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

NAME
     mktemp, mkstemp, mkstemps,	mkostemp, mkostemps, mkdtemp --	make unique
     temporary file or directory name

LIBRARY
     Standard C	Library	(libc, -lc)

SYNOPSIS
     #include <stdlib.h>

     char *
     mktemp(char *template);

     int
     mkstemp(char *template);

     int
     mkostemp(char *template, int oflags);

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

     char *
     mkdtemp(char *template);

     #include <unistd.h>

     int
     mkstemps(char *template, int suffixlen);

DESCRIPTION
     The mktemp() function takes the given file	name template and overwrites a
     portion of	it to create a file name.  This	file name is unique and	suit-
     able for use by the application.  The template may	be any file name with
     some number of `X's appended to it, for example /tmp/temp.XXXXXX.	The
     trailing `X's are replaced	with the current process number	and/or a
     unique letter combination.	 The number of unique file names mktemp() can
     return depends on the number of `X's provided.  Although the NetBSD im-
     plementation of the functions will	accept any number of trailing `X's,
     for portability reasons one should	use only six.  Using six `X's will re-
     sult in mktemp() testing roughly 26 ** 6 (308915776) combinations.

     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 is like
     mkstemp() but allows specifying additional	open(2)	flags (defined in
     <fcntl.h>).  The permitted	flags are O_APPEND, O_DIRECT, O_SHLOCK,
     O_EXLOCK, O_SYNC and O_CLOEXEC.

     The mkstemps() and	mkostemps() functions act the same as mkstemp()	and
     mkostemp()	respectively, except they permit a suffix to exist in the tem-
     plate.  The template should be of the form	/tmp/tmpXXXXXXsuffix.  The
     mkstemps()	and mkostemps()	function are told the length of	the suffix
     string.

     The mkdtemp() function is similar to mkstemp(), but it creates a mode
     0700 directory instead and	returns	the path.

     Please note that the permissions of the file or directory being created
     are subject to the	restrictions imposed by	the umask(2) system call.  It
     may thus happen that the created file is unreadable and/or	unwritable.

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

EXAMPLES
     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[15]	= "";
	   FILE	*sfp;

	   strlcpy(sfn,	"/tmp/ed.XXXXXX", sizeof sfn);
	   if (mktemp(sfn) == NULL || (sfp = fopen(sfn,	"w+")) == NULL)	{
		   fprintf(stderr, "%s:	%s\n", sfn, strerror(errno));
		   return (NULL);
	   }
	   return (sfp);

     should be rewritten like this:

	   char	sfn[15]	= "";
	   FILE	*sfp;
	   int fd = -1;

	   strlcpy(sfn,	"/tmp/ed.XXXXXX", sizeof sfn);
	   if ((fd = mkstemp(sfn)) == -1 ||
	       (sfp = fdopen(fd, "w+"))	== NULL) {
		   if (fd != -1) {
			   unlink(sfn);
			   close(fd);
		   }
		   fprintf(stderr, "%s:	%s\n", sfn, strerror(errno));
		   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 filename 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.

ERRORS
     The mkstemp(), mkostemp(),	mkstemps(), mkostemps()	and mkdtemp() func-
     tions may set errno to one	of the following values:

     [ENOTDIR]		The pathname portion of	the template is	not an exist-
			ing directory.

     The mktemp(), mkstemp() and mkdtemp() functions may also set errno	to any
     value specified by	the stat(2) function.

     The mkstemp() function 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.

SEE ALSO
     chmod(2), getpid(2), open(2), stat(2), umask(2)

STANDARDS
     The mktemp() conforms to IEEE Std 1003.1-2001 ("POSIX.1").	 It was	how-
     ever removed from the specification in the	IEEE Std 1003.1-2008
     ("POSIX.1") revision.  The	mkstemp() and mkdtemp()	functions conform to
     IEEE Std 1003.1-2004 ("POSIX.1") and IEEE Std 1003.1-2008 ("POSIX.1"),
     respectively.

HISTORY
     A mktemp()	function appeared in Version 7 AT&T UNIX.

     The mkstemp() function appeared in	4.4BSD.

     The mkdtemp() function appeared in	NetBSD 1.4.  The mkstemps() function
     first appeared in OpenBSD 2.4, and	later in FreeBSD 3.4 and NetBSD	7.0.
     The mkostemp() and	mkostemps() functions appeared in FreeBSD 10.0 and
     NetBSD 7.0.

BUGS
     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 filenames	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().

     The mkdtemp() function is nonstandard and should not be used if portabil-
     ity is required.

SECURITY CONSIDERATIONS
     The use of	mktemp() should	generally be avoided, as a hostile process can
     exploit a race condition in the time between the generation of a tempo-
     rary filename by mktemp() and the invoker's use of	the temporary name.  A
     link-time warning will be issued advising the use of mkstemp() or
     mkdtemp() instead.

FreeBSD	13.0			 June 18, 2014			  FreeBSD 13.0

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUES | EXAMPLES | ERRORS | SEE ALSO | STANDARDS | HISTORY | BUGS | SECURITY CONSIDERATIONS

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