char *tmpnam(char *s);
char *tempnam(const char *dir, const char *pfx);
The tmpnam function always generates a filename using the path-prefix defined as P_tmpdir in the <stdio.h> header file. The generated filename is different each time that tmpnam is called from the same process, up to TMP_MAX times. If s is NULL, tmpnam leaves its result in an internal static area and returns a pointer to that area. The next call to tmpnam destroys the contents of the area. If s is not NULL, it is assumed to be the address of an array of at least L_tmpnam bytes, where L_tmpnam is a constant defined in <stdio.h>; tmpnam places its result in that array and returns s.
tempnam allows choosing a directory. The argument dir points to the name of the directory in which the file is to be created. If dir is NULL or points to a string that is not a name for an appropriate directory, the path-prefix defined as P_tmpdir in the <stdio.h> header file is used. If that directory is not accessible, \/tmp is used as a last resort. This entire sequence can be up-staged by providing an environment variable TMPDIR in the user's environment, whose value is the name of the desired temporary-file directory.
Many applications prefer their temporary files to have certain favorite initial letter sequences in their names. Use the pfx argument for this. This argument may be NULL or point to a string of up to five characters to be used as the first few characters of the temporary-filename.
If tempnam cannot return the expected result for any reason, i.e., malloc(S) failed, or none of the above mentioned attempts to find an appropriate directory was successful, a NULL pointer is returned.
Files created using these functions and either fopen(S) or creat(S) are temporary only in the sense that they reside in a directory intended for temporary use, and their names are unique. It is the user's responsibility to use unlink(S) to remove the file when its use is ended.
If called more than 17,576 times in a single process, these functions recycle previously used names.
Between the time a filename is created and the file is opened, it is possible for some other process to create a file with the same name. This can never happen if that other process is using these functions or mktemp, and the filenames are chosen to render duplication by other means unlikely.
X/Open Portability Guide, Issue 3, 1989
ANSI X3.159-1989 Programming Language -- C ;
IEEE POSIX Std 1003.1-1990 System Application Program Interface (API) [C Language] (ISO/IEC 9945-1) ;
and NIST FIPS 151-1 .