NAME
mktemp —
make temporary file name
(unique)
LIBRARY
library “libc”
SYNOPSIS
#include
<stdlib.h>
char *
mktemp(char
*template);
int
mkstemp(char
*template);
int
mkostemp(char
*template, int
oflags);
int
mkostemps(char
*template, int
suffixlen, int
oflags);
char *
mkdtemp(char
*template);
#include
<unistd.h>
int
mkstemps(char
*template, int
suffixlen);
DESCRIPTION
Themktemp()
function takes the given file name template and overwrites a portion of it to
create a file name. This file name is guaranteed not to exist at the time of
function invocation and is suitable for use by the application. The template
may be any file name with some number of
‘Xs’ appended to it, for example
/tmp/temp.XXXXXX. The trailing
‘Xs’ are replaced with a unique
alphanumeric combination. The number of unique file names
mktemp() can return depends on the number of
‘Xs’ provided; six
‘Xs’ will result in
mktemp() selecting one of 56800235584 (62 ** 6)
possible temporary file names.
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 is like mkstemp() but allows specifying
additional open(2) flags (defined in
<fcntl.h>). The permitted
flags are O_APPEND,
O_DIRECT, O_SHLOCK,
O_EXLOCK, O_SYNC and
O_CLOEXEC.
The
mkstemps()
and
mkostemps()
functions act the same as mkstemp() and
mkostemp() respectively, except they permit a suffix
to exist in the template. The template should be of the form
/tmp/tmpXXXXXXsuffix. The
mkstemps() and mkostemps()
function are told the length of the suffix string.
The
mkdtemp()
function makes the same replacement to the template as in
mktemp() and creates the template directory, mode
0700.
RETURN VALUES
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 either call
fails an error code is placed in the global variable
errno.
ERRORS
The mkstemp(),
mkostemp(), mkstemps(),
mkostemps() and mkdtemp()
functions may set errno to one of the following
values:
- [
ENOTDIR] - The pathname portion of the template is not an existing directory.
The mkostemp() and
mkostemps() functions may also set
errno to the following value:
- [
EINVAL] - The oflags argument is invalid.
The mkstemp(),
mkostemp(), mkstemps(),
mkostemps() and mkdtemp()
functions may also set errno to any value specified by
the stat(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.
NOTES
A common problem that results in a core dump is that the
programmer passes in a read-only string to
mktemp(),
mkstemp(), mkstemps() or
mkdtemp(). This is common with programs that were
developed before ISO/IEC 9899:1990
(“ISO C90”) compilers were common. For example,
calling mkstemp() with an argument of
"/tmp/tempfile.XXXXXX" will result in a core dump due to
mkstemp() attempting to modify the string constant
that was given.
The
mkdtemp(),
mkstemp() and mktemp()
function prototypes are also available from
<unistd.h>.
SEE ALSO
STANDARDS
The mkstemp() and
mkdtemp() functions are expected to conform to
IEEE Std 1003.1-2008 (“POSIX.1”). The
mktemp() function is expected to conform to
IEEE Std 1003.1-2001 (“POSIX.1”) and
is not specified by IEEE Std 1003.1-2008
(“POSIX.1”). The mkostemp(),
mkstemps() and mkostemps()
functions do not conform to any standard.
HISTORY
A mktemp() function appeared in
Version 7 AT&T UNIX. The
mkstemp() function appeared in
4.4BSD. The mkdtemp()
function first appeared in OpenBSD 2.2, and later in
FreeBSD 3.2. The mkstemps()
function first appeared in OpenBSD 2.4, and later in
FreeBSD 3.4. The mkostemp()
and mkostemps() functions appeared in
FreeBSD 10.0.
BUGS
This family of functions produces filenames which can be guessed,
though the risk is minimized when large numbers of
‘Xs’ are used to increase the number
of possible temporary filenames. This makes the race in
mktemp(), between testing for a file's existence (in
the mktemp() function call) and opening it for use
(later in the user application) particularly dangerous from a security
perspective. Whenever it is possible, mkstemp() or
mkostemp() should be used instead, since it does not
have the race condition. If mkstemp() cannot be
used, the filename created by mktemp() should be
created using the O_EXCL flag to
open(2) and the return status of the call should be tested for
failure. This will ensure that the program does not continue blindly in the
event that an attacker has already created the file with the intention of
manipulating or reading its contents.