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unistd.h(0P) POSIX Programmer's Manual unistd.h(0P)
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 may not be implemented on Linux.
unistd.h — standard symbolic constants and types
#include <unistd.h>
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.
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.
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.
None.
limits.h(0p), stddef.h(0p), stdint.h(0p), stdio.h(0p),
sys_socket.h(0p), sys_types.h(0p), termios.h(0p), wctype.h(0p)
The System Interfaces volume of POSIX.1‐2017, access(3p),
alarm(3p), chown(3p), close(3p), confstr(3p), crypt(3p),
ctermid(3p), dup(3p), _Exit(3p), encrypt(3p), exec(1p),
fchdir(3p), fchown(3p), fdatasync(3p), fork(3p), fpathconf(3p),
fsync(3p), ftruncate(3p), getcwd(3p), getegid(3p), geteuid(3p),
getgid(3p), getgroups(3p), gethostid(3p), gethostname(3p),
getlogin(3p), getopt(3p), getpgid(3p), getpgrp(3p), getpid(3p),
getppid(3p), getsid(3p), getuid(3p), isatty(3p), lchown(3p),
link(3p), lockf(3p), lseek(3p), nice(3p), pause(3p), pipe(3p),
read(3p), readlink(3p), rmdir(3p), setegid(3p), seteuid(3p),
setgid(3p), setpgid(3p), setpgrp(3p), setregid(3p), setreuid(3p),
setsid(3p), setuid(3p), sleep(3p), swab(3p), symlink(3p),
sync(3p), sysconf(3p), tcgetpgrp(3p), tcsetpgrp(3p),
truncate(3p), ttyname(3p), unlink(3p), write(3p)
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 .
IEEE/The Open Group 2017 unistd.h(0P)
Pages that refer to this page: fcntl.h(0p), limits.h(0p), stropts.h(0p), ar(1p), access(3p), alarm(3p), chdir(3p), chown(3p), close(3p), confstr(3p), crypt(3p), dup(3p), encrypt(3p), exec(3p), _Exit(3p), fchdir(3p), fchown(3p), fdatasync(3p), fork(3p), fpathconf(3p), fsync(3p), ftruncate(3p), getcwd(3p), getegid(3p), geteuid(3p), getgid(3p), getgroups(3p), gethostid(3p), gethostname(3p), getlogin(3p), getopt(3p), getpgid(3p), getpgrp(3p), getpid(3p), getppid(3p), getsid(3p), getuid(3p), isatty(3p), lchown(3p), link(3p), lockf(3p), lseek(3p), nice(3p), pause(3p), pipe(3p), read(3p), readlink(3p), rmdir(3p), setegid(3p), setenv(3p), seteuid(3p), setgid(3p), setpgid(3p), setpgrp(3p), setregid(3p), setreuid(3p), setsid(3p), setuid(3p), sleep(3p), stdin(3p), swab(3p), symlink(3p), sync(3p), sysconf(3p), tcgetpgrp(3p), tcsetpgrp(3p), truncate(3p), ttyname(3p), unlink(3p), write(3p)
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