char tmpnam(char s);
char tempnam(const char dir, const char pfx);
tmpnam always generates a file name using the path-prefix defined as P_tmpdir in the stdio.h header file. 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.
tempnam 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 file is used. If that directory is not accessible, /tmp will be 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-file name.
tempnam uses malloc to get space for the constructed file name, and returns a pointer to this area. Thus, any pointer value returned from tempnam may serve as an argument to free [see malloc(3C)]. If tempnam cannot return the expected result for any reason-- for example, malloc failed--or none of the above mentioned attempts to find an appropriate directory was successful, a NULL pointer will be returned.
tempnam fails if there is not enough space.
Files created using these functions and either fopen or creat 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 remove the file when its use is ended.
If called more than TMP_MAX (defined in stdio.h) times in a single process, these functions start recycling previously used names.
Between the time a file name 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 file names are chosen to render duplication by other means unlikely.
When used in a multi-threaded application, tmpnam should have a non-NULL argument.