MKTEMP(3)

NAME
mktemp - make temporary file name (unique)

LIBRARY
libc

SYNOPSIS
#include <unistd.h>
char *
mktemp(char *template);
int
mkstemp(char *template);
int
mkstemps(char *template, int suffixlen);
char *
mkdtemp(char *template);

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 ‘Xs’ appended to it, for example /tmp/temp.XXXX. The trailing ‘Xs’ 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 ‘Xs’ provided; six ‘Xs’ will result in mktemp() testing roughly 26 ** 6 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 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/tmpXXXXXXsuffix. mkstemps() is told the length of the suffix string.

The mkdtemp() function makes the same replacement to the template as in mktemp(3) 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 return -1 if no suitable file could be created. If either call fails an error code is placed in the global variable errno.

ERRORS
The mkstemp(), mkstemps() 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 mkstemp(), mkstemps() and mkdtemp() functions may also set errno to any value specified by the stat(2) function.

The mkstemp() and mkstemps() 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.

NOTES
A common problem that results in a core dump is that the programmer passes in a read-only string to mktemp(), mkstemp(), mkstemps() or mkdtemp(). This is common with programs that were developed before ansiC compilers were common. For example, calling mkstemp() with an argument of "/tmp/tempfile.XXXXXX" will result in a core dump due to mkstemp() attempting to modify the string constant that was given. If the program in question makes heavy use of that type of function call, you do have the option of compiling the program so that it will store string constants in a writable segment of memory. See gcc(1) for more information.

BUGS
This family of functions produces filenames which can be guessed. This makes the race in mktemp(), between testing for a file's existence and opening it for use, particularly dangerous from a security perspective. Whenever it is possible, mkstemp() should be used instead.

SEE ALSO
chmod(2), getpid(2), mkdir(2), open(2), stat(2)

HISTORY
A mktemp() function appeared in AT&T UNIX. The mkdtemp() function first appeared in OpenBSD 2.2, and later in FreeBSD 3.2. The mkstemps() function first appeared in OpenBSD 2.4, and later in FreeBSD 3.4.