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\*(Ba 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 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 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).
RETURN VALUES
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.
ENVIRONMENT
TMPDIR
.Pf [ Fn tempnam
only]
If set,
the directory in which the temporary file is stored.
TMPDIR
is ignored for processes
for which
issetugid(2)
is true.
ERRORS
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).
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();.