---

Manual Reference Pages  - <unistd.h> (P)

---

unistd.h - standard symbolic constants and types

CONTENTS

Synopsis
Description
     Version Test Macros
     Constants for Options and Option Groups
     Execution-Time Symbolic Constants
     Constants for Functions
     Type Definitions
     Declarations
Application Usage
Rationale
     Version Test Macros
     Compile-Time Symbolic Constants for System-Wide Options
Future Directions
See Also
Copyright

#include <unistd.h>

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

The following symbolic constants shall be defined:

_POSIX_VERSION
	Integer value indicating version of IEEE Std 1003.1 (C-language binding) to which the implementation conforms. For implementations conforming to IEEE Std 1003.1-2001, the value shall be 200112L.
_POSIX2_VERSION
	Integer value indicating version of the Shell and Utilities volume of IEEE Std 1003.1 to which the implementation conforms. For implementations conforming to IEEE Std 1003.1-2001, the value shall be 200112L.
The following symbolic constant shall be defined only if the implementation supports the XSI option; see XSI Conformance .
_XOPEN_VERSION
	Integer value indicating version of the X/Open Portability Guide to which the implementation conforms. The value shall be 600.

The following symbolic constants, if defined in <unistd.h>, shall have a value of -1, 0, or greater, unless otherwise specified below. If these are undefined, the fpathconf(), pathconf(), or sysconf() functions can be used to determine whether the option is provided for a particular invocation of the application.

If a symbolic constant is defined with the value -1, the option is not supported. Headers, data types, and function interfaces required only for the option need not be supplied. An application that attempts to use anything associated only with the option is considered to be requiring an extension.

If a symbolic constant is defined with a value greater than zero, the option shall always be supported when the application is executed. All headers, data types, and functions shall be present and shall operate as specified.

If a symbolic constant is defined with the value zero, all headers, data types, and functions shall be present. The application can check at runtime to see whether the option is supported by calling fpathconf(), pathconf(), or sysconf() with the indicated name parameter.

Unless explicitly specified otherwise, the behavior of functions associated with an unsupported option is unspecified, and an application that uses such functions without first checking fpathconf(), pathconf(), or sysconf() is considered to be requiring an extension.

For conformance requirements, refer to Conformance .

