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

NAME
     mktemp, mkstemp, mkdtemp, mkdtempat_np, mkstemps, mkstempsat_np,
     mkostemp, mkostemps, mkostempsat_np -- make temporary file name (unique)

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <unistd.h>

     char *
     mktemp(char *template);

     int
     mkstemp(char *template);

     char *
     mkdtemp(char *template);

     char *
     mkdtempat_np(int dfd, char *template);

     int
     mkstemps(char *template, int suffixlen);

     int
     mkstempsat_np(int dfd, char *template, int suffixlen);

     int
     mkostemp(char *template, int oflags);

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

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

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 mkdtemp() function makes the same replacement to the template as in
     mktemp() and creates the template directory, mode 0700.

     The mkostemp() function is like mkstemp() but allows specifying addi-
     tional open(2) flags (defined in <fcntl.h>).  The permitted flags are
     O_APPEND, O_SHLOCK, O_EXLOCK 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 tem-
     plate.  The template should be of the form /tmp/tmpXXXXXXsuffix.  The
     mkstemps() and mkostemps() functions are told the length of the suffix
     string.

     The mkdtempat_np(), mkstempsat_np(), and mkostempsat_np() functions act
     the same as mkdtemp(), mkstemps(), and mkostemps() respectively, except
     in the case where the path specifies a relative path.  In this case, path
     is treated as relative to the directory associated with the file descrip-
     tor fd instead of the current working directory.

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() 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.

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.

LEGACY SYNOPSIS
     #include <unistd.h>

     The include file <unistd.h> is necessary and sufficient for all func-
     tions.

SEE ALSO
     chmod(2), getpid(2), mkdir(2), open(2), stat(2), compat(5)

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() 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
     appeared in OpenBSD 2.4, and later in FreeBSD 3.4.  The mkostemp() and
     mkostemps() functions appeared in OS X 10.12.

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 dan-
     gerous 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 pro-
     gram does not continue blindly in the event that an attacker has already
     created the file with the intention of manipulating or reading its con-
     tents.

BSD                            December 18, 2015                           BSD