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PROLOG

       This  manual  page  is part of the POSIX Programmer's Manual.  The Linux implementation of
       this interface may differ (consult the corresponding Linux  manual  page  for  details  of
       Linux behavior), or the interface may not be implemented on Linux.

NAME

       unistd.h — standard symbolic constants and types

SYNOPSIS

       #include <unistd.h>

DESCRIPTION

       The  <unistd.h>  header  defines  miscellaneous symbolic constants and types, and declares
       miscellaneous functions. The actual values of the  constants  are  unspecified  except  as
       shown. The contents of this header are shown below.

   Version Test Macros
       The  <unistd.h>  header shall define the following symbolic constants. The values shall be
       suitable for use in #if preprocessing directives.

       _POSIX_VERSION
             Integer value indicating version of this standard (C-language binding) to which  the
             implementation  conforms.  For implementations conforming to POSIX.1‐2008, the value
             shall be 200809L.

       _POSIX2_VERSION
             Integer value indicating version of the Shell and Utilities  volume  of  POSIX.1  to
             which  the  implementation conforms. For implementations conforming to POSIX.1‐2008,
             the  value   shall   be   200809L.   For   profile   implementations   that   define
             _POSIX_SUBPROFILE  (see Section 2.1.5.1, Subprofiling Considerations) in <unistd.h>,
             _POSIX2_VERSION may be left undefined or be defined with the value  -1  to  indicate
             that  the  Shell  and  Utilities volume of POSIX.1 is not supported. In this case, a
             call to sysconf(_SC_2_VERSION) shall return either 200809L or -1 indicating that the
             Shell  and  Utilities  volume  of  POSIX.1  is or is not, respectively, supported at
             runtime.

       The  <unistd.h>  header  shall  define  the  following  symbolic  constant  only  if   the
       implementation  supports  the XSI option; see Section 2.1.4, XSI Conformance.  If defined,
       its value shall be suitable for use in #if preprocessing directives.

       _XOPEN_VERSION
             Integer value indicating version of  the  X/Open  Portability  Guide  to  which  the
             implementation conforms. The value shall be 700.

   Constants for Options and Option Groups
       The  following  symbolic constants, if defined in <unistd.h>, shall have a value of -1, 0,
       or greater, unless otherwise specified below.  For  profile  implementations  that  define
       _POSIX_SUBPROFILE  (see  Section  2.1.5.1,  Subprofiling  Considerations)  in  <unistd.h>,
       constants described below as always having a value greater than zero need not  be  defined
       and,  if defined, may have a value of -1, 0, or greater.  The values shall be suitable for
       use in #if preprocessing directives.

       If a symbolic constant is not defined or is defined with the value -1, the option  is  not
       supported  for  compilation.  If  it is defined with a value greater than zero, the option
       shall always be supported when the application is executed. If  it  is  defined  with  the
       value  zero,  the  option  shall  be  supported  for compilation and might or might not be
       supported at runtime. See  Section  2.1.6,  Options  for  further  information  about  the
       conformance requirements of these three categories of support.

       _POSIX_ADVISORY_INFO
             The  implementation  supports  the  Advisory  Information option.  If this symbol is
             defined in <unistd.h>, it shall be defined to be -1, 0, or  200809L.  The  value  of
             this symbol reported by sysconf() shall either be -1 or 200809L.

       _POSIX_ASYNCHRONOUS_IO
             The implementation supports asynchronous input and output.  This symbol shall always
             be set to the value 200809L.

       _POSIX_BARRIERS
             The implementation supports barriers.  This symbol shall always be set to the  value
             200809L.

       _POSIX_CHOWN_RESTRICTED
             The  use  of  chown()  and  fchown()  is  restricted  to  a process with appropriate
             privileges, and to changing the group ID of a file only to the effective group ID of
             the  process  or to one of its supplementary group IDs. This symbol shall be defined
             with a value other than -1.

