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MKTEMP(3)	       FreeBSD Library Functions Manual		     MKTEMP(3)

     mktemp, mkstemp, mkostemp,	mkstemps, mkostemps, mkdtemp --	make temporary
     file name (unique)

     #include <stdlib.h>

     char *
     mktemp(char *template);

     mkstemp(char *template);

     mkstemps(char *template, int suffixlen);

     char *
     mkdtemp(char *template);

     #include <stdlib.h>
     #include <fcntl.h>

     mkostemp(char *template, int flags);

     mkostemps(char *template, int suffixlen, int flags);

     The mktemp() family of functions take the given file name template	and
     overwrite a portion of it to create a new file name.  This	file name is
     unique and	suitable for use by the	application.  The template may be any
     file name with at least six trailing Xs, for example /tmp/temp.XXXXXXXX.
     The trailing Xs are replaced with a unique	digit and letter combination.
     The number	of unique file names that can be returned depends on the num-
     ber of Xs provided; mktemp() will try at least 2 ** 31 combinations be-
     fore giving up.  At least six Xs must be used, though 10 is much better.

     The mktemp() function generates a temporary file name based on a template
     as	described above.  Because mktemp() does	not actually create the	tempo-
     rary file there is	a window of opportunity	during which another process
     can open the file instead.	 Because of this race condition, mktemp()
     should not	be used	where mkstemp()	can be used instead.  mktemp() was
     marked as a legacy	interface in IEEE Std 1003.1-2001 ("POSIX.1").

     The mkstemp() function makes the same replacement to the template and
     creates the template file,	mode 0600, returning a file descriptor opened
     for reading and writing.  This avoids the race between testing for	a
     file's existence and opening it for use.

     The mkostemp() function acts the same as mkstemp(), except	that the flags
     argument may contain zero or more of the following	flags for the underly-
     ing open(2) system	call:

	   O_APPEND	Append on each write.
	   O_CLOEXEC	Set the	close-on-exec flag on the new file descriptor.
	   O_SYNC	Perform	synchronous I/O	operations.

     The mkstemps() and	mkostemps() functions act the same as mkstemp()	and
     mkostemp(), except	they permit a suffix to	exist in the template.	The
     template should be	of the form /tmp/tmpXXXXXXXXXXsuffix.  mkstemps() and
     mkostemps() are told the length of	the suffix string, i.e.,

     The mkdtemp() function makes the same replacement to the template as in
     mktemp() and creates the template directory, mode 0700.

     The mktemp() and mkdtemp()	functions return a pointer to the template on
     success and NULL on failure.  The mkstemp(), mkostemp(), mkstemps(), and
     mkostemps() functions return -1 if	no suitable file could be created.  If
     any call fails, an	error code is placed in	the global variable errno.

     Quite often a programmer will want	to replace a use of mktemp() with
     mkstemp(),	usually	to avoid the problems described	above.	Doing this
     correctly requires	a good understanding of	the code in question.

     For instance, code	of this	form:

	   char	sfn[19];
	   FILE	*sfp;

	   strlcpy(sfn,	"/tmp/ed.XXXXXXXXXX", sizeof(sfn));
	   if (mktemp(sfn) == NULL || (sfp = fopen(sfn,	"w+")) == NULL)	{
		   warn("%s", sfn);
		   return (NULL);
	   return (sfp);

     should be rewritten like this:

	   char	sfn[19];
	   FILE	*sfp;
	   int fd;

	   strlcpy(sfn,	"/tmp/ed.XXXXXXXXXX", sizeof(sfn));
	   if ((fd = mkstemp(sfn)) == -1 ||
	       (sfp = fdopen(fd, "w+"))	== NULL) {
		   if (fd != -1) {
		   warn("%s", sfn);
		   return (NULL);
	   return (sfp);

     Often one will find code which uses mktemp() very early on, perhaps to
     globally initialize the template nicely, but the code which calls open(2)
     or	fopen(3) on that file name will	occur much later.  (In almost all
     cases, the	use of fopen(3)	will mean that the flags O_CREAT | O_EXCL are
     not given to open(2), and thus a symbolic link race becomes possible,
     hence making necessary the	use of fdopen(3) as seen above.)  Furthermore,
     one must be careful about code which opens, closes, and then re-opens the
     file in question.	Finally, one must ensure that upon error the temporary
     file is removed correctly.

     There are also cases where	modifying the code to use mktemp(), in concert
     with open(2) using	the flags O_CREAT | O_EXCL, is better, as long as the
     code retries a new	template if open(2) fails with an errno	of EEXIST.

     The mktemp(), mkstemp(), mkostemp(), and mkdtemp()	functions may set
     errno to one of the following values:

     [EINVAL]		The template argument has fewer	than six trailing Xs.

     [EEXIST]		All file names tried are already in use.  Consider ap-
			pending	more Xs	to the template.

     The mkstemps() and	mkostemps() functions may set errno to

     [EINVAL]		The template argument length is	less than suffixlen or
			it has fewer than six Xs before	the suffix.

     [EEXIST]		All file names tried are already in use.  Consider ap-
			pending	more Xs	to the template.

     In	addition, the mkostemp() and mkostemps() functions may also set	errno

     [EINVAL]		flags is invalid.

     The mktemp() function may also set	errno to any value specified by	the
     lstat(2) function.

     The mkstemp(), mkostemp(),	mkstemps(), and	mkostemps() functions may also
     set errno to any value specified by the open(2) function.

     The mkdtemp() function may	also set errno to any value specified by the
     mkdir(2) function.

     chmod(2), lstat(2), mkdir(2), open(2), tempnam(3),	tmpfile(3), tmpnam(3)

     The mkdtemp() and mkstemp() functions conform to the IEEE Std 1003.1-2008
     ("POSIX.1") specification.	 The ability to	specify	more than six Xs is an
     extension to that standard.  The mkostemp() function is expected to con-
     form to a future revision of that standard.

     The mktemp() function conforms to IEEE Std	1003.1-2001 ("POSIX.1"); as of
     IEEE Std 1003.1-2008 ("POSIX.1") it is no longer a	part of	the standard.

     The mkstemps() and	mkostemps() functions are non-standard and should not
     be	used if	portability is required.

     A mktemp()	function appeared in Version 7 AT&T UNIX.  The mkdtemp() func-
     tion appeared in OpenBSD 2.2.  The	mkstemp() function appeared in 4.4BSD.
     The mkstemps() function appeared in OpenBSD 2.3.  The mkostemp() and
     mkostemps() functions appeared in OpenBSD 5.7.

     For mktemp() there	is an obvious race between file	name selection and
     file creation and deletion: the program is	typically written to call
     tmpnam(3),	tempnam(3), or mktemp().  Subsequently,	the program calls
     open(2) or	fopen(3) and erroneously opens a file (or symbolic link, FIFO
     or	other device) that the attacker	has created in the expected file loca-
     tion.  Hence mkstemp() is recommended, since it atomically	creates	the
     file.  An attacker	can guess the file names produced by mktemp().	When-
     ever it is	possible, mkstemp() or mkdtemp() should	be used	instead.

     For this reason, ld(1) will output	a warning message whenever it links
     code that uses mktemp().

FreeBSD	13.0		       October 26, 2014			  FreeBSD 13.0


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