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MKTEMP(3) Library Functions Manual MKTEMP(3)

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

#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);

The () 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 () 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 () 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 () 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:

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

The () and () 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 () 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(), 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.

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.

The mktemp(), mkstemp(), mkostemp(), and mkdtemp() functions may set errno to one of the following values:

[]
The template argument has fewer than six trailing Xs.
[]
All file names tried are already in use. Consider appending more Xs to the template.

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

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

[]
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.

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

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.

A mktemp() function appeared in Version 7 AT&T UNIX. The mkdtemp() function appeared in OpenBSD 2.2. The mkstemp() function appeared in 4.3BSD. The mkstemps() function appeared in OpenBSD 2.3. The mkostemp() and mkostemps() functions appeared in OpenBSD 5.7.

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().

August 4, 2022 OpenBSD-7.2