MKTEMP(3) BSD Library Functions Manual MKTEMP(3)

NAME
     mmkktteemmpp - make temporary file name (unique)

LLIIBBRRAARRYY
     Standard C Library (libc, -lc)

SYNOPSIS
     ##iinncclluuddee <>

     char *
     mmkktteemmpp(char *template);

     int
     mmkksstteemmpp(char *template);

     int
     mmkksstteemmppss(char *template, int suffixlen);

     char *
     mmkkddtteemmpp(char *template);

DESCRIPTION
     The mmkktteemmpp() 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 mmkktteemmpp() can return depends on the number of `Xs' provided;
     six `Xs' will result in mmkktteemmpp() selecting one of 56800235584 (62 ** 6)
     possible temporary file names.

     The mmkksstteemmpp() 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 mmkksstteemmppss() function acts the same as mmkksstteemmpp(), except it permits a
     suffix to exist in the template.  The template should be of the form
     /tmp/tmpXXXXXXsuffix.  The mmkksstteemmppss() function is told the length of the
     suffix string.

     The mmkkddtteemmpp() function makes the same replacement to the template as in
     mmkktteemmpp() and creates the template directory, mode 0700.

RETURN VALUES
     The mmkktteemmpp() and mmkkddtteemmpp() functions return a pointer to the template on
     success and NULL on failure.  The mmkksstteemmpp() and mmkksstteemmppss() functions
     return -1 if no suitable file could be created.  If either call fails an
     error code is placed in the global variable errno.

EERRRROORRSS
     The mmkksstteemmpp(), mmkksstteemmppss() and mmkkddtteemmpp() functions may set errno to one of
     the following values:

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

     The mmkksstteemmpp(), mmkksstteemmppss() and mmkkddtteemmpp() functions may also set errno to
     any value specified by the stat(2) function.

     The mmkksstteemmpp() and mmkksstteemmppss() functions may also set errno to any value
     specified by the open(2) function.

     The mmkkddtteemmpp() function may also set errno to any value specified by the
     mkdir(2) function.

NNOOTTEESS
     A common problem that results in a core dump is that the programmer
     passes in a read-only string to mmkktteemmpp(), mmkksstteemmpp(), mmkksstteemmppss() or
     mmkkddtteemmpp().  This is common with programs that were developed before
     ISO/IEC 9899:1990 (``ISO C90'') compilers were common.  For example,
     calling mmkksstteemmpp() with an argument of "/tmp/tempfile.XXXXXX" will result
     in a core dump due to mmkksstteemmpp() attempting to modify the string constant
     that was given.  If the program in question makes heavy use of that type
     of function call, you do have the option of compiling the program so that
     it will store string constants in a writable segment of memory.  See
     gcc(1) for more information.

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 mmkktteemmpp(),
     between testing for a file's existence (in the mmkktteemmpp() function call)
     and opening it for use (later in the user application) particularly dan-
     gerous from a security perspective.  Whenever it is possible, mmkksstteemmpp()
     should be used instead, since it does not have the race condition.  If
     mmkksstteemmpp() cannot be used, the filename created by mmkktteemmpp() should be cre-
     ated using the OEXCL 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.

     The implementation of these functions calls arc4random(3), which is not
     reentrant.  You must provide your own locking around this and other con-
     sumers of the arc4random(3) API.

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

HISTORY
     A mmkktteemmpp() function appeared in Version 7 AT&T UNIX.  The mmkksstteemmpp() func-
     tion appeared in 4.4BSD.  The mmkkddtteemmpp() function first appeared in
     OpenBSD 2.2, and later in FreeBSD 3.2.  The mmkksstteemmppss() function first
     appeared in OpenBSD 2.4, and later in FreeBSD 3.4.

BSD                            February 11, 1998                           BSD