The Compilation Environment

Applications should ensure that the feature test macro _XOPEN_SOURCE is defined with the value 500 before inclusion of any header. This is needed to enable the functionality described in this specification, and possibly to enable functionality defined elsewhere in the Common Applications Environment.

Identifiers in this specification may only be undefined using the #undef directive as described in Use and Implementation of Interfaces or The X/Open Name Space . These #undef directives must follow all #include directives of any XSI headers.

Most strictly conforming POSIX and ISO C applications will compile on systems compliant to this specification. However, an application which uses any of the items marked as an extension to POSIX and ISO C, for any purpose other than that shown here, will not necessarily compile. In such cases, it may be necessary to alter those applications to use alternative identifiers.

Since this specification is aligned with the ISO C standard, and since all functionality enabled by _POSIX_C_SOURCE set greater than zero and less than or equal to 199506L should be enabled by _XOPEN_SOURCE set equal to 500, there should be no need to define either _POSIX_SOURCE or _POSIX_C_SOURCE if _XOPEN_SOURCE is so defined. Therefore if _XOPEN_SOURCE is set equal to 500 and _POSIX_SOURCE is defined, or _POSIX_C_SOURCE is set greater than zero and less than or equal to 199506L, the behaviour is the same as if only _XOPEN_SOURCE is defined and set equal to 500. However, should _POSIX_C_SOURCE be set to a value greater than 199506L, the behaviour is undefined.

 The X/Open Name Space

All identifiers in this specification except environ are defined in at least one of the headers, as shown in . When _XOPEN_SOURCE is defined, each header defines or declares some identifiers, potentially conflicting with identifiers used by the application. The set of identifiers visible to the application consists of precisely those identifiers from the header pages of the included headers, as well as additional identifiers reserved for the implementation. In addition, some headers may make visible identifiers from other headers as indicated on the relevant header pages.

An implementation conforming to the referenced Networking Services, Issue 5 specification will define or declare an additional set of identifiers as shown in the XNS Name Space or the XNS reference pages.

The identifiers reserved for use by the implementation are described below.

1. Each identifier with external linkage described in the header section is reserved for use as an identifier with external linkage if the header is included.

2. Each macro name described in the header section is reserved for any use if the header is included.

3. Each identifier with file scope described in the header section is reserved for use as an identifier with file scope in the same name space if the header is included.

If any header in the following table is included, identifiers with the prefixes, suffixes or complete names shown are reserved for any use by the implementation.

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Header	Prefix	Suffix	Complete Name
<aio.h>	aio_, lio_, AIO_, LIO_
<dirent.h>	d_
<errno.h>	E
<fcntl.h>	l_
<glob.h>	gl_
<grp.h>	gr_
<limits.h>		_MAX
<locale.h>	LC_[A-Z]
<mqueue.h>	mq_, MQ_
<ndbm.h>	dbm_
<poll.h>	pd_, ph_, ps_
<pthread.h>	pthread_, PTHREAD_
<pwd.h>	pw_
<regex.h>	re_, rm_
<sched.h>	sched_, SCHED_
<semaphore.h>	sem_, SEM_
<signal.h>	sa_, SIG[A-Z], SIG_[A-Z], ss_, sv_, si_, SI_, sigev_, SIGEV_, sival_, uc_
<stropts.h>	bi_, ic_, l_, sl_, str_
<sys/ipc.h>	ipc_		key, pad, seq
<sys/mman.h>	shm_, MAP_, MCL_, MS_, PROT_
<sys/msg.h>	msg		msg
<sys/resource.h>	rlim_, ru_
<sys/sem.h>	sem		sem
<sys/shm.h>	shm
<sys/stat.h>	st_
<sys/statvfs.h>	f_
<sys/time.h>	fds_, it_, tv_, FD_
<sys/times.h>	tms_
<sys/uio.h>	iov_
<sys/utsname.h>	uts_
<sys/wait.h>	si_, W[A-Z], P_
<termios.h>	c_
<time.h>	tm_, clock_, timer_, it_, tv_, CLOCK_, TIMER_
<ucontext.h>	uc_, ss_
<ulimit.h>	UL_
<utime.h>	utim_
<utmpx.h>	ut_	_LVL, _TIME, _PROCESS
<wordexp.h>	we_
ANY header		_t

Note:

The notation [A-Z] indicates any upper-case letter in the portable character set. The notation [a-z] indicates any lower-case letter in the portable character set. Commas and spaces in the lists of prefixes and complete names in the above table are not part of any prefix or complete name.

If any header in the following table is included, macros with the prefixes shown may be defined. After the last inclusion of a given header, an application may use identifiers with the corresponding prefixes for its own purpose, provided their use is preceded by an #undef of the corresponding macro.

Header	Prefix
<dlfcn.h>	RTLD_
<fcntl.h>	F_	O_	S_
<fmtmsg.h>	MM_
<fnmatch.h>	FNM_
<ftw.h>	FTW
<glob.h>	GLOB_
<ndbm.h>	DBM_
<nl_types.h>	NL_
<poll.h>	POLL
<regex.h>	REG_
<signal.h>	SA_	SIG_[0-9a-z_]
	BUS_	CLD_	FPE_	ILL_	POLL_
	SEGV_	SI_	SS_	SV_	TRAP_
<stropts.h>	FLUSH[A-Z]	I_	M_	MUXID_R[A-Z]
	S_	SND[A-Z]	STR
<syslog.h>	LOG_
<sys/ipc.h>	IPC_
<sys/mman.h>	PROT_	MAP_	MS_
<sys/msg.h>	MSG[A-Z]	MSG_[A-Z]
<sys/resource.h>	PRIO_	RLIM_	RLIMIT_	RUSAGE_
<sys/sem.h>	SEM_
<sys/shm.h>	SHM[A-Z]	SHM_[A-Z]
<sys/socket.h>	AF_	MSG_	PF_	SO
<sys/stat.h>	S_
<sys/statvfs.h>	ST_
<sys/time.h>	FD_	ITIMER_
<sys/uio.h>	IOV_
<sys/wait.h>	BUS_	CLD_	FPE_	ILL_	POLL_
	SEGV_	SI_	TRAP_
<termios.h>	V	I	O	TC	B[0-9]
<wordexp.h>	WRDE_

