The Fn tmpnam function returns a pointer to a file name, in the P_tmpdir directory, which did not reference an existing file at some indeterminate point in the past. P_tmpdir is defined in the include file Aq Pa stdio.h . If the argument Fa s is non- NULL the file name is copied to the buffer it references. Otherwise, the file name is copied to a static buffer. In either case, Fn tmpnam returns a pointer to the file name.
The buffer referenced by Fa s is expected to be at least L_tmpnam bytes in length. L_tmpnam is defined in the include file Aq Pa stdio.h .
The Fn tempnam function is similar to Fn tmpnam , but provides the ability to specify the directory which will contain the temporary file and the file name prefix.
The environment variable TMPDIR (if set), the argument Fa tmpdir (if non- NULL and not empty), the directory P_tmpdir and the directory /tmp are tried, in the listed order, as directories in which to store the temporary file.
The argument Fa prefix , if non- NULL is used to specify a file name prefix, which will be the first part of the created file name. Fn Tempnam allocates memory in which to store the file name; the returned pointer may be used as a subsequent argument to free(3).
The Fn tmpnam and Fn tempfile functions return a pointer to a file name on success, and a NULL pointer on error.
The Fn tmpnam function may fail and set errno for any of the errors specified for the library function mktemp(3).
The Fn tempnam function may fail and set errno for any of the errors specified for the library functions malloc(3) or mktemp(3).
There are four important problems with these interfaces (as well as with the historic mktemp(3) interface). First, there is an obvious race between file name selection and file creation and deletion. Second, most historic implementations provide only a limited number of possible temporary file names (usually 26) before file names will start being recycled. Third, the System V implementations of these functions (and of mktemp) use the access(2) function to determine whether or not the temporary file may be created. This has obvious ramifications for setuid or setgid programs, complicating the portable use of these interfaces in such programs. Finally, there is no specification of the permissions with which the temporary files are created.
This implementation does not have these flaws, but portable software cannot depend on that. In particular, the Fn tmpfile interface should not be used in software expected to be used on other systems if there is any possibility that the user does not wish the temporary file to be publicly readable and writable.