unistd.h(0p) — Linux manual page

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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 of the standard utilities that are provided in a
             manner accessible via the exec family of functions.

       _CS_POSIX_V7_ILP32_OFF32_CFLAGS
             If sysconf(_SC_V7_ILP32_OFF32) returns -1, the meaning of
             this value is unspecified. Otherwise, this value is the set
             of initial options to be given to the c99 utility to build
             an application using a programming model with 32-bit int,
             long, pointer, and off_t types.

       _CS_POSIX_V7_ILP32_OFF32_LDFLAGS
             If sysconf(_SC_V7_ILP32_OFF32) returns -1, the meaning of
             this value is unspecified. Otherwise, this value is the set
             of final options to be given to the c99 utility to build an
             application using a programming model with 32-bit int,
             long, pointer, and off_t types.

       _CS_POSIX_V7_ILP32_OFF32_LIBS
             If sysconf(_SC_V7_ILP32_OFF32) returns -1, the meaning of
             this value is unspecified. Otherwise, this value is the set
             of libraries to be given to the c99 utility to build an
             application using a programming model with 32-bit int,
             long, pointer, and off_t types.

       _CS_POSIX_V7_ILP32_OFFBIG_CFLAGS
             If sysconf(_SC_V7_ILP32_OFFBIG) returns -1, the meaning of
             this value is unspecified. Otherwise, this value is the set
             of initial options to be given to the c99 utility to build
             an application using a programming model with 32-bit int,
             long, and pointer types, and an off_t type using at least
             64 bits.

       _CS_POSIX_V7_ILP32_OFFBIG_LDFLAGS
             If sysconf(_SC_V7_ILP32_OFFBIG) returns -1, the meaning of
             this value is unspecified. Otherwise, this value is the set
             of final options to be given to the c99 utility to build an
             application using a programming model with 32-bit int,
             long, and pointer types, and an off_t type using at least
             64 bits.

       _CS_POSIX_V7_ILP32_OFFBIG_LIBS
             If sysconf(_SC_V7_ILP32_OFFBIG) returns -1, the meaning of
             this value is unspecified. Otherwise, this value is the set
             of libraries to be given to the c99 utility to build an
             application using a programming model with 32-bit int,
             long, and pointer types, and an off_t type using at least
             64 bits.

       _CS_POSIX_V7_LP64_OFF64_CFLAGS
             If sysconf(_SC_V7_LP64_OFF64) returns -1, the meaning of
             this value is unspecified. Otherwise, this value is the set
             of initial options to be given to the c99 utility to build
             an application using a programming model with 32-bit int
             and 64-bit long, pointer, and off_t types.

       _CS_POSIX_V7_LP64_OFF64_LDFLAGS
             If sysconf(_SC_V7_LP64_OFF64) returns -1, the meaning of
             this value is unspecified. Otherwise, this value is the set
             of final options to be given to the c99 utility to build an
             application using a programming model with 32-bit int and
             64-bit long, pointer, and off_t types.

       _CS_POSIX_V7_LP64_OFF64_LIBS
             If sysconf(_SC_V7_LP64_OFF64) returns -1, the meaning of
             this value is unspecified. Otherwise, this value is the set
             of libraries to be given to the c99 utility to build an
             application using a programming model with 32-bit int and
             64-bit long, pointer, and off_t types.

       _CS_POSIX_V7_LPBIG_OFFBIG_CFLAGS
             If sysconf(_SC_V7_LPBIG_OFFBIG) returns -1, the meaning of
             this value is unspecified. Otherwise, this value is the set
             of initial options to be given to the c99 utility to build
             an application using a programming model with an int type
             using at least 32 bits and long, pointer, and off_t types
             using at least 64 bits.

       _CS_POSIX_V7_LPBIG_OFFBIG_LDFLAGS
             If sysconf(_SC_V7_LPBIG_OFFBIG) returns -1, the meaning of
             this value is unspecified. Otherwise, this value is the set
             of final options to be given to the c99 utility to build an
             application using a programming model with an int type
             using at least 32 bits and long, pointer, and off_t types
             using at least 64 bits.

       _CS_POSIX_V7_LPBIG_OFFBIG_LIBS
             If sysconf(_SC_V7_LPBIG_OFFBIG) returns -1, the meaning of
             this value is unspecified. Otherwise, this value is the set
             of libraries to be given to the c99 utility to build an
             application using a programming model with an int type
             using at least 32 bits and long, pointer, and off_t types
             using at least 64 bits.

       _CS_POSIX_V7_THREADS_CFLAGS
             If sysconf(_SC_POSIX_THREADS) returns -1, the meaning of
             this value is unspecified. Otherwise, this value is the set
             of initial options to be given to the c99 utility to build
             a multi-threaded application. These flags are in addition
             to those associated with any of the other
             _CS_POSIX_V7_*_CFLAGS values used to specify particular
             type size programing environments.

