mkstemp



MKTEMP(3)                  Library Functions Manual                  MKTEMP(3)


NAME

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


SYNOPSIS

     #include <stdlib.h>

     char *
     mktemp(char *template);

     int
     mkstemp(char *template);

     int
     mkstemps(char *template, int suffixlen);

     char *
     mkdtemp(char *template);

     #include <stdlib.h>
     #include <fcntl.h>

     int
     mkostemp(char *template, int flags);

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


DESCRIPTION

     The mktemp() family of functions take the given file name template and
     overwrite a portion of it to create a new file name.  This file name is
     unique and suitable for use by the application.  The template may be any
     file name with at least six trailing Xs, for example /tmp/temp.XXXXXXXX.
     The trailing Xs are replaced with a unique digit and letter combination.
     The number of unique file names that can be returned depends on the
     number of Xs provided; mktemp() will try at least 2 ** 31 combinations
     before giving up.  At least six Xs must be used, though 10 is much
     better.

     The mktemp() function generates a temporary file name based on a template
     as described above.  Because mktemp() does not actually create the
     temporary file there is a window of opportunity during which another
     process can open the file instead.  Because of this race condition,
     mktemp() should not be used where mkstemp() can be used instead.
     mktemp() was marked as a legacy interface in IEEE Std 1003.1-2001
     ("POSIX.1").

     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 acts the same as mkstemp(), except that the flags
     argument may contain zero or more of the following flags for the
     underlying open(2) system call:

           O_APPEND     Append on each write.
           O_CLOEXEC    Set the close-on-exec flag on the new file descriptor.
           O_SYNC       Perform synchronous I/O operations.

     The mkstemps() and mkostemps() functions act the same as mkstemp() and
     mkostemp(), except they permit a suffix to exist in the template.  The
     template should be of the form /tmp/tmpXXXXXXXXXXsuffix.  mkstemps() and
     mkostemps() are 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.


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
     any 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[19];
           FILE *sfp;

           strlcpy(sfn, "/tmp/ed.XXXXXXXXXX", sizeof(sfn));
           if (mktemp(sfn) == NULL || (sfp = fopen(sfn, "w+")) == NULL) {
                   warn("%s", sfn);
                   return (NULL);
           }
           return (sfp);

     should be rewritten like this:

           char sfn[19];
           FILE *sfp;
           int fd;

           strlcpy(sfn, "/tmp/ed.XXXXXXXXXX", sizeof(sfn));
           if ((fd = mkstemp(sfn)) == -1 ||
               (sfp = fdopen(fd, "w+")) == NULL) {
                   if (fd != -1) {
                           unlink(sfn);
                           close(fd);
                   }
                   warn("%s", sfn);
                   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 file name 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 mktemp(), mkstemp(), mkostemp(), and mkdtemp() functions may set
     errno to one of the following values:

     [EINVAL]           The template argument has fewer than six trailing Xs.

     [EEXIST]           All file names tried are already in use.  Consider
                        appending more Xs to the template.

     The mkstemps() and mkostemps() functions may set errno to

     [EINVAL]           The template argument length is less than suffixlen or
                        it has fewer than six Xs before the suffix.

     [EEXIST]           All file names tried are already in use.  Consider
                        appending more Xs to the template.

     In addition, the mkostemp() and mkostemps() functions may also set errno
     to

     [EINVAL]           flags is invalid.

     The mktemp() function may also set errno to any value specified by the
     lstat(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.


SEE ALSO

     chmod(2), lstat(2), mkdir(2), open(2), tempnam(3), tmpfile(3), tmpnam(3)


STANDARDS

     The mkdtemp() and mkstemp() functions conform to the IEEE Std 1003.1-2008
     ("POSIX.1") specification.  The ability to specify more than six Xs is an
     extension to that standard.  The mkostemp() function is expected to
     conform to a future revision of that standard.

     The mktemp() function conforms to IEEE Std 1003.1-2001 ("POSIX.1"); as of
     IEEE Std 1003.1-2008 ("POSIX.1") it is no longer a part of the standard.

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


HISTORY

     A mktemp() function appeared in Version 7 AT&T UNIX.  The mkdtemp()
     function appeared in OpenBSD 2.2.  The mkstemp() function appeared in
     4.4BSD.  The mkstemps() function appeared in OpenBSD 2.3.  The mkostemp()
     and mkostemps() functions appeared in OpenBSD 5.7.


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
     location.  Hence mkstemp() is recommended, since it atomically creates
     the file.  An attacker can guess the file names 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 mktemp().

OpenBSD 6.4                    October 26, 2014                    OpenBSD 6.4

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