MKTEMP(3) BSD Library Functions Manual MKTEMP(3)
mktemp -- make temporary file name (unique)
Standard C Library (libc, -lc)
mkstemps(char *template, int suffixlen);
The mktemp() function takes the given file name template and overwrites a portion of it to
create a file name. This file name is guaranteed not to exist at the time of function invo-
cation and is 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.XXXXXX. The trailing 'Xs' are
replaced with a unique alphanumeric combination. The number of unique file names mktemp()
can return depends on the number of 'Xs' provided; six 'Xs' will result in mktemp() select-
ing one of 56800235584 (62 ** 6) possible temporary file names.
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. The mkstemps()
function is told the length of the suffix string.
The mkdtemp() 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() and mkstemps() functions return -1 if no suitable file could be cre-
ated. If either call fails an error code is placed in the global variable errno.
The mkstemp(), mkstemps() and mkdtemp() functions may set errno to one of the following val-
[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
The mkdtemp() function may also set errno to any value specified by the mkdir(2) function.
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 ISO/IEC 9899:1990 (``ISO C90'') 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.
chmod(2), getpid(2), mkdir(2), open(2), stat(2)
A mktemp() function appeared in Version 7 AT&T UNIX. The mkstemp() function appeared in
4.4BSD. 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.
This family of functions produces filenames which can be guessed, though the risk is mini-
mized when large numbers of 'Xs' are used to increase the number of possible temporary file-
names. This makes the race in mktemp(), between testing for a file's existence (in the
mktemp() function call) and opening it for use (later in the user application) particularly
dangerous from a security perspective. Whenever it is possible, mkstemp() should be used
instead, since it does not have the race condition. If mkstemp() cannot be used, the file-
name created by mktemp() should be created using the O_EXCL flag to open(2) and the return
status of the call should be tested for failure. This will ensure that the program does not
continue blindly in the event that an attacker has already created the file with the inten-
tion of manipulating or reading its contents.
The implementation of these functions calls arc4random(3), which is not reentrant. You must
provide your own locking around this and other consumers of the arc4random(3) API.
BSD February 11, 1998 BSD