MKTEMP(3)

MKTEMP(3) | Library Functions Manual | MKTEMP(3)

# NAME

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

# LIBRARY

Standard C Library (libc, -lc)

# SYNOPSIS

`#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);

# DESCRIPTION

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.XXXXXX. 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. Although the NetBSD implementation of the functions will accept any number of trailing ‘`X`’s, for portability reasons one should use only six. Using six ‘`X`’s will result in `mktemp`() testing roughly 26 ** 6 (308915776) combinations.

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 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` and `O_CLOEXEC`.

The `mkstemps`() and `mkostemps`() 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 `mkdtemp`() 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.

# 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 returns -1 if no suitable file could be created. If either 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.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`.

# ERRORS

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

[`ENOTDIR`]
  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.

# SEE ALSO

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

# STANDARDS

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`() and `mkdtemp`() functions conform to IEEE Std 1003.1-2004 (“POSIX.1”) and IEEE Std 1003.1-2008 (“POSIX.1”), respectively.

# HISTORY

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.

# 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 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`() function is nonstandard and should not be used if portability is required.

# SECURITY CONSIDERATIONS

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.

June 18, 2014 | NetBSD 9.5