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unistd.h(0P)              POSIX Programmer's Manual             unistd.h(0P)

PROLOG         top

       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.

NAME         top

       unistd.h — standard symbolic constants and types

SYNOPSIS         top

       #include <unistd.h>

DESCRIPTION         top

       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 standard utilities.

       _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
       <inttypes.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;

       The following sections are informative.

APPLICATION USAGE         top

       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         top

       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         top

       None.

SEE ALSO         top

       inttypes.h(0p), limits.h(0p), stddef.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.1‐2008, 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)

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The Open
       Group Base Specifications Issue 7, Copyright (C) 2013 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
       applied.) 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.unix.org/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                 2013                        unistd.h(0P)