       _POSIX_CLOCK_SELECTION
             The implementation supports clock selection.  This symbol shall always be set to the
             value 200809L.

       _POSIX_CPUTIME
             The  implementation  supports the Process CPU-Time Clocks option.  If this symbol is
             defined in <unistd.h>, it shall be defined to be -1, 0, or  200809L.  The  value  of
             this symbol reported by sysconf() shall either be -1 or 200809L.

       _POSIX_FSYNC
             The  implementation  supports  the  File  Synchronization option.  If this symbol is
             defined in <unistd.h>, it shall be defined to be -1, 0, or  200809L.  The  value  of
             this symbol reported by sysconf() shall either be -1 or 200809L.

       _POSIX_IPV6
             The  implementation  supports  the  IPv6  option.   If  this  symbol  is  defined in
             <unistd.h>, it shall be defined to be -1, 0, or 200809L. The value  of  this  symbol
             reported by sysconf() shall either be -1 or 200809L.

       _POSIX_JOB_CONTROL
             The  implementation supports job control. This symbol shall always be set to a value
             greater than zero.

       _POSIX_MAPPED_FILES
             The implementation supports memory mapped Files.  This symbol shall always be set to
             the value 200809L.

       _POSIX_MEMLOCK
             The  implementation  supports  the Process Memory Locking option.  If this symbol is
             defined in <unistd.h>, it shall be defined to be -1, 0, or  200809L.  The  value  of
             this symbol reported by sysconf() shall either be -1 or 200809L.

       _POSIX_MEMLOCK_RANGE
             The  implementation  supports  the  Range  Memory Locking option.  If this symbol is
             defined in <unistd.h>, it shall be defined to be -1, 0, or  200809L.  The  value  of
             this symbol reported by sysconf() shall either be -1 or 200809L.

       _POSIX_MEMORY_PROTECTION
             The  implementation  supports memory protection.  This symbol shall always be set to
             the value 200809L.

       _POSIX_MESSAGE_PASSING
             The implementation supports the Message Passing option.  If this symbol  is  defined
             in <unistd.h>, it shall be defined to be -1, 0, or 200809L. The value of this symbol
             reported by sysconf() shall either be -1 or 200809L.

       _POSIX_MONOTONIC_CLOCK
             The implementation supports the Monotonic Clock option.  If this symbol  is  defined
             in <unistd.h>, it shall be defined to be -1, 0, or 200809L. The value of this symbol
             reported by sysconf() shall either be -1 or 200809L.

       _POSIX_NO_TRUNC
             Pathname components longer than {NAME_MAX} generate an error. This symbol  shall  be
             defined with a value other than -1.

       _POSIX_PRIORITIZED_IO
             The implementation supports the Prioritized Input and Output option.  If this symbol
             is defined in <unistd.h>, it shall be defined to be -1, 0, or 200809L. The value  of
             this symbol reported by sysconf() shall either be -1 or 200809L.

       _POSIX_PRIORITY_SCHEDULING
             The  implementation  supports  the  Process  Scheduling  option.   If this symbol is
             defined in <unistd.h>, it shall be defined to be -1, 0, or  200809L.  The  value  of
             this symbol reported by sysconf() shall either be -1 or 200809L.

       _POSIX_RAW_SOCKETS
             The  implementation  supports  the Raw Sockets option.  If this symbol is defined in
             <unistd.h>, it shall be defined to be -1, 0, or 200809L. The value  of  this  symbol
             reported by sysconf() shall either be -1 or 200809L.

       _POSIX_READER_WRITER_LOCKS
             The  implementation  supports  read-write locks.  This symbol shall always be set to
             the value 200809L.

       _POSIX_REALTIME_SIGNALS
             The implementation supports realtime signals.  This symbol shall always  be  set  to
             the value 200809L.

       _POSIX_REGEXP
             The  implementation  supports  the  Regular Expression Handling option.  This symbol
             shall always be set to a value greater than zero.

