TMPFILE(3) BSD Library Functions Manual TMPFILE(3)
tempnam, tmpfile, tmpnam -- temporary file routines
Standard C Library (libc, -lc)
tempnam(const char *tmpdir, const char *prefix);
The tmpfile() function returns a pointer to a stream associated with a file descriptor returned by the routine mkstemp(3). The created file
is unlinked before tmpfile() returns, causing the file to be automatically deleted when the last reference to it is closed. The file is
opened with the access value 'w+'.
The tmpnam() function returns a pointer to a file name, in the P_tmpdir directory, which did not reference an existing file at some indeter-
minate point in the past. P_tmpdir is defined in the include file <stdio.h>. If the argument s is non-NULL, the file name is copied to the
buffer it references. Otherwise, the file name is copied to a static buffer. In either case, tmpnam() returns a pointer to the file name.
The buffer referenced by s is expected to be at least L_tmpnam bytes in length. L_tmpnam is defined in the include file <stdio.h>.
The tempnam() function is similar to tmpnam(), but provides the ability to specify the directory which will contain the temporary file and
the file name prefix.
The environment variable TMPDIR (if set), the argument tmpdir (if non-NULL), the directory P_tmpdir, and the directory /tmp are tried, in the
listed order, as directories in which to store the temporary file.
The argument prefix, if non-NULL, is used to specify a file name prefix, which will be the first part of the created file name. tempnam()
allocates memory in which to store the file name; the returned pointer may be used as a subsequent argument to free(3).
The tmpfile() function returns a pointer to an open file stream on success, and a NULL pointer on error.
The tmpnam() and tempnam() functions return a pointer to a file name on success, and a NULL pointer on error.
The tmpfile() function may fail and set the global variable errno for any of the errors specified for the library functions fdopen(3) or
The tmpnam() function may fail and set errno for any of the errors specified for the library function mktemp(3).
The tempnam() function may fail and set errno for any of the errors specified for the library functions malloc(3) or mktemp(3).
The tmpfile() and tmpnam() functions conform to ANSI X3.159-1989 (``ANSI C89''). All described functions also conform to IEEE Std
1003.1-2001 (``POSIX.1''), albeit the tempnam() and tmpnam() functions have been marked as obsolete in the IEEE Std 1003.1-2008 (``POSIX.1'')
These interfaces are provided for AT&T System V UNIX and ANSI compatibility only. The mkstemp(3) interface is strongly preferred.
There are four important problems with these interfaces (as well as with the historic mktemp(3) interface). First, there is an obvious race
between file name selection and file creation and deletion: the program is typically written to call tmpnam(), tempnam(), or mktemp(3). Sub-
sequently, the program calls open(2) or fopen(3) and erroneously opens a file (or symbolic link, or fifo or other device) that the attacker
has placed in the expected file location. Hence mkstemp(3) is recommended, since it atomically creates the file.
Second, most historic implementations provide only a limited number of possible temporary file names (usually 26) before file names will
start being recycled. Third, the AT&T System V UNIX implementations of these functions (and of mktemp(3)) use the access(2) system call to
determine whether or not the temporary file may be created. This has obvious ramifications for setuid or setgid programs, complicating the
portable use of these interfaces in such programs. Finally, there is no specification of the permissions with which the temporary files are
This implementation of tmpfile() does not have these flaws, and that of tmpnam() and tempnam() only have the first limitation, but portable
software cannot depend on that. In particular, the tmpfile() interface should not be used in software expected to be used on other systems
if there is any possibility that the user does not wish the temporary file to be publicly readable and writable.
A link-time warning will be issued if tmpnam() or tempnam() is used, and advises the use of mkstemp() instead.
April 30, 2010 BSD