TMPFILE(3) Library Routines TMPFILE(3)
tempnam, tmpfile, tmpnam - temporary file routines
FILE * tmpfile(void);
char * tmpnam(char *str);
char * tempnam(const char *tmpdir, const char *prefix);
The tmpfile function returns a pointer to a stream associated with a
file descriptor returned by the routine mkstemp(3). The created file
is unlinked before tmpfile returns, causing the file to be automati-
cally deleted when the last reference to it is closed. The file is
opened with the access value w+b.
The tmpnam function returns a pointer to a file name, in the P_tmpdir
directory, which did not reference an existing file at some indetermi-
nate point in the past. P_tmpdir is defined in the include file
stdio.h. If the argument str 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, tmpnam returns a pointer to the file
The buffer referenced by str is expected to be at least L_tmpnam bytes
in length. L_tmpnam is defined in the include file stdio.h.
The tempnam function is similar to 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 dir (if non-
NULL), 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 prefix, if non-NULL, is used to specify a file name pre-
fix, which will be the first part of the created file name. Tempnam
allocates memory in which to store the file name; the returned pointer
may be used as a subsequent argument to free(3).
The tmpfile function returns a pointer to an open file stream on suc-
cess, and a NULL pointer on error.
The tmpnam and tempfile functions return a pointer to a file name on
success, and a NULL pointer on error.
The tmpfile function may fail and set the global variable errno for any
of the errors specified for the library functions fdopen(3) or
The tmpnam function may fail and set errno for any of the errors speci-
fied for the library function mktemp(3).
The tempnam function may fail and set errno for any of the errors spec-
ified for the library functions malloc(3) or mktemp (3).
The tmpfile and tmpnam functions conform to ANSI C.
These interfaces are provided for System V and ANSI compatibility only.
The mkstemp(3) interface is strongly preferred.
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. Sec-
ond, 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 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.
GNO 26 November 1995 TMPFILE(3)
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