#include <stdio.h> char *tempnam(const char *dir, const char *pfx);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
The tempnam() function returns a pointer to a string that is a valid filename, and such that a file with this name did not exist when tempnam() checked. The filename suffix of the pathname generated will start with pfx in case pfx is a non-NULL string of at most five bytes. The directory prefix part of the pathname generated is required to be "appropriate" (often that at least implies writable).
Attempts to find an appropriate directory go through the following steps:
|tempnam()||Thread safety||MT-Safe env|
SUSv2 does not mention the use of TMPDIR; glibc will use it only when the program is not set-user-ID. On SVr4, the directory used under d) is /tmp (and this is what glibc does).
Because it dynamically allocates memory used to return the pathname, tempnam() is reentrant, and thus thread safe, unlike tmpnam(3).
The tempnam() function generates a different string each time it is called, up to TMP_MAX (defined in <stdio.h>) times. If it is called more than TMP_MAX times, the behavior is implementation defined.
tempnam() uses at most the first five bytes from pfx.