MKTEMP(3) BSD Programmer's Manual MKTEMP(3)NAME
mktemp, mkstemp, mkstemps, 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);
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 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 that can be returned 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 mktemp() function generates a temporary file name based on a template
as described above. Because mktemp() does not actually create the tem-
porary file there is a window of opportunity during which another process
can open the file instead. Because of this race condition the mktemp()
should not be used in new code. mktemp() was marked as a legacy interface
in IEEE Std 1003.1-2001 ("POSIX") and may be removed in a future release
of OpenBSD.
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
The mktemp() and mkdtemp() functions return a pointer to the template on
success and NULL on failure. The mkstemp() and mkstemps() functions re-
turn -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[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) {
unlink(sfn);
close(fd);
}
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 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 mkstemp() and mkdtemp() functions may set errno to one of the follow-
ing values:
[ENOTDIR] The path name 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 length.
The mkdtemp() function may also set errno to any value specified by the
mkdir(2) function.
SEE ALSOchmod(2), getpid(2), mkdir(2), open(2), stat(2), tempnam(3), tmpfile(3),
tmpnam(3)HISTORY
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.
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 loca-
tion. 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().
The mkdtemp() and mkstemps() functions are non-standard and should not be
used if portability is required.
MirOS BSD #10-current June 4, 1993 2