       _POSIX_SAVED_IDS
             Each process has a saved set-user-ID and a saved set-group-ID.   This  symbol  shall
             always be set to a value greater than zero.

       _POSIX_SEMAPHORES
             The  implementation  supports  semaphores.   This  symbol shall always be set to the
             value 200809L.

       _POSIX_SHARED_MEMORY_OBJECTS
             The implementation supports the Shared Memory Objects option.   If  this  symbol  is
             defined  in  <unistd.h>,  it  shall be defined to be -1, 0, or 200809L. The value of
             this symbol reported by sysconf() shall either be -1 or 200809L.

       _POSIX_SHELL
             The implementation supports the POSIX shell. This symbol shall always be  set  to  a
             value greater than zero.

       _POSIX_SPAWN
             The  implementation  supports  the  Spawn  option.   If  this  symbol  is defined in
             <unistd.h>, it shall be defined to be -1, 0, or 200809L. The value  of  this  symbol
             reported by sysconf() shall either be -1 or 200809L.

       _POSIX_SPIN_LOCKS
             The  implementation  supports  spin  locks.   This symbol shall always be set to the
             value 200809L.

       _POSIX_SPORADIC_SERVER
             The implementation supports the Process Sporadic Server option.  If this  symbol  is
             defined  in  <unistd.h>,  it  shall be defined to be -1, 0, or 200809L. The value of
             this symbol reported by sysconf() shall either be -1 or 200809L.

       _POSIX_SYNCHRONIZED_IO
             The implementation supports the Synchronized  Input  and  Output  option.   If  this
             symbol  is  defined  in <unistd.h>, it shall be defined to be -1, 0, or 200809L. The
             value of this symbol reported by sysconf() shall either be -1 or 200809L.

       _POSIX_THREAD_ATTR_STACKADDR
             The implementation supports the Thread Stack  Address  Attribute  option.   If  this
             symbol  is  defined  in <unistd.h>, it shall be defined to be -1, 0, or 200809L. The
             value of this symbol reported by sysconf() shall either be -1 or 200809L.

       _POSIX_THREAD_ATTR_STACKSIZE
             The implementation supports the Thread Stack Size Attribute option.  If this  symbol
             is  defined in <unistd.h>, it shall be defined to be -1, 0, or 200809L. The value of
             this symbol reported by sysconf() shall either be -1 or 200809L.

       _POSIX_THREAD_CPUTIME
             The implementation supports the Thread CPU-Time Clocks option.  If  this  symbol  is
             defined  in  <unistd.h>,  it  shall be defined to be -1, 0, or 200809L. The value of
             this symbol reported by sysconf() shall either be -1 or 200809L.

       _POSIX_THREAD_PRIO_INHERIT
             The implementation supports the Non-Robust Mutex  Priority  Inheritance  option.  If
             this  symbol  is defined in <unistd.h>, it shall be defined to be -1, 0, or 200809L.
             The value of this symbol reported by sysconf() shall either be -1 or 200809L.

       _POSIX_THREAD_PRIO_PROTECT
             The implementation supports the Non-Robust Mutex Priority Protection option. If this
             symbol  is  defined  in <unistd.h>, it shall be defined to be -1, 0, or 200809L. The
             value of this symbol reported by sysconf() shall either be -1 or 200809L.

       _POSIX_THREAD_PRIORITY_SCHEDULING
             The implementation supports the Thread Execution Scheduling option.  If this  symbol
             is  defined in <unistd.h>, it shall be defined to be -1, 0, or 200809L. The value of
             this symbol reported by sysconf() shall either be -1 or 200809L.

       _POSIX_THREAD_PROCESS_SHARED
             The implementation supports the Thread Process-Shared  Synchronization  option.   If
             this  symbol  is defined in <unistd.h>, it shall be defined to be -1, 0, or 200809L.
             The value of this symbol reported by sysconf() shall either be -1 or 200809L.

