MKTEMP(3) BSD Library Functions Manual MKTEMP(3)
mkdtemp, mkstemp, mkstemps, 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 invocation and is suitable for use by the application. The template may be any file name with some num-
ber 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() selecting 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 created. 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 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.
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 con-
stant 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 pro-
gram so that it will store string constants in a writable segment of memory. See gcc(1) for more information.
The include file <unistd.h> is necessary and sufficient for all functions.
chmod(2), getpid(2), mkdir(2), open(2), stat(2), compat(5)
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 minimized when large numbers of 'Xs' are used to
increase the number of possible temporary filenames. 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 filename cre-
ated 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 intention of
manipulating or reading its contents.
February 11, 1998 BSD