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

NAME
     mktemp -- make temporary file name	(unique)

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 guaranteed	not to
     exist at the time of function invocation and is suitable for use by the
     application.  The template	may be any file	name with some number of `Xs'
     appended to it, for example /tmp/temp.XXXXXX.  The	trailing `Xs' are
     replaced with a unique alphanumeric combination.  The number of unique
     file names	mktemp() can return depends on the number of `Xs' provided;
     six `Xs' will result in mktemp() selecting	one of 56800235584 (62 ** 6)
     possible temporary	file names.

     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 addi-
     tional 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 makes the same replacement to the template as in
     mktemp() and creates the template directory, mode 0700.

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 return -1 if	no suitable file could be created.  If
     either call fails an error	code is	placed in the global variable errno.

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 mkostemp() and	mkostemps() functions may also set errno to the	fol-
     lowing value:

     [EINVAL]		The oflags argument is invalid.

     The mkstemp(), mkostemp(),	mkstemps(), mkostemps()	and mkdtemp() func-
     tions may also set	errno to any value specified by	the stat(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.

NOTES
     A common problem that results in a	core dump is that the programmer
     passes in a read-only string to mktemp(), mkstemp(), mkstemps() or
     mkdtemp().	 This is common	with programs that were	developed before
     ISO/IEC 9899:1990 (``ISO C90'') compilers were common.  For example,
     calling mkstemp() with an argument	of "/tmp/tempfile.XXXXXX" will result
     in	a core dump due	to mkstemp() attempting	to modify the string constant
     that was given.

     The mkdtemp(), mkstemp() and mktemp() function prototypes are also	avail-
     able from <unistd.h>.

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

STANDARDS
     The mkstemp() and mkdtemp() functions are expected	to conform to IEEE Std
     1003.1-2008 (``POSIX.1'').	 The mktemp() function is expected to conform
     to	IEEE Std 1003.1-2001 (``POSIX.1'') and is not specified	by IEEE	Std
     1003.1-2008 (``POSIX.1'').	 The mkostemp(), mkstemps() and	mkostemps()
     functions do not conform to any standard.

HISTORY
     A mktemp()	function appeared in Version 7 AT&T UNIX.  The mkstemp() func-
     tion appeared in 4.4BSD.  The mkdtemp() function first appeared in
     OpenBSD 2.2, and later in FreeBSD 3.2.  The mkstemps() function first
     appeared in OpenBSD 2.4, and later	in FreeBSD 3.4.	 The mkostemp()	and
     mkostemps() functions appeared in FreeBSD 10.0.

BUGS
     This family of functions produces filenames which can be guessed, though
     the risk is minimized when	large numbers of `Xs' are used to increase the
     number of possible	temporary filenames.  This makes the race in mktemp(),
     between testing for a file's existence (in	the mktemp() function call)
     and opening it for	use (later in the user application) particularly dan-
     gerous from a security perspective.  Whenever it is possible, mkstemp()
     or	mkostemp() should be used instead, since it does not have the race
     condition.	 If mkstemp() cannot be	used, the filename created by mktemp()
     should be created using the O_EXCL	flag to	open(2)	and the	return status
     of	the call should	be tested for failure.	This will ensure that the pro-
     gram does not continue blindly in the event that an attacker has already
     created the file with the intention of manipulating or reading its	con-
     tents.

FreeBSD	9.3			August 8, 2013			   FreeBSD 9.3

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUES | ERRORS | NOTES | SEE ALSO | STANDARDS | HISTORY | BUGS

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