       _POSIX_THREAD_ROBUST_PRIO_INHERIT
             The implementation supports the Robust Mutex Priority Inheritance  option.  If  this
             symbol  is  defined  in <unistd.h>, it shall be defined to be -1, 0, or 200809L. The
             value of this symbol reported by sysconf() shall either be -1 or 200809L.

       _POSIX_THREAD_ROBUST_PRIO_PROTECT
             The implementation supports the Robust Mutex Priority  Protection  option.  If  this
             symbol  is  defined  in <unistd.h>, it shall be defined to be -1, 0, or 200809L. The
             value of this symbol reported by sysconf() shall either be -1 or 200809L.

       _POSIX_THREAD_SAFE_FUNCTIONS
             The implementation supports thread-safe functions.  This symbol shall always be  set
             to the value 200809L.

       _POSIX_THREAD_SPORADIC_SERVER
             The  implementation  supports  the Thread Sporadic Server option.  If this symbol is
             defined in <unistd.h>, it shall be defined to be -1, 0, or  200809L.  The  value  of
             this symbol reported by sysconf() shall either be -1 or 200809L.

       _POSIX_THREADS
             The  implementation  supports threads.  This symbol shall always be set to the value
             200809L.

       _POSIX_TIMEOUTS
             The implementation supports timeouts.  This symbol shall always be set to the  value
             200809L.

       _POSIX_TIMERS
             The  implementation  supports  timers.  This symbol shall always be set to the value
             200809L.

       _POSIX_TRACE
             The implementation supports  the  Trace  option.   If  this  symbol  is  defined  in
             <unistd.h>,  it  shall  be defined to be -1, 0, or 200809L. The value of this symbol
             reported by sysconf() shall either be -1 or 200809L.

       _POSIX_TRACE_EVENT_FILTER
             The implementation supports the Trace  Event  Filter  option.   If  this  symbol  is
             defined  in  <unistd.h>,  it  shall be defined to be -1, 0, or 200809L. The value of
             this symbol reported by sysconf() shall either be -1 or 200809L.

       _POSIX_TRACE_INHERIT
             The implementation supports the Trace Inherit option.  If this symbol is defined  in
             <unistd.h>,  it  shall  be defined to be -1, 0, or 200809L. The value of this symbol
             reported by sysconf() shall either be -1 or 200809L.

       _POSIX_TRACE_LOG
             The implementation supports the Trace Log option.  If  this  symbol  is  defined  in
             <unistd.h>,  it  shall  be defined to be -1, 0, or 200809L. The value of this symbol
             reported by sysconf() shall either be -1 or 200809L.

       _POSIX_TYPED_MEMORY_OBJECTS
             The implementation supports the Typed Memory Objects  option.   If  this  symbol  is
             defined  in  <unistd.h>,  it  shall be defined to be -1, 0, or 200809L. The value of
             this symbol reported by sysconf() shall either be -1 or 200809L.

       _POSIX_V6_ILP32_OFF32
             The implementation provides a C-language compilation environment  with  32-bit  int,
             long, pointer, and off_t types.

       _POSIX_V6_ILP32_OFFBIG
             The  implementation  provides  a C-language compilation environment with 32-bit int,
             long, and pointer types and an off_t type using at least 64 bits.

       _POSIX_V6_LP64_OFF64
             The implementation provides a C-language compilation environment with 32-bit int and
             64-bit long, pointer, and off_t types.

       _POSIX_V6_LPBIG_OFFBIG
             The  implementation  provides  a C-language compilation environment with an int type
             using at least 32 bits and long, pointer, and off_t types using at least 64 bits.

       _POSIX_V7_ILP32_OFF32
             The implementation provides a C-language compilation environment  with  32-bit  int,
             long, pointer, and off_t types.

