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MKTEMP(3)	       FreeBSD Library Functions Manual		     MKTEMP(3)

     mktemp -- make temporary file name	(unique)

     Standard C	Library	(libc, -lc)

     #include <stdlib.h>

     char *
     mktemp(char *template);

     mkstemp(char *template);

     mkostemp(char *template, int oflags);

     mkostemps(char *template, int suffixlen, int oflags);

     mkostempsat(int dfd, char *template, int suffixlen, int oflags);

     char *
     mkdtemp(char *template);

     #include <unistd.h>

     mkstemps(char *template, int suffixlen);

     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 re-
     placed 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) pos-
     sible 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 addi-
     tional open(2) flags (defined in <fcntl.h>).  The permitted flags are

     The mkstemps() and	mkostemps() functions act the same as mkstemp()	and
     mkostemp()	respectively, except they permit a suffix to exist in the tem-
     plate.  The template should be of the form	/tmp/tmpXXXXXXsuffix.  The
     mkstemps()	and mkostemps()	function are told the length of	the suffix

     The mkostempsat() function	acts the same as mkostemps() but takes an ad-
     ditional directory	descriptor as a	parameter.  The	temporary file is cre-
     ated 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.

     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.

     The mkstemp(), mkostemp(),	mkstemps(), mkostemps()	and mkdtemp() func-
     tions may set errno to one	of the following values:

     [ENOTDIR]		The pathname portion of	the template is	not an exist-
			ing directory.

     The mkostemp() and	mkostemps() functions may also set errno to the	fol-
     lowing value:

     [EINVAL]		The oflags argument is invalid.

     The mkstemp(), mkostemp(),	mkstemps(), mkostemps()	and mkdtemp() func-
     tions 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.

     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,	call-
     ing 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	avail-
     able from <unistd.h>.

     chmod(2), getpid(2), mkdir(2), open(2), stat(2)

     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.

     A mktemp()	function appeared in Version 7 AT&T UNIX.  The mkstemp() func-
     tion appeared in 4.4BSD.  The mkdtemp() function first appeared in
     OpenBSD 2.2, and later in FreeBSD 3.2.  The mkstemps() function first ap-
     peared in OpenBSD 2.4, and	later in FreeBSD 3.4.  The mkostemp() and
     mkostemps() functions appeared in FreeBSD 10.0.  The mkostempsat()	func-
     tion appeared in FreeBSD 13.0.

     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 dan-
     gerous 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 cre-
     ated 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	13.0			 July 29, 2019			  FreeBSD 13.0


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