_POSIX_ADVISORY_INFO
	The implementation supports the Advisory Information option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_POSIX_ASYNCHRONOUS_IO
	The implementation supports the Asynchronous Input and Output option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_POSIX_BARRIERS
	The implementation supports the Barriers option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_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 always be set to a value other than -1.
_POSIX_CLOCK_SELECTION
	The implementation supports the Clock Selection option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_POSIX_CPUTIME
	The implementation supports the Process CPU-Time Clocks option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_POSIX_FSYNC
	The implementation supports the File Synchronization option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_POSIX_IPV6
	The implementation supports the IPv6 option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_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 the Memory Mapped Files option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_POSIX_MEMLOCK
	The implementation supports the Process Memory Locking option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_POSIX_MEMLOCK_RANGE
	The implementation supports the Range Memory Locking option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_POSIX_MEMORY_PROTECTION
	The implementation supports the Memory Protection option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_POSIX_MESSAGE_PASSING
	The implementation supports the Message Passing option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_POSIX_MONOTONIC_CLOCK
	The implementation supports the Monotonic Clock option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_POSIX_NO_TRUNC
	Pathname components longer than {NAME_MAX} generate an error. This symbol shall always be set to a value other than -1.
_POSIX_PRIORITIZED_IO
	The implementation supports the Prioritized Input and Output option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_POSIX_PRIORITY_SCHEDULING
	The implementation supports the Process Scheduling option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_POSIX_RAW_SOCKETS
	The implementation supports the Raw Sockets option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_POSIX_READER_WRITER_LOCKS
	The implementation supports the Read-Write Locks option. This is always set to a value greater than zero if the Threads option is supported. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_POSIX_REALTIME_SIGNALS
	The implementation supports the Realtime Signals Extension option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_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 the Semaphores option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_POSIX_SHARED_MEMORY_OBJECTS
	The implementation supports the Shared Memory Objects option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_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 has a value other than -1 or 0, it shall have the value 200112L.
_POSIX_SPIN_LOCKS
	The implementation supports the Spin Locks option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_POSIX_SPORADIC_SERVER
	The implementation supports the Process Sporadic Server option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_POSIX_SYNCHRONIZED_IO
	The implementation supports the Synchronized Input and Output option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_POSIX_THREAD_ATTR_STACKADDR
	The implementation supports the Thread Stack Address Attribute option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_POSIX_THREAD_ATTR_STACKSIZE
	The implementation supports the Thread Stack Size Attribute option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_POSIX_THREAD_CPUTIME
	The implementation supports the Thread CPU-Time Clocks option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_POSIX_THREAD_PRIO_INHERIT
	The implementation supports the Thread Priority Inheritance option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_POSIX_THREAD_PRIO_PROTECT
	The implementation supports the Thread Priority Protection option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_POSIX_THREAD_PRIORITY_SCHEDULING
	The implementation supports the Thread Execution Scheduling option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_POSIX_THREAD_PROCESS_SHARED
	The implementation supports the Thread Process-Shared Synchronization option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_POSIX_THREAD_SAFE_FUNCTIONS
	The implementation supports the Thread-Safe Functions option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_POSIX_THREAD_SPORADIC_SERVER
	The implementation supports the Thread Sporadic Server option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_POSIX_THREADS
	The implementation supports the Threads option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_POSIX_TIMEOUTS
	The implementation supports the Timeouts option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_POSIX_TIMERS
	The implementation supports the Timers option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_POSIX_TRACE
	The implementation supports the Trace option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_POSIX_TRACE_EVENT_FILTER
	The implementation supports the Trace Event Filter option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_POSIX_TRACE_INHERIT
	The implementation supports the Trace Inherit option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_POSIX_TRACE_LOG
	The implementation supports the Trace Log option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_POSIX_TYPED_MEMORY_OBJECTS
	The implementation supports the Typed Memory Objects option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_POSIX_VDISABLE
	This symbol shall be defined to be the value of a character that shall disable terminal special character handling as described in <termios.h> . This symbol shall always be set to a value other than -1.
_POSIX2_C_BIND
	The implementation supports the C-Language Binding option. This symbol shall always have the value 200112L.
_POSIX2_C_DEV
	The implementation supports the C-Language Development Utilities option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_POSIX2_CHAR_TERM
	The implementation supports at least one terminal type.
_POSIX2_FORT_DEV
	The implementation supports the FORTRAN Development Utilities option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_POSIX2_FORT_RUN
	The implementation supports the FORTRAN Runtime Utilities option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_POSIX2_LOCALEDEF
	The implementation supports the creation of locales by the localedef utility. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_POSIX2_PBS
	The implementation supports the Batch Environment Services and Utilities option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_POSIX2_PBS_ACCOUNTING
	The implementation supports the Batch Accounting option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_POSIX2_PBS_CHECKPOINT
	The implementation supports the Batch Checkpoint/Restart option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_POSIX2_PBS_LOCATE
	The implementation supports the Locate Batch Job Request option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_POSIX2_PBS_MESSAGE
	The implementation supports the Batch Job Message Request option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_POSIX2_PBS_TRACK
	The implementation supports the Track Batch Job Request option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_POSIX2_SW_DEV
	The implementation supports the Software Development Utilities option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_POSIX2_UPE
	The implementation supports the User Portability Utilities option. If this symbol has a value other than -1 or 0, it shall have the value 200112L.
_V6_ILP32_OFF32
	The implementation provides a C-language compilation environment with 32-bit int, long, pointer, and off_t types.
_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.
_V6_LP64_OFF64
	The implementation provides a C-language compilation environment with 32-bit int and 64-bit long, pointer, and off_t types.
_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.
_XBS5_ILP32_OFF32 (LEGACY)
	The implementation provides a C-language compilation environment with 32-bit int, long, pointer, and off_t types.
_XBS5_ILP32_OFFBIG (LEGACY)
	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.
_XBS5_LP64_OFF64 (LEGACY)
	The implementation provides a C-language compilation environment with 32-bit int and 64-bit long, pointer, and off_t types.
_XBS5_LPBIG_OFFBIG (LEGACY)
	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.
_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_LEGACY
	The implementation supports the Legacy Option Group.
_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 extension.

