tmpnam, tmpnam_r — create a name for a temporary file


#include <stdio.h>
char *tmpnam( char *s);


The tmpnam() function returns a pointer to a string that is a valid filename, and such that a file with this name did not exist at some point in time, so that naive programmers may think it a suitable name for a temporary file. If the argument s is NULL this name is generated in an internal static buffer and may be overwritten by the next call to tmpnam(). If s is not NULL, the name is copied to the character array (of length at least L_tmpnam) pointed to by s and the value s is returned in case of success.

The pathname that is created, has a directory prefix P_tmpdir. (Both L_tmpnam and P_tmpdir are defined in <stdio.h> just like the TMP_MAX mentioned below.)


The tmpnam() function returns a pointer to a unique temporary filename, or NULL if a unique name cannot be generated.


No errors are defined.


Multithreading (see pthreads(7))

The tmpnam() function is thread-safe with exceptions. It is not thread-safe if called with a NULL parameter.

The tmpnam_r() function is thread-safe.


SVr4, 4.3BSD, C89, C99, POSIX.1-2001. POSIX.1-2008 marks tmpnam() as obsolete.


The tmpnam() function generates a different string each time it is called, up to TMP_MAX times. If it is called more than TMP_MAX times, the behavior is implementation defined.

Although tmpnam() generates names that are difficult to guess, it is nevertheless possible that between the time that tmpnam() returns a pathname, and the time that the program opens it, another program might create that pathname using open(2), or create it as a symbolic link. This can lead to security holes. To avoid such possibilities, use the open(2) O_EXCL flag to open the pathname. Or better yet, use mkstemp(3) or tmpfile(3).

Portable applications that use threads cannot call tmpnam() with a NULL argument if either _POSIX_THREADS or _POSIX_THREAD_SAFE_FUNCTIONS is defined.

A POSIX draft proposed to use a function tmpnam_r() defined by

char *
tmpnam_r(char *s)
    return s ? tmpnam(s) : NULL;

apparently as a warning not to use NULL. A few systems implement it. To get a glibc prototype for this function from <stdio.h> define _SVID_SOURCE or _BSD_SOURCE (before including any header file).


Never use this function. Use mkstemp(3) or tmpfile(3) instead.


mkstemp(3), mktemp(3), tempnam(3), tmpfile(3)


This page is part of release 3.52 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at−pages/.

  Copyright (c) 1999 Andries Brouwer (

Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the
entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

Since the Linux kernel and libraries are constantly changing, this
manual page may be incorrect or out-of-date.  The author(s) assume no
responsibility for errors or omissions, or for damages resulting from
the use of the information contained herein.  The author(s) may not
have taken the same level of care in the production of this manual,
which is licensed free of charge, as they might when working

Formatted or processed versions of this manual, if unaccompanied by
the source, must acknowledge the copyright and authors of this work.

2003-11-15, aeb, added tmpnam_r