. In particular, it
cannot require that a function associated with an unsupported option will fail if it does not perform as
specified. However, this policy does not prevent a standard from requiring certain functions to always be
present, but that they shall always fail on some implementations. The setpgid() function in the
POSIX.1‐1990 standard, for example, is considered appropriate.
The POSIX standards include various options, and the C-language binding support for an option implies
that the implementation must supply data types and function interfaces. An application must be able to
discover whether the implementation supports each option.
Any application must consider the following three cases for each option:
1. Option never supported.
The implementation advertises at compile time that the option will never be supported. In this case,
it is not necessary for the implementation to supply any of the data types or function interfaces
that are provided only as part of the option. The implementation might provide data types and
functions that are similar to those defined by POSIX.1‐2008, but there is no guarantee for any
particular behavior.
2. Option always supported.
The implementation advertises at compile time that the option will always be supported. In this case,
all data types and function interfaces shall be available and shall operate as specified.
3. Option might or might not be supported.
Some implementations might not provide a mechanism to specify support of options at compile time. In
addition, the implementation might be unable or unwilling to specify support or non-support at
compile time. In either case, any application that might use the option at runtime must be able to
compile and execute. The implementation must provide, at compile time, all data types and function
interfaces that are necessary to allow this. In this situation, there must be a mechanism that allows
the application to query, at runtime, whether the option is supported. If the application attempts to
use the option when it is not supported, the result is unspecified unless explicitly specified
otherwise in POSIX.1‐2008.
FUTURE DIRECTIONS
None.
SEE ALSO
<limits.h>, <stddef.h>, <stdint.h>, <stdio.h>, <sys_socket.h>, <sys_types.h>, <termios.h>, <wctype.h>
The System Interfaces volume of POSIX.1‐2017, access(), alarm(), chown(), close(), confstr(), crypt(),
ctermid(), dup(), _Exit(), encrypt(), exec, fchdir(), fchown(), fdatasync(), fork(), fpathconf(),
fsync(), ftruncate(), getcwd(), getegid(), geteuid(), getgid(), getgroups(), gethostid(), gethostname(),
getlogin(), getopt(), getpgid(), getpgrp(), getpid(), getppid(), getsid(), getuid(), isatty(), lchown(),
link(), lockf(), lseek(), nice(), pause(), pipe(), read(), readlink(), rmdir(), setegid(), seteuid(),
setgid(), setpgid(), setpgrp(), setregid(), setreuid(), setsid(), setuid(), sleep(), swab(), symlink(),
sync(), sysconf(), tcgetpgrp(), tcsetpgrp(), truncate(), ttyname(), unlink(), write()
COPYRIGHT
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 .