       _POSIX_V7_ILP32_OFFBIG
             The  implementation  provides  a C-language compilation environment with 32-bit int,
             long, and pointer types and an off_t type using at least 64 bits.

       _POSIX_V7_LP64_OFF64
             The implementation provides a C-language compilation environment with 32-bit int and
             64-bit long, pointer, and off_t types.

       _POSIX_V7_LPBIG_OFFBIG
             The  implementation  provides  a C-language compilation environment with an int type
             using at least 32 bits and long, pointer, and off_t types using at least 64 bits.

       _POSIX2_C_BIND
             The implementation supports the C-Language Binding option. This symbol shall  always
             have the value 200809L.

       _POSIX2_C_DEV
             The  implementation  supports  the C-Language Development Utilities option.  If this
             symbol is defined in <unistd.h>, it shall be defined to be -1, 0,  or  200809L.  The
             value of this symbol reported by sysconf() shall either be -1 or 200809L.

       _POSIX2_CHAR_TERM
             The  implementation supports the Terminal Characteristics option.  The value of this
             symbol reported by sysconf() shall either be -1 or a value greater than zero.

       _POSIX2_FORT_DEV
             The implementation supports the  FORTRAN  Development  Utilities  option.   If  this
             symbol  is  defined  in <unistd.h>, it shall be defined to be -1, 0, or 200809L. The
             value of this symbol reported by sysconf() shall either be -1 or 200809L.

       _POSIX2_FORT_RUN
             The implementation supports the FORTRAN Runtime Utilities option.  If this symbol is
             defined  in  <unistd.h>,  it  shall be defined to be -1, 0, or 200809L. The value of
             this symbol reported by sysconf() shall either be -1 or 200809L.

       _POSIX2_LOCALEDEF
             The implementation supports the creation of locales by the  localedef  utility.   If
             this  symbol  is defined in <unistd.h>, it shall be defined to be -1, 0, or 200809L.
             The value of this symbol reported by sysconf() shall either be -1 or 200809L.

       _POSIX2_PBS
             The implementation supports the Batch Environment Services and Utilities option.  If
             this  symbol  is defined in <unistd.h>, it shall be defined to be -1, 0, or 200809L.
             The value of this symbol reported by sysconf() shall either be -1 or 200809L.

       _POSIX2_PBS_ACCOUNTING
             The implementation supports the Batch Accounting option.  If this symbol is  defined
             in <unistd.h>, it shall be defined to be -1, 0, or 200809L. The value of this symbol
             reported by sysconf() shall either be -1 or 200809L.

       _POSIX2_PBS_CHECKPOINT
             The implementation supports the Batch Checkpoint/Restart option.  If this symbol  is
             defined  in  <unistd.h>,  it  shall be defined to be -1, 0, or 200809L. The value of
             this symbol reported by sysconf() shall either be -1 or 200809L.

       _POSIX2_PBS_LOCATE
             The implementation supports the Locate Batch Job Request option.  If this symbol  is
             defined  in  <unistd.h>,  it  shall be defined to be -1, 0, or 200809L. The value of
             this symbol reported by sysconf() shall either be -1 or 200809L.

       _POSIX2_PBS_MESSAGE
             The implementation supports the Batch Job Message Request option.  If this symbol is
             defined  in  <unistd.h>,  it  shall be defined to be -1, 0, or 200809L. The value of
             this symbol reported by sysconf() shall either be -1 or 200809L.

       _POSIX2_PBS_TRACK
             The implementation supports the Track Batch Job Request option.  If this  symbol  is
             defined  in  <unistd.h>,  it  shall be defined to be -1, 0, or 200809L. The value of
             this symbol reported by sysconf() shall either be -1 or 200809L.

       _POSIX2_SW_DEV
             The implementation supports the Software  Development  Utilities  option.   If  this
             symbol  is  defined  in <unistd.h>, it shall be defined to be -1, 0, or 200809L. The
             value of this symbol reported by sysconf() shall either be -1 or 200809L.

