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MKTEMP(3)                 OpenBSD Programmer's Manual                MKTEMP(3)

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

     #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 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.XXXX.  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; six  `X's will result in
     mktemp() testing roughly 26 ** 6 combinations.  At least 6 `X's should be
     used, though 10 is much better.

     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/tmpXXXXXXXXXXsuffix.  mkstemps() is told the length of the suffix
     string, i.e., strlen("suffix");

     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() 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.XXXXXXXXXX", 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.XXXXXXXXXX", 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 cas-
     es, 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 mkstemp() and mkdtemp() functions may set errno to one of the follow-
     ing values:

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

     The 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 mkstemps() function may also set errno to any value specified by the
     open(2) function or,

     [EINVAL]      The suffix length is longer than the template length.

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

     chmod(2), getpid(2), mkdir(2), open(2), stat(2), tempnam(3), tmpfile(3),

     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.

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

OpenBSD 3.4                      June 4, 1993                                3


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