If any of the following constants are not defined in the <unistd.h> header, the value shall vary depending on the file to which it is applied.

If any of the following 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 constants, whether defined 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.

The following symbolic constant shall be defined:

NULL	Null pointer
The following symbolic constants shall be defined for the access() function:
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 following symbolic constants shall be defined for the confstr() function:
_CS_PATH
	This is the value for the PATH environment variable that finds all standard utilities.
_CS_POSIX_V6_ILP32_OFF32_CFLAGS
	If sysconf(_SC_V6_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_V6_ILP32_OFF32_LDFLAGS
	If sysconf(_SC_V6_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_V6_ILP32_OFF32_LIBS
	If sysconf(_SC_V6_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_V6_ILP32_OFFBIG_CFLAGS
	If sysconf(_SC_V6_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_V6_ILP32_OFFBIG_LDFLAGS
	If sysconf(_SC_V6_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_V6_ILP32_OFFBIG_LIBS
	If sysconf(_SC_V6_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_V6_LP64_OFF64_CFLAGS
	If sysconf(_SC_V6_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_V6_LP64_OFF64_LDFLAGS
	If sysconf(_SC_V6_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_V6_LP64_OFF64_LIBS
	If sysconf(_SC_V6_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_V6_LPBIG_OFFBIG_CFLAGS
	If sysconf(_SC_V6_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_V6_LPBIG_OFFBIG_LDFLAGS
	If sysconf(_SC_V6_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_V6_LPBIG_OFFBIG_LIBS
	If sysconf(_SC_V6_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_V6_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, useconds_t, wchar_t, and wint_t types are no greater than the width of type long.
The following symbolic constants are reserved for compatibility with Issue 5:
_CS_XBS5_ILP32_OFF32_CFLAGS (LEGACY) _CS_XBS5_ILP32_OFF32_LDFLAGS (LEGACY) _CS_XBS5_ILP32_OFF32_LIBS (LEGACY) _CS_XBS5_ILP32_OFF32_LINTFLAGS (LEGACY) _CS_XBS5_ILP32_OFFBIG_CFLAGS (LEGACY) _CS_XBS5_ILP32_OFFBIG_LDFLAGS (LEGACY) _CS_XBS5_ILP32_OFFBIG_LIBS (LEGACY) _CS_XBS5_ILP32_OFFBIG_LINTFLAGS (LEGACY) _CS_XBS5_LP64_OFF64_CFLAGS (LEGACY) _CS_XBS5_LP64_OFF64_LDFLAGS (LEGACY) _CS_XBS5_LP64_OFF64_LIBS (LEGACY) _CS_XBS5_LP64_OFF64_LINTFLAGS (LEGACY) _CS_XBS5_LPBIG_OFFBIG_CFLAGS (LEGACY) _CS_XBS5_LPBIG_OFFBIG_LDFLAGS (LEGACY) _CS_XBS5_LPBIG_OFFBIG_LIBS (LEGACY) _CS_XBS5_LPBIG_OFFBIG_LINTFLAGS (LEGACY)
The following symbolic constants shall be defined for the lseek() and fcntl() functions and shall have distinct values:
SEEK_CUR
	Set file offset to current plus offset.
SEEK_END
	Set file offset to EOF plus offset.
SEEK_SET
	Set file offset to offset.
The following symbolic constants shall be defined 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 following symbolic constants shall be defined for pathconf():
_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_MIN_XFER_SIZE _PC_REC_XFER_ALIGN _PC_SYMLINK_MAX _PC_SYNC_IO _PC_VDISABLE
The following symbolic constants shall be defined for sysconf():
_SC_2_C_BIND _SC_2_C_DEV _SC_2_C_VERSION _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_ARG_MAX _SC_AIO_LISTIO_MAX _SC_AIO_MAX _SC_AIO_PRIO_DELTA_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_FILE_LOCKING _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_SEMAPHORES _SC_SEM_NSEMS_MAX _SC_SEM_VALUE_MAX _SC_SHARED_MEMORY_OBJECTS _SC_SHELL _SC_SIGQUEUE_MAX _SC_SPAWN _SC_SPIN_LOCKS _SC_SPORADIC_SERVER _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_SAFE_FUNCTIONS _SC_THREAD_SPORADIC_SERVER _SC_THREAD_STACK_MIN _SC_THREAD_THREADS_MAX _SC_TIMEOUTS _SC_THREADS _SC_TIMER_MAX _SC_TIMERS _SC_TRACE _SC_TRACE_EVENT_FILTER _SC_TRACE_INHERIT _SC_TRACE_LOG _SC_TTY_NAME_MAX _SC_TYPED_MEMORY_OBJECTS _SC_TZNAME_MAX _SC_V6_ILP32_OFF32 _SC_V6_ILP32_OFFBIG _SC_V6_LP64_OFF64 _SC_V6_LPBIG_OFFBIG _SC_VERSION _SC_XBS5_ILP32_OFF32 (LEGACY) _SC_XBS5_ILP32_OFFBIG (LEGACY) _SC_XBS5_LP64_OFF64 (LEGACY) _SC_XBS5_LPBIG_OFFBIG (LEGACY) _SC_XOPEN_CRYPT _SC_XOPEN_ENH_I18N _SC_XOPEN_LEGACY _SC_XOPEN_REALTIME _SC_XOPEN_REALTIME_THREADS _SC_XOPEN_SHM _SC_XOPEN_STREAMS _SC_XOPEN_UNIX _SC_XOPEN_VERSION _SC_XOPEN_XCU_VERSION
The two constants _SC_PAGESIZE and _SC_PAGE_SIZE may be defined to have the same value.
The following symbolic constants shall be defined 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 size_t, ssize_t, uid_t, gid_t, off_t, pid_t, and useconds_t types shall be defined as described in <sys/types.h>.

