TMPNAM(3) Linux Programmer's Manual TMPNAM(3)


tmpnam, tmpnam_r - create a name for a temporary file



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)


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