MKTEMP(3) Library Functions Manual MKTEMP(3)

mktemp, mkstemp, mkstemps, mkostemp, mkostemps, mkdtemp — make unique temporary file or directory name

Standard C Library (libc, -lc)

#include <stdlib.h>

char *
mktemp(char *template);

int
mkstemp(char *template);

int
mkostemp(char *template, int oflags);

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

char *
mkdtemp(char *template);

#include <unistd.h>

int
mkstemps(char *template, int suffixlen);

The () 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’ characters appended to it, for example /tmp/temp.XXXXXX. The trailing ‘X’ characters in the template are replaced with a unique letter and number combination. mktemp() can return depends on the number of ‘X’ characters provided. Although the NetBSD implementation of the functions will accept any number of trailing ‘X’ characters, for portability reasons one should use only six. Using six ‘X’ characters will result in mktemp() testing roughly 62 ** 6 (56800235584) combinations.

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 is like mkstemp() but allows specifying additional open(2) flags (defined in <fcntl.h>). The permitted flags are O_APPEND, O_DIRECT, O_SHLOCK, O_EXLOCK, O_SYNC, O_CLOEXEC and O_CLOFORK.

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

The () function is similar to mkstemp(), but it creates a mode 0700 directory instead and returns the path.

Please note that the permissions of the file or directory being created are subject to the restrictions imposed by the umask(2) system call. It may thus happen that the created file is unreadable and/or unwritable.

The mktemp() and mkdtemp() functions return a pointer to the template on success and NULL on failure. The mkstemp(), mkostemp(), mkstemps() and mkostemps() functions 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.XXXXXX", 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.XXXXXX", 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 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 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(), mkostemp(), mkstemps(), mkostemps() and mkdtemp() functions may set errno to one of the following values:

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

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

chmod(2), getpid(2), open(2), stat(2), umask(2)

The mktemp() conforms to IEEE Std 1003.1-2001 (“POSIX.1”). It was however removed from the specification in the IEEE Std 1003.1-2008 (“POSIX.1”) revision. The mkstemp() function conforms to IEEE Std 1003.1-2004 (“POSIX.1”). The mkdtemp() function conforms to IEEE Std 1003.1-2008 (“POSIX.1”). The mkostemp() function conforms to IEEE Std 1003.1-2024 (“POSIX.1”).

A mktemp() function appeared in Version 7 AT&T UNIX.

The mkstemp() function appeared in 4.4BSD.

The mkdtemp() function appeared in NetBSD 1.4. The mkstemps() function first appeared in OpenBSD 2.4, and later in FreeBSD 3.4 and NetBSD 7.0. The mkostemp() and mkostemps() functions appeared in FreeBSD 10.0 and NetBSD 7.0.

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 use of mktemp() should generally be avoided, as a hostile process can exploit a race condition in the time between the generation of a temporary filename by mktemp() and the invoker's use of the temporary name. A link-time warning will be issued advising the use of mkstemp() or mkdtemp() instead.

November 2, 2024 NetBSD 11.0