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: (i) In case the
environment variable TMPDIR exists and contains the name of an appropriate
directory, that is used. (ii) Otherwise, if the dir argument is
non-NULL and appropriate, it is used. (iii) Otherwise, P_tmpdir (as
defined in <stdio.h>) is used when appropriate. (iv) Finally an
implementation-defined directory may be used.
SUSv2 does not mention the use of TMPDIR; glibc will use it only when the
program is not suid. SVID2 specifies that the directory used under (iv) is
/tmp. SVID2 specifies that the string returned by tempnam() was
allocated using malloc(3) and hence can be freed by free(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 behaviour is implementation defined.
In case the pfx argument has length larger than five, glibc will use the
first five bytes. Upon failure to find a unique name, glibc will return