Note:

The notation [0-9] indicates any digit. The notation [A-Z] indicates any upper-case letter in the portable character set. The notation [0-9a-z_] indicates any digit, any lower-case letter in the portable character set or underscore.

The following identifiers are reserved regardless of the inclusion of headers.

1. All identifiers that begin with an underscore and either an upper-case letter or another underscore are always reserved for any use by the implementation.

2. All identifiers that begin with an underscore are always reserved for use as identifiers with file scope in both the ordinary identifier and tag name spaces.

3. All identifiers in the table below are reserved for use as identifiers with external linkage. Some of these identifiers do not appear in this specification, but are reserved for future use by the ISO C standard.

   abort abs acos acosf acosl asctime asin asinf asinl atan atan2 atan2f atan2l atanf atanl atexit atof atoi atol bsearch calloc ceil ceilf ceill clearerr clock cos cosf cosh coshf coshl

   cosl ctime difftime div errno exit exp expf expl fabs fabsf fabsl fclose feof ferror fflush fgetc fgetpos fgets fgetwc fgetws floor floorf floorl fmod fmodf fmodl fopen fprintf fputc fputs

   fputwc fputws fread free freopen frexp frexpf frexpl fscanf fseek fsetpos ftell fwide fwprintf fwrite fwscanf getc getchar getenv gets getwc getwchar gmtime is[a-z]* labs ldexp ldexpf ldexpl ldiv localeconv localtime

   log log10 log10f log10l logf logl longjmp malloc mblen mbrlen mbrtowc mbsinit mbsrtowcs mbstowcs mbtowc mem[a-z]* mktime modf modff modfl perror pow powf powl printf putc putchar puts putwc putwchar qsort

   raise rand realloc remove rename rewind scanf setbuf setjmp setlocale setvbuf signal sin sinf sinh sinhf sinhl sinl sprintf sqrt sqrtf sqrtl srand sscanf str[a-z]* swprintf swscanf system tan tanf tanh

   tanhf tanhl tanl time tmpfile tmpnam to[a-z]* ungetc ungetwc va_end vfprintf vfwprintf vprintf vsprintf vswprintf vwprintf wcrtomb wcs[a-z]* wctob wctomb wctrans wctype wcwidth wmem[a-z]* wprintf wscanf

   Note:

   The notation [a-z] indicates any lower-case letter in the portable character set. The notation * indicates any combination of digits, letters in the portable character set, and underscore.

---

4.

The following identifiers are also reserved for use as identifiers with external linkage:

a64l	endservent	getpgid	logb	regex	sigset
acosh	endutxent	getpmsg	lstat	remainder	sigstack
asinh	expm1	getpriority	makecontext	remque	srandom
atanh	fattach	getpwent	mknod	rindex	statvfs
basename	fchdir	getrlimit	mkstemp	rint	strcasecmp
bcmp	fchmod	getrusage	mktemp	sbrk	strdup
bcopy	fchown	getsid	mmap	scalb	strncasecmp
brk	fcvt	getsubopt	mprotect	select	swapcontext
bsd_signal	fdetach	gettimeofday	msync	setcontext	symlink
bzero	ffs	getutxent	munmap	setgrent	sync
cbrt	fmtmsg	getutxid	nextafter	setitimer	syslog
closelog	fstatvfs	getutxline	nftw	setlogmask	tcgetsid
dbm_clearerr	ftime	getwd	openlog	setpgrp	truncate
dbm_close	ftok	grantpt	poll	setpriority	ttyslot
dbm_delete	ftruncate	ilogb	ptsname	setpwent	ualarm
dbm_error	gcvt	index	putmsg	setreuid	unlockpt
dbm_fetch	getcontext	initstate	putpmsg	setrlimit	usleep
dbm_firstkey	getdate	insque	pututxline	setstate	utimes
dbm_nextkey	getdtablesize	ioctl	random	setutxent	valloc
dbm_open	getgrent	isastream	re_comp	sigaltstack	vfork
dbm_store	getgrgid	killpg	re_exec	sighold	wait3
dirname	gethostid	l64a	readlink	sigignore	waitid
ecvt	getitimer	lchown	readv	siginterrupt	writev

All the identifiers defined in this specification that have external linkage are always reserved for use as identifiers with external linkage.

No other identifiers are reserved.

Applications must not declare or define identifiers with the same name as an identifier reserved in the same context. Since macro names are replaced whenever found, independent of scope and name space, macro names matching any of the reserved identifier names must not be defined if any associated header is included.

Except that the effect of each inclusion of <assert.h> depends on the definition of NDEBUG, headers may be included in any order, and each may be included more than once in a given scope, with no difference in effect from that of being included only once.

If used, a header must be included outside of any external declaration or definition, and it must be first included before the first reference to any type or macro it defines, or to any function or object it declares. However, if an identifier is declared or defined in more than one header, the second and subsequent associated headers may be included after the initial reference to the identifier. Prior to the inclusion of a header, the program must not define any macros with names lexically identical to symbols defined by that header.