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NetBSD 6.1.5 - man page for mktemp (netbsd section 3)

MKTEMP(3)			   BSD Library Functions Manual 			MKTEMP(3)

     mktemp, mkstemp, mkdtemp -- make unique temporary file or directory name

     Standard C Library (libc, -lc)

     #include <stdlib.h>

     char *
     mktemp(char *template);

     mkstemp(char *template);

     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 unique and suitable 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 implementation of the functions will
     accept any number of trailing 'X's, for portability reasons one should use only six.  Using
     six 'X's will result 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 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.

     The mktemp() and mkdtemp() functions return a pointer to the template on success and NULL on
     failure.  The mkstemp() function returns -1 if no suitable file could be created.	If either
     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[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) {
		   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 care-
     ful 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() and mkdtemp() functions may set errno to one of the following val-

     [ENOTDIR]		The pathname portion of the template is not an existing 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.

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

     The mktemp() conforms to IEEE Std 1003.1-2001 (``POSIX.1'').  It was however 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.

     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.

     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().  Sub-
     sequently, 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 location.
     Hence mkstemp() is recommended, since it atomically creates the file.  An attacker can guess
     the filenames produced by mktemp().  Whenever 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

     The mkdtemp() function is nonstandard and should not be used if portability is required.

     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 temporary 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.

BSD					  April 29, 2010				      BSD

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