       _CS_POSIX_V7_THREADS_LDFLAGS
             If sysconf(_SC_POSIX_THREADS) returns -1, the meaning of
             this value is unspecified. Otherwise, this value is the set
             of final options to be given to the c99 utility to build a
             multi-threaded application. These flags are in addition to
             those associated with any of the other
             _CS_POSIX_V7_*_LDFLAGS values used to specify particular
             type size programing environments.

       _CS_POSIX_V7_WIDTH_RESTRICTED_ENVS
             This value is a <newline>-separated list of names of
             programming environments supported by the implementation in
             which the widths of the blksize_t, cc_t, mode_t, nfds_t,
             pid_t, ptrdiff_t, size_t, speed_t, ssize_t, suseconds_t,
             tcflag_t, wchar_t, and wint_t types are no greater than the
             width of type long.  The format of each name shall be
             suitable for use with the getconf -v option.

       _CS_V7_ENV
             This is the value that provides the environment variable
             information (other than that provided by _CS_PATH) that is
             required by the implementation to create a conforming
             environment, as described in the implementation's
             conformance documentation.

       The following symbolic constants are reserved for compatibility
       with Issue 6:

       _CS_POSIX_V6_ILP32_OFF32_CFLAGS
       _CS_POSIX_V6_ILP32_OFF32_LDFLAGS
       _CS_POSIX_V6_ILP32_OFF32_LIBS
       _CS_POSIX_V6_ILP32_OFFBIG_CFLAGS
       _CS_POSIX_V6_ILP32_OFFBIG_LDFLAGS
       _CS_POSIX_V6_ILP32_OFFBIG_LIBS
       _CS_POSIX_V6_LP64_OFF64_CFLAGS
       _CS_POSIX_V6_LP64_OFF64_LDFLAGS
       _CS_POSIX_V6_LP64_OFF64_LIBS
       _CS_POSIX_V6_LPBIG_OFFBIG_CFLAGS
       _CS_POSIX_V6_LPBIG_OFFBIG_LDFLAGS
       _CS_POSIX_V6_LPBIG_OFFBIG_LIBS
       _CS_POSIX_V6_WIDTH_RESTRICTED_ENVS
       _CS_V6_ENV

       The <unistd.h> header shall define SEEK_CUR, SEEK_END, and
       SEEK_SET as described in <stdio.h>.

       The <unistd.h> header shall define the following symbolic
       constants as possible values for the function argument to the
       lockf() function:

       F_LOCK      Lock a section for exclusive use.

       F_TEST      Test section for locks by other processes.

       F_TLOCK     Test and lock a section for exclusive use.

       F_ULOCK     Unlock locked sections.

       The <unistd.h> header shall define the following symbolic
       constants for pathconf():

       _PC_2_SYMLINKS
       _PC_ALLOC_SIZE_MIN
       _PC_ASYNC_IO
       _PC_CHOWN_RESTRICTED
       _PC_FILESIZEBITS
       _PC_LINK_MAX
       _PC_MAX_CANON
       _PC_MAX_INPUT
       _PC_NAME_MAX
       _PC_NO_TRUNC
       _PC_PATH_MAX
       _PC_PIPE_BUF
       _PC_PRIO_IO
       _PC_REC_INCR_XFER_SIZE
       _PC_REC_MAX_XFER_SIZE
       _PC_REC_MIN_XFER_SIZE
       _PC_REC_XFER_ALIGN
       _PC_SYMLINK_MAX
       _PC_SYNC_IO
       _PC_TIMESTAMP_RESOLUTION
       _PC_VDISABLE

       The <unistd.h> header shall define the following symbolic
       constants for sysconf():

       _SC_2_C_BIND
       _SC_2_C_DEV
       _SC_2_CHAR_TERM
       _SC_2_FORT_DEV
       _SC_2_FORT_RUN
       _SC_2_LOCALEDEF
       _SC_2_PBS
       _SC_2_PBS_ACCOUNTING
       _SC_2_PBS_CHECKPOINT
       _SC_2_PBS_LOCATE
       _SC_2_PBS_MESSAGE
       _SC_2_PBS_TRACK
       _SC_2_SW_DEV
       _SC_2_UPE
       _SC_2_VERSION
       _SC_ADVISORY_INFO
       _SC_AIO_LISTIO_MAX
       _SC_AIO_MAX
       _SC_AIO_PRIO_DELTA_MAX
       _SC_ARG_MAX
       _SC_ASYNCHRONOUS_IO
       _SC_ATEXIT_MAX
       _SC_BARRIERS
       _SC_BC_BASE_MAX
       _SC_BC_DIM_MAX
       _SC_BC_SCALE_MAX
       _SC_BC_STRING_MAX
       _SC_CHILD_MAX
       _SC_CLK_TCK
       _SC_CLOCK_SELECTION
       _SC_COLL_WEIGHTS_MAX
       _SC_CPUTIME
       _SC_DELAYTIMER_MAX
       _SC_EXPR_NEST_MAX
       _SC_FSYNC
       _SC_GETGR_R_SIZE_MAX
       _SC_GETPW_R_SIZE_MAX
       _SC_HOST_NAME_MAX
       _SC_IOV_MAX
       _SC_IPV6
       _SC_JOB_CONTROL
       _SC_LINE_MAX
       _SC_LOGIN_NAME_MAX
       _SC_MAPPED_FILES
       _SC_MEMLOCK
       _SC_MEMLOCK_RANGE
       _SC_MEMORY_PROTECTION
       _SC_MESSAGE_PASSING
       _SC_MONOTONIC_CLOCK
       _SC_MQ_OPEN_MAX
       _SC_MQ_PRIO_MAX
       _SC_NGROUPS_MAX
       _SC_OPEN_MAX
       _SC_PAGE_SIZE
       _SC_PAGESIZE
       _SC_PRIORITIZED_IO
       _SC_PRIORITY_SCHEDULING
       _SC_RAW_SOCKETS
       _SC_RE_DUP_MAX
       _SC_READER_WRITER_LOCKS
       _SC_REALTIME_SIGNALS
       _SC_REGEXP
       _SC_RTSIG_MAX
       _SC_SAVED_IDS
       _SC_SEM_NSEMS_MAX
       _SC_SEM_VALUE_MAX
       _SC_SEMAPHORES
       _SC_SHARED_MEMORY_OBJECTS
       _SC_SHELL
       _SC_SIGQUEUE_MAX
       _SC_SPAWN
       _SC_SPIN_LOCKS
       _SC_SPORADIC_SERVER
       _SC_SS_REPL_MAX
       _SC_STREAM_MAX
       _SC_SYMLOOP_MAX
       _SC_SYNCHRONIZED_IO
       _SC_THREAD_ATTR_STACKADDR
       _SC_THREAD_ATTR_STACKSIZE
       _SC_THREAD_CPUTIME
       _SC_THREAD_DESTRUCTOR_ITERATIONS
       _SC_THREAD_KEYS_MAX
       _SC_THREAD_PRIO_INHERIT
       _SC_THREAD_PRIO_PROTECT
       _SC_THREAD_PRIORITY_SCHEDULING
       _SC_THREAD_PROCESS_SHARED
       _SC_THREAD_ROBUST_PRIO_INHERIT
       _SC_THREAD_ROBUST_PRIO_PROTECT
       _SC_THREAD_SAFE_FUNCTIONS
       _SC_THREAD_SPORADIC_SERVER
       _SC_THREAD_STACK_MIN
       _SC_THREAD_THREADS_MAX
       _SC_THREADS
       _SC_TIMEOUTS
       _SC_TIMER_MAX
       _SC_TIMERS
       _SC_TRACE
       _SC_TRACE_EVENT_FILTER
       _SC_TRACE_EVENT_NAME_MAX
       _SC_TRACE_INHERIT
       _SC_TRACE_LOG
       _SC_TRACE_NAME_MAX
       _SC_TRACE_SYS_MAX
       _SC_TRACE_USER_EVENT_MAX
       _SC_TTY_NAME_MAX
       _SC_TYPED_MEMORY_OBJECTS
       _SC_TZNAME_MAX
       _SC_V7_ILP32_OFF32
       _SC_V7_ILP32_OFFBIG
       _SC_V7_LP64_OFF64
       _SC_V7_LPBIG_OFFBIG
       _SC_V6_ILP32_OFF32
       _SC_V6_ILP32_OFFBIG
       _SC_V6_LP64_OFF64
       _SC_V6_LPBIG_OFFBIG
       _SC_VERSION
       _SC_XOPEN_CRYPT
       _SC_XOPEN_ENH_I18N
       _SC_XOPEN_REALTIME
       _SC_XOPEN_REALTIME_THREADS
       _SC_XOPEN_SHM
       _SC_XOPEN_STREAMS
       _SC_XOPEN_UNIX
       _SC_XOPEN_UUCP
       _SC_XOPEN_VERSION

       The two constants _SC_PAGESIZE and _SC_PAGE_SIZE may be defined
       to have the same value.

       The <unistd.h> header shall define the following symbolic
       constants for file streams:

       STDERR_FILENO File number of stderr; 2.

