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NAME    [Toc]    [Back]

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

SYNOPSIS    [Toc]    [Back]

     #include <unistd.h>

     char *
     mktemp(char *template);

     mkstemp(char *template);

     mkstemps(char *template, int suffixlen);

     char *
     mkdtemp(char *template);

DESCRIPTION    [Toc]    [Back]

     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.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.

RETURN VALUES    [Toc]    [Back]

     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.

EXAMPLES    [Toc]    [Back]

     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+")) ==
                   fprintf(stderr,  "%s:  %s0,  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:  %s0,  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 careful about code which opens, closes, and then reopens 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

ERRORS    [Toc]    [Back]

     The mkstemp() and mkdtemp() functions may set errno  to  one
of the following

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

     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

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

SEE ALSO    [Toc]    [Back]

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

HISTORY    [Toc]    [Back]

     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.

BUGS    [Toc]    [Back]

     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  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 mktemp().

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

OpenBSD      3.6                           June      4,      1993
[ Back ]
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