tempnam, tmpfile, tmpnam - temporary file routines
#include <stdio.h>
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
automatically
deleted when the last reference to it is closed. Since mkstemp(3) creates
the file with mode S_IRUSR S_IWUSR, after the unlink
fchown(2) and
umask(2) are used to set the file mode to the expected value. The file
is opened with the access value `w+'.
The 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
<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 name.
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
tmpdir (if nonnull),
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 prefix,
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 success,
and a null pointer on error.
The tmpnam() and tempnam() 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
mkstemp(3).
The tmpnam() function may fail and set errno for any of the
errors specified
for the library function mktemp(3).
The tempnam() function may fail and set errno for any of the
errors specified
for the library functions malloc(3) or mktemp(3).
mkstemp(3), mktemp(3)
The tmpfile() and tmpnam() functions conform to ANSI
X3.159-1989 (``ANSI
C'').
tmpnam() and tempnam() are provided for System V and ANSI
compatibility
only. These interfaces are typically not used in safe ways.
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:
the program is
typically written to call tmpnam(), tmpname(), or mktemp(3).
Subsequently,
the program calls open(2) or fopen(3) and erroneously
opens a file
(or symbolic link, or FIFO or other device) that the attacker has placed
in the expected file location. Hence mkstemp(3) is recommended, since it
atomically creates the file.
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(3)) use the access(2) function to determine
whether or not
the temporary file may be created. This has obvious ramifications for
daemons or setuid/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.
For these reasons, ld(1) will output a warning message whenever it links
code that uses the functions tmpnam() or tempnam().
OpenBSD 3.6 November 17, 1993
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