Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages


home | help
MKTEMP(3)	       FreeBSD Library Functions Manual		     MKTEMP(3)

     mktemp -- make temporary file name	(unique)

     Standard C	Library	(libc, -lc)

     #include <unistd.h>

     char *
     mktemp(char *template);

     mkstemp(char *template);

     mkstemps(char *template, int suffixlen);

     char *
     mkdtemp(char *template);

     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 mkstemps() function acts the same as mkstemp(), except	it permits a
     suffix to exist in	the template.  The template should be of the form
     /tmp/tmpXXXXXXsuffix.  The	mkstemps() function is 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.

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

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

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

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

     The mkstemp() and mkstemps() 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.

     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.  If the program in	question makes heavy use of that type
     of	function call, you do have the option of compiling the program so that
     it	will store string constants in a writable segment of memory.  See
     gcc(1) for	more information.

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

     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.

     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()
     should be used instead, since it does not have the	race condition.	 If
     mkstemp() cannot be used, the filename created by mktemp()	should be cre-
     ated 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 program does not
     continue blindly in the event that	an attacker has	already	created	the
     file with the intention of	manipulating or	reading	its contents.

     The implementation	of these functions calls arc4random(3),	which is not
     reentrant.	 You must provide your own locking around this and other con-
     sumers of the arc4random(3) API.

FreeBSD	6.0		       February	11, 1998		   FreeBSD 6.0


Want to link to this manual page? Use this URL:

home | help