The intptr_t type shall be defined as described in <inttypes.h>.

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 *);
char        *ctermid(char *);

int          dup(int);



int          dup2(int, 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 []);
void        _exit(int);
int          fchown(int, uid_t, gid_t);

int          fchdir(int);


int          fdatasync(int);

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);

char        *getwd(char *); (LEGACY )

int          isatty(int);

int          lchown(const char *, uid_t, gid_t);

int          link(const char *, const char *);

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);
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 *);
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);

useconds_t   ualarm(useconds_t, useconds_t);

int          unlink(const char *);

int          usleep(useconds_t);
pid_t        vfork(void);

ssize_t      write(int, const void *, size_t);

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

The following external variables shall be declared:

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

The following sections are informative.

IEEE Std 1003.1-2001 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 IEEE Std 1003.1 (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 IEEE Std 1003.1 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

IEEE Std 1003.1-2001 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) < 200112L) {
    fprintf(stderr, "POSIX.1-2001 system required, terminating \n");
    exit(1);
}

New applications should not use _XOPEN_SHM or _XOPEN_ENH_I18N.

As IEEE Std 1003.1-2001 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.

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 IEEE Std 1003.1-2001.

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.

IEEE Std 1003.1-2001 now 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. IEEE Std 1003.1-2001 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 IEEE Std 1003.1-2001, 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 IEEE Std 1003.1-2001.

None.

<inttypes.h> , <limits.h> , <sys/socket.h> , <sys/types.h> , <termios.h> , <wctype.h> , the System Interfaces volume of IEEE Std 1003.1-2001, access(), alarm(), chdir(), chown(), close(), crypt(), ctermid(), dup(), encrypt(), environ, exec, exit(), fchdir(), fchown(), fcntl(), fork(), fpathconf(), fsync(), ftruncate(), getcwd(), getegid(), geteuid(), getgid(), getgroups(), gethostid(), gethostname(), getlogin(), getpgid(), getpgrp(), getpid(), getppid(), getsid(), getuid(), isatty(), lchown(), link(), lockf(), lseek(), nice(), pathconf(), pause(), pipe(), read(), readlink(), rmdir(), setgid(), setpgid(), setpgrp(), setregid(), setreuid(), setsid(), setuid(), sleep(), swab(), symlink(), sync(), sysconf(), tcgetpgrp(), tcsetpgrp(), truncate(), ttyname(), ualarm(), unlink(), usleep(), vfork(), write()

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 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 .