       _POSIX2_UPE
             The implementation supports the User Portability Utilities option.  If  this  symbol
             is  defined in <unistd.h>, it shall be defined to be -1, 0, or 200809L. The value of
             this symbol reported by sysconf() shall either be -1 or 200809L.

       _XOPEN_CRYPT
             The implementation supports the X/Open Encryption Option Group.

       _XOPEN_ENH_I18N
             The implementation supports the Issue 4,  Version  2  Enhanced  Internationalization
             Option Group. This symbol shall always be set to a value other than -1.

       _XOPEN_REALTIME
             The implementation supports the X/Open Realtime Option Group.

       _XOPEN_REALTIME_THREADS
             The implementation supports the X/Open Realtime Threads Option Group.

       _XOPEN_SHM
             The  implementation supports the Issue 4, Version 2 Shared Memory Option Group. This
             symbol shall always be set to a value other than -1.

       _XOPEN_STREAMS
             The implementation supports the XSI STREAMS Option Group.

       _XOPEN_UNIX
             The implementation supports the XSI option.

       _XOPEN_UUCP
             The implementation supports the UUCP Utilities option. If this symbol is defined  in
             <unistd.h>,  it  shall  be defined to be -1, 0, or 200809L. The value of this symbol
             reported by sysconf() shall be either -1 or 200809L.

   Execution-Time Symbolic Constants
       If any of the following symbolic constants are not defined in the <unistd.h>  header,  the
       value shall vary depending on the file to which it is applied. If defined, they shall have
       values suitable for use in #if preprocessing directives.

       If any of the following symbolic constants are defined to have value -1 in the  <unistd.h>
       header, the implementation shall not provide the option on any file; if any are defined to
       have a value other than -1 in the <unistd.h> header, the implementation shall provide  the
       option on all applicable files.

       All  of  the following values, whether defined as symbolic constants in <unistd.h> or not,
       may be queried with respect to  a  specific  file  using  the  pathconf()  or  fpathconf()
       functions:

       _POSIX_ASYNC_IO
             Asynchronous input or output operations may be performed for the associated file.

       _POSIX_PRIO_IO
             Prioritized input or output operations may be performed for the associated file.

       _POSIX_SYNC_IO
             Synchronized input or output operations may be performed for the associated file.

       If  the  following  symbolic constants are defined in the <unistd.h> header, they apply to
       files and all paths in all file systems on the implementation:

       _POSIX_TIMESTAMP_RESOLUTION
             The resolution in nanoseconds for all file timestamps.

       _POSIX2_SYMLINKS
             Symbolic links can be created.

   Constants for Functions
       The <unistd.h> header shall define NULL as described in <stddef.h>.

       The <unistd.h> header shall define the following  symbolic  constants  for  use  with  the
       access() function. The values shall be suitable for use in #if preprocessing directives.

       F_OK        Test for existence of file.

       R_OK        Test for read permission.

       W_OK        Test for write permission.

       X_OK        Test for execute (search) permission.

       The  constants  F_OK,  R_OK,  W_OK, and X_OK and the expressions R_OK|W_OK, R_OK|X_OK, and
       R_OK|W_OK|X_OK shall all have distinct values.

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

       _CS_PATH
             This  is  the value for the PATH environment variable that finds all of the standard
             utilities that are provided in a manner accessible via the exec family of functions.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       F_LOCK      Lock a section for exclusive use.

       F_TEST      Test section for locks by other processes.

       F_TLOCK     Test and lock a section for exclusive use.

       F_ULOCK     Unlock locked sections.

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

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

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

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

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

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

       STDERR_FILENO File number of stderr; 2.

       STDIN_FILENO  File number of stdin; 0.

       STDOUT_FILENO File number of stdout; 1.

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

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

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

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

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

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

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

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

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

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

       The following sections are informative.

