MKTEMP(3) FreeBSD Library Functions Manual MKTEMP(3)
NAME
mktemp - make temporary file name (unique)
LIBRARY
Standard C Library (libc, -lc)
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);
int
mkostempsat(int dfd, char *template, int suffixlen, int oflags);
char *
mkdtemp(char *template);
#include <unistd.h>
int
mkstemps(char *template, int suffixlen);
DESCRIPTION
The mktemp() 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 mkostempsat() function acts the same as mkostemps() but takes an
additional directory descriptor as a parameter. The temporary file is
created relative to the corresponding directory, or to the current
working directory if the special value AT_FDCWD is specified. If the
template path is an absolute path, the dfd parameter is ignored and the
behavior is identical to mkostemps().
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
chmod(2), getpid(2), mkdir(2), open(2), stat(2)
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(), mkostemps() and
mkostempsat() 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. The mkostempsat()
function appeared in FreeBSD 13.0.
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(), mkostemp() or mkostempsat() should be used instead, since they
do 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.
FreeBSD 13.1-RELEASE-p6 July 29, 2019 FreeBSD 13.1-RELEASE-p6
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