The tmpnam() function always generates a file name using the path prefix defined as P_tmpdir in the <stdio.h> header. On Solaris systems, the default value for P_tmpdir is /var/tmp. 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() will destroy 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.

The tmpnam_r() function has the same functionality as tmpnam() except that if s is a null pointer, the function returns NULL.

The tempnam() function allows the user to control the choice of 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 is used. If that directory is not accessible, /tmp is used. If, however, the TMPDIR environment variable is set in the user's environment, its value is used as the temporary-file directory.

Many applications prefer that temporary files have certain initial character sequences in their names. The pfx argument may be NULL or point to a string of up to five characters to be used as the initial characters of the temporary-file name.

Upon successful completion, tempnam() uses malloc(3C) to allocate space for a string, puts the generated pathname in that space, and returns a pointer to it. The pointer is suitable for use in a subsequent call to free(). If tempnam() cannot return the expected result for any reason (for example, malloc() failed), or if none of the above-mentioned attempts to find an appropriate directory was successful, a null pointer is returned and errno is set to indicate the error.