APPLICATION USAGE

       POSIX.1‐2008 only describes the behavior of systems that claim conformance to it. However,
       application developers who want to write applications that adapt to other versions of this
       standard  (or  to systems that do not conform to any POSIX standard) may find it useful to
       code them so as to  conditionally  compile  different  code  depending  on  the  value  of
       _POSIX_VERSION, for example:

           #if _POSIX_VERSION >= 200112L
           /* Use the newer function that copes with large files. */
           off_t pos=ftello(fp);
           #else
           /* Either this is an old version of POSIX, or _POSIX_VERSION is
              not even defined, so use the traditional function. */
           long pos=ftell(fp);
           #endif

       Earlier versions of POSIX.1‐2008 and of the Single UNIX Specification can be identified by
       the following macros:

       POSIX.1‐1988 standard
             _POSIX_VERSION==198808L

       POSIX.1‐1990 standard
             _POSIX_VERSION==199009L

       ISO POSIX‐1:1996 standard
             _POSIX_VERSION==199506L

       Single UNIX Specification, Version 1
             _XOPEN_UNIX and _XOPEN_VERSION==4

       Single UNIX Specification, Version 2
             _XOPEN_UNIX and _XOPEN_VERSION==500

       ISO POSIX‐1:2001 and Single UNIX Specification, Version 3
             _POSIX_VERSION==200112L, plus (if the  XSI  option  is  supported)  _XOPEN_UNIX  and
             _XOPEN_VERSION==600

       POSIX.1‐2008  does  not make any attempt to define application binary interaction with the
       underlying operating system. However, application developers may find it useful  to  query
       _SC_VERSION  at  runtime  via  sysconf()  to  determine whether the current version of the
       operating system  supports  the  necessary  functionality  as  in  the  following  program
       fragment:

           if (sysconf(_SC_VERSION) < 200809L) {
               fprintf(stderr, "POSIX.1-2008 system required, terminating \n");
               exit(1);
           }

       New applications should not use _XOPEN_SHM or _XOPEN_ENH_I18N.

RATIONALE

       As  POSIX.1‐2008  evolved,  certain  options  became sufficiently standardized that it was
       concluded that simply requiring one of the option choices was simpler than  retaining  the
       option.  However,  for  backwards-compatibility,  the option flags (with required constant
       values) are retained.

   Version Test Macros
       The standard developers considered altering the definition of _POSIX_VERSION and  removing
       _SC_VERSION  from  the  specification of sysconf() since the utility to an application was
       deemed by some to be minimal,  and  since  the  implementation  of  the  functionality  is
       potentially  problematic.  However,  they recognized that support for existing application
       binaries is  a  concern  to  manufacturers,  application  developers,  and  the  users  of
       implementations conforming to POSIX.1‐2008.

       While  the example using _SC_VERSION in the APPLICATION USAGE section does not provide the
       greatest degree of imaginable utility to the application developer or user, it is arguably
       better  than  a  core  file or some other equally obscure result. (It is also possible for
       implementations  to  encode  and  recognize  application  binaries  compiled  in   various
       POSIX.1-conforming  environments,  and  modify  the  semantics of the underlying system to
       conform to the expectations of the application.) For the reasons outlined in the preceding
       paragraphs and in the APPLICATION USAGE section, the standard developers elected to retain
       the _POSIX_VERSION and _SC_VERSION functionality.

   Compile-Time Symbolic Constants for System-Wide Options
       POSIX.1‐2008 includes support in certain areas for  the  newly  adopted  policy  governing
       options and stubs.

       This  policy provides flexibility for implementations in how they support options. It also
       specifies how conforming applications can adapt to different implementations that  support
       different sets of options. It allows the following:

        1. If  an  implementation  has  no  interest in supporting an option, it does not have to
           provide anything associated with that option beyond the announcement that it does  not
           support it.

        2. An  implementation  can  support  a partial or incompatible version of an option (as a
           non-standard extension) as long as it does not claim to support the option.