       STDIN_FILENO  File number of stdin; 0.

       STDOUT_FILENO File number of stdout; 1.

       The <unistd.h> header shall define the following symbolic
       constant for terminal special character handling:

       _POSIX_VDISABLE
                     This symbol shall be defined to be the value of a
                     character that shall disable terminal special
                     character handling as described in Section 11.2.6,
                     Special Control Characters.  This symbol shall
                     always be set to a value other than -1.

   Type Definitions
       The <unistd.h> header shall define the size_t, ssize_t, uid_t,
       gid_t, off_t, and pid_t types as described in <sys/types.h>.

       The <unistd.h> header shall define the intptr_t type as described
       in <stdint.h>.

   Declarations
       The following shall be declared as functions and may also be
       defined as macros. Function prototypes shall be provided.

           int          access(const char *, int);
           unsigned     alarm(unsigned);
           int          chdir(const char *);
           int          chown(const char *, uid_t, gid_t);
           int          close(int);
           size_t       confstr(int, char *, size_t);
           char        *crypt(const char *, const char *);
           int          dup(int);
           int          dup2(int, int);
           void         _exit(int);
           void         encrypt(char [64], int);
           int          execl(const char *, const char *, ...);
           int          execle(const char *, const char *, ...);
           int          execlp(const char *, const char *, ...);
           int          execv(const char *, char *const []);
           int          execve(const char *, char *const [], char *const []);
           int          execvp(const char *, char *const []);
           int          faccessat(int, const char *, int, int);
           int          fchdir(int);
           int          fchown(int, uid_t, gid_t);
           int          fchownat(int, const char *, uid_t, gid_t, int);
           int          fdatasync(int);
           int          fexecve(int, char *const [], char *const []);
           pid_t        fork(void);
           long         fpathconf(int, int);
           int          fsync(int);
           int          ftruncate(int, off_t);
           char        *getcwd(char *, size_t);
           gid_t        getegid(void);
           uid_t        geteuid(void);
           gid_t        getgid(void);
           int          getgroups(int, gid_t []);
           long         gethostid(void);
           int          gethostname(char *, size_t);
           char        *getlogin(void);
           int          getlogin_r(char *, size_t);
           int          getopt(int, char * const [], const char *);
           pid_t        getpgid(pid_t);
           pid_t        getpgrp(void);
           pid_t        getpid(void);
           pid_t        getppid(void);
           pid_t        getsid(pid_t);
           uid_t        getuid(void);
           int          isatty(int);
           int          lchown(const char *, uid_t, gid_t);
           int          link(const char *, const char *);
           int          linkat(int, const char *, int, const char *, int);
           int          lockf(int, int, off_t);
           off_t        lseek(int, off_t, int);
           int          nice(int);
           long         pathconf(const char *, int);
           int          pause(void);
           int          pipe(int [2]);
           ssize_t      pread(int, void *, size_t, off_t);
           ssize_t      pwrite(int, const void *, size_t, off_t);
           ssize_t      read(int, void *, size_t);
           ssize_t      readlink(const char *restrict, char *restrict, size_t);
           ssize_t      readlinkat(int, const char *restrict, char *restrict, size_t);
           int          rmdir(const char *);
           int          setegid(gid_t);
           int          seteuid(uid_t);
           int          setgid(gid_t);
           int          setpgid(pid_t, pid_t);
           pid_t        setpgrp(void);
           int          setregid(gid_t, gid_t);
           int          setreuid(uid_t, uid_t);
           pid_t        setsid(void);
           int          setuid(uid_t);
           unsigned     sleep(unsigned);
           void         swab(const void *restrict, void *restrict, ssize_t);
           int          symlink(const char *, const char *);
           int          symlinkat(const char *, int, const char *);
           void         sync(void);
           long         sysconf(int);
           pid_t        tcgetpgrp(int);
           int          tcsetpgrp(int, pid_t);
           int          truncate(const char *, off_t);
           char        *ttyname(int);
           int          ttyname_r(int, char *, size_t);
           int          unlink(const char *);
           int          unlinkat(int, const char *, int);
           ssize_t      write(int, const void *, size_t);

       Implementations may also include the pthread_atfork() prototype
       as defined in <pthread.h>.  Implementations may also include the
       ctermid() prototype as defined in <stdio.h>.

       The <unistd.h> header shall declare the following external
       variables:

           extern char  *optarg;
           extern int    opterr, optind, optopt;

       Inclusion of the <unistd.h> header may make visible all symbols
       from the headers <stddef.h>, <stdint.h>, and <stdio.h>.

       The following sections are informative.

APPLICATION USAGE         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

       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)

COPYRIGHT         top

       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)

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