MKTEMP(3) | Library Functions Manual | MKTEMP(3) |
mktemp
, mkstemp
,
mkostemp
, mkstemps
,
mkostemps
, mkdtemp
—
make temporary file name (unique)
#include
<stdlib.h>
char *
mktemp
(char
*template);
int
mkstemp
(char
*template);
int
mkstemps
(char
*template, int
suffixlen);
char *
mkdtemp
(char
*template);
#include
<stdlib.h>
#include <fcntl.h>
int
mkostemp
(char
*template, int
flags);
int
mkostemps
(char
*template, int
suffixlen, int
flags);
The
mktemp
()
family of functions take the given file name template and overwrite a
portion of it to create a new file name. This file name is unique and
suitable for use by the application. The template may be any file name with
at least six trailing Xs, for example
/tmp/temp.XXXXXXXX. The trailing
Xs are replaced with a unique digit and letter
combination. The number of unique file names that can be returned depends on
the number of Xs provided;
mktemp
() will try at least 2 ** 31 combinations
before giving up. At least six Xs must be used, though 10
is much better.
The
mktemp
()
function generates a temporary file name based on a template as described
above. Because mktemp
() does not actually create the
temporary file there is a window of opportunity during which another process
can open the file instead. Because of this race condition,
mktemp
() should not be used where
mkstemp
() can be used instead.
mktemp
() was marked as a legacy interface in
IEEE Std 1003.1-2001 (“POSIX.1”).
The
mkstemp
()
function makes the same replacement to the template and creates the template
file, mode 0600, returning a file descriptor opened for reading and writing.
This avoids the race between testing for a file's existence and opening it
for use.
The
mkostemp
()
function acts the same as mkstemp
(), except that the
flags argument may contain zero or more of the
following flags for the underlying
open(2) system call:
The
mkstemps
()
and
mkostemps
()
functions act the same as mkstemp
() and
mkostemp
(), except they permit a suffix to exist in
the template. The template should be of the form
/tmp/tmpXXXXXXXXXXsuffix.
mkstemps
() and mkostemps
()
are told the length of the suffix string, i.e.,
strlen("suffix")
.
The
mkdtemp
()
function makes the same replacement to the template as in
mktemp
() and creates the template directory, mode
0700.
The mktemp
() and
mkdtemp
() functions return a pointer to the template
on success and NULL
on failure. The
mkstemp
(), mkostemp
(),
mkstemps
(), and mkostemps
()
functions return -1 if no suitable file could be created. If any call fails,
an error code is placed in the global variable
errno.
Quite often a programmer will want to replace a use of
mktemp
() with mkstemp
(),
usually to avoid the problems described above. Doing this correctly requires
a good understanding of the code in question.
For instance, code of this form:
char sfn[19]; FILE *sfp; strlcpy(sfn, "/tmp/ed.XXXXXXXXXX", sizeof(sfn)); if (mktemp(sfn) == NULL || (sfp = fopen(sfn, "w+")) == NULL) { warn("%s", sfn); return (NULL); } return (sfp);
should be rewritten like this:
char sfn[19]; FILE *sfp; int fd; strlcpy(sfn, "/tmp/ed.XXXXXXXXXX", sizeof(sfn)); if ((fd = mkstemp(sfn)) == -1 || (sfp = fdopen(fd, "w+")) == NULL) { if (fd != -1) { unlink(sfn); close(fd); } warn("%s", sfn); return (NULL); } return (sfp);
Often one will find code which uses
mktemp
() very early on, perhaps to globally
initialize the template nicely, but the code which calls
open(2) or
fopen(3) on that file name
will occur much later. (In almost all cases, the use of
fopen(3) will mean that the
flags O_CREAT
| O_EXCL
are
not given to open(2), and thus
a symbolic link race becomes possible, hence making necessary the use of
fdopen(3) as seen above.)
Furthermore, one must be careful about code which opens, closes, and then
re-opens the file in question. Finally, one must ensure that upon error the
temporary file is removed correctly.
There are also cases where modifying the code to use
mktemp
(), in concert with
open(2) using the flags
O_CREAT
| O_EXCL
, is better,
as long as the code retries a new template if
open(2) fails with an
errno of EEXIST
.
The mktemp
(),
mkstemp
(), mkostemp
(), and
mkdtemp
() functions may set
errno to one of the following values:
EINVAL
]EEXIST
]The mkstemps
() and
mkostemps
() functions may set
errno to
EINVAL
]EEXIST
]In addition, the mkostemp
() and
mkostemps
() functions may also set
errno to
EINVAL
]The mktemp
() function may also set
errno to any value specified by the
lstat(2) function.
The mkstemp
(),
mkostemp
(), mkstemps
(), and
mkostemps
() functions may also set
errno to any value specified by the
open(2) function.
The mkdtemp
() function may also set
errno to any value specified by the
mkdir(2) function.
chmod(2), lstat(2), mkdir(2), open(2), tempnam(3), tmpfile(3), tmpnam(3)
The mkdtemp
() and
mkstemp
() functions conform to the
IEEE Std 1003.1-2008 (“POSIX.1”)
specification. The ability to specify more than six Xs is
an extension to that standard. The mkostemp
()
function is expected to conform to a future revision of that standard.
The mktemp
() function conforms to
IEEE Std 1003.1-2001 (“POSIX.1”); as
of IEEE Std 1003.1-2008 (“POSIX.1”) it
is no longer a part of the standard.
The mkstemps
() and
mkostemps
() functions are non-standard and should
not be used if portability is required.
A mktemp
() function appeared in
Version 7 AT&T UNIX. The
mkdtemp
() function appeared in
OpenBSD 2.2. The mkstemp
()
function appeared in 4.4BSD. The
mkstemps
() function appeared in
OpenBSD 2.3. The mkostemp
()
and mkostemps
() functions appeared in
OpenBSD 5.7.
For mktemp
() there is an obvious race
between file name selection and file creation and deletion: the program is
typically written to call
tmpnam(3),
tempnam(3), or
mktemp
(). Subsequently, the program calls
open(2) or
fopen(3) and erroneously opens
a file (or symbolic link, FIFO or other device) that the attacker has
created in the expected file location. Hence
mkstemp
() is recommended, since it atomically
creates the file. An attacker can guess the file names produced by
mktemp
(). Whenever it is possible,
mkstemp
() or mkdtemp
()
should be used instead.
For this reason, ld(1)
will output a warning message whenever it links code that uses
mktemp
().
October 26, 2014 | OpenBSD-6.8 |