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MKTEMP(3)                  Library Functions Manual                  MKTEMP(3)

     mktemp, mkstemp, mkstemps, mkostemp, mkostemps, mkdtemp - make unique
     temporary file or directory name

     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);

     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 unique and
     suitable for use by the application.  The template may be any file name
     with some number of `X' characters appended to it, for example
     /tmp/temp.XXXXXX.  The trailing `X' characters in the template are
     replaced with a unique letter and number combination.  mktemp() can
     return depends on the number of `X' characters provided.  Although the
     NetBSD implementation of the functions will accept any number of trailing
     `X' characters, for portability reasons one should use only six.  Using
     six `X' characters will result in mktemp() testing roughly 62 ** 6
     (56800235584) combinations.

     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,

     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

     The mkdtemp() function is similar to mkstemp(), but it creates a mode
     0700 directory instead and returns the path.

     Please note that the permissions of the file or directory being created
     are subject to the restrictions imposed by the umask(2) system call.  It
     may thus happen that the created file is unreadable and/or unwritable.

     The mktemp() and mkdtemp() functions return a pointer to the template on
     success and NULL on failure.  The mkstemp(), mkostemp(), mkstemps() and
     mkostemps() functions returns -1 if no suitable file could be created.
     If either call fails an error code is placed in the global variable

     Quite often a programmer will want to replace a use of mktemp() with
     mkstemp(), usually to avoid the problems described above.  Doing this
     correctly requires a good understanding of the code in question.

     For instance, code of this form:

           char sfn[15] = "";
           FILE *sfp;

           strlcpy(sfn, "/tmp/ed.XXXXXX", sizeof sfn);
           if (mktemp(sfn) == NULL || (sfp = fopen(sfn, "w+")) == NULL) {
                   fprintf(stderr, "%s: %s\n", sfn, strerror(errno));
                   return (NULL);
           return (sfp);

     should be rewritten like this:

           char sfn[15] = "";
           FILE *sfp;
           int fd = -1;

           strlcpy(sfn, "/tmp/ed.XXXXXX", sizeof sfn);
           if ((fd = mkstemp(sfn)) == -1 ||
               (sfp = fdopen(fd, "w+")) == NULL) {
                   if (fd != -1) {
                   fprintf(stderr, "%s: %s\n", sfn, strerror(errno));
                   return (NULL);
           return (sfp);

     Often one will find code which uses mktemp() very early on, perhaps to
     globally initialize the template nicely, but the code which calls open(2)
     or fopen(3) on that filename will occur much later.  (In almost all
     cases, the use of fopen(3) will mean that the flags O_CREAT | O_EXCL are
     not given to open(2), and thus a symbolic link race becomes possible,
     hence making necessary the use of fdopen(3) as seen above).  Furthermore,
     one must be careful about code which opens, closes, and then re-opens the
     file in question.  Finally, one must ensure that upon error the temporary
     file is removed correctly.

     There are also cases where modifying the code to use mktemp(), in concert
     with open(2) using the flags O_CREAT | O_EXCL, is better, as long as the
     code retries a new template if open(2) fails with an errno of EEXIST.

     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 mktemp(), mkstemp() and mkdtemp() functions may also set errno to any
     value specified by the stat(2) function.

     The mkstemp() function 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.

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

     The mktemp() conforms to IEEE Std 1003.1-2001 ("POSIX.1").  It was
     however removed from the specification in the IEEE Std 1003.1-2008
     ("POSIX.1") revision.  The mkstemp() and mkdtemp() functions conform to
     IEEE Std 1003.1-2004 ("POSIX.1") and IEEE Std 1003.1-2008 ("POSIX.1"),

     A mktemp() function appeared in Version 7 AT&T UNIX.

     The mkstemp() function appeared in 4.4BSD.

     The mkdtemp() function appeared in NetBSD 1.4.  The mkstemps() function
     first appeared in OpenBSD 2.4, and later in FreeBSD 3.4 and NetBSD 7.0.
     The mkostemp() and mkostemps() functions appeared in FreeBSD 10.0 and
     NetBSD 7.0.

     For mktemp() there is an obvious race between file name selection and
     file creation and deletion: the program is typically written to call
     tmpnam(3), tempnam(3), or mktemp().  Subsequently, the program calls
     open(2) or fopen(3) and erroneously opens a file (or symbolic link, fifo
     or other device) that the attacker has created in the expected file
     location.  Hence mkstemp() is recommended, since it atomically creates
     the file.  An attacker can guess the filenames produced by mktemp().
     Whenever it is possible, mkstemp() or mkdtemp() should be used instead.

     For this reason, ld(1) will output a warning message whenever it links
     code that uses mktemp().

     The mkdtemp() function is nonstandard and should not be used if
     portability is required.

     The use of mktemp() should generally be avoided, as a hostile process can
     exploit a race condition in the time between the generation of a
     temporary filename by mktemp() and the invoker's use of the temporary
     name.  A link-time warning will be issued advising the use of mkstemp()
     or mkdtemp() instead.

NetBSD 10.99                     July 25, 2021                    NetBSD 10.99