        3. An application can determine whether the option is supported.  A  strictly  conforming
           application  must  check  this  announcement  mechanism  before  first  using anything
           associated with the option.

       There is an important implication of this policy. POSIX.1‐2008 cannot dictate the behavior
       of  interfaces associated with an option when the implementation does not claim to support
       the option.  In  particular,  it  cannot  require  that  a  function  associated  with  an
       unsupported  option  will  fail  if it does not perform as specified. However, this policy
       does not prevent a standard from requiring certain functions to  always  be  present,  but
       that  they  shall  always  fail  on  some  implementations.  The setpgid() function in the
       POSIX.1‐1990 standard, for example, is considered appropriate.

       The POSIX standards include various options, and the C-language  binding  support  for  an
       option  implies that the implementation must supply data types and function interfaces. An
       application must be able to discover whether the implementation supports each option.

       Any application must consider the following three cases for each option:

        1. Option never supported.

           The implementation advertises at compile time that the option will never be supported.
           In  this  case,  it  is not necessary for the implementation to supply any of the data
           types or function interfaces that are  provided  only  as  part  of  the  option.  The
           implementation  might  provide  data  types  and  functions  that are similar to those
           defined by POSIX.1‐2008, but there is no guarantee for any particular behavior.

        2. Option always supported.

           The implementation  advertises  at  compile  time  that  the  option  will  always  be
           supported. In this case, all data types and function interfaces shall be available and
           shall operate as specified.

        3. Option might or might not be supported.

           Some implementations might not provide a mechanism to specify support  of  options  at
           compile  time. In addition, the implementation might be unable or unwilling to specify
           support or non-support at compile time.  In either case, any  application  that  might
           use the option at runtime must be able to compile and execute. The implementation must
           provide, at compile time, all data types and function interfaces that are necessary to
           allow  this.  In this situation, there must be a mechanism that allows the application
           to query, at runtime, whether the option is supported. If the application attempts  to
           use  the  option when it is not supported, the result is unspecified unless explicitly
           specified otherwise in POSIX.1‐2008.

FUTURE DIRECTIONS

       None.

SEE ALSO

       <limits.h>, <stddef.h>, <stdint.h>, <stdio.h>, <sys_socket.h>, <sys_types.h>, <termios.h>,
       <wctype.h>

       The System Interfaces volume of POSIX.1‐2017, access(), alarm(), chown(), close(),
       confstr(), crypt(), ctermid(), dup(), _Exit(), encrypt(), exec, fchdir(), fchown(),
       fdatasync(), fork(), fpathconf(), fsync(), ftruncate(), getcwd(), getegid(), geteuid(),
       getgid(), getgroups(), gethostid(), gethostname(), getlogin(), getopt(), getpgid(),
       getpgrp(), getpid(), getppid(), getsid(), getuid(), isatty(), lchown(), link(), lockf(),
       lseek(), nice(), pause(), pipe(), read(), readlink(), rmdir(), setegid(), seteuid(),
       setgid(), setpgid(), setpgrp(), setregid(), setreuid(), setsid(), setuid(), sleep(),
       swab(), symlink(), sync(), sysconf(), tcgetpgrp(), tcsetpgrp(), truncate(), ttyname(),
       unlink(), write()

COPYRIGHT

       Portions  of  this  text  are  reprinted  and  reproduced in electronic form from IEEE Std
       1003.1-2017, Standard for Information Technology -- Portable  Operating  System  Interface
       (POSIX),  The  Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by
       the Institute of Electrical and Electronics Engineers, Inc and The  Open  Group.   In  the
       event  of  any  discrepancy  between this version and the original IEEE and The Open Group
       Standard, the original IEEE and The Open Group  Standard  is  the  referee  document.  The
       original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

       Any  typographical  or  formatting errors that appear in this page are most likely to have
       been introduced during the conversion of the source files to man page  format.  To  report
       such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .