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mkstemp(3)

NAME

     mktemp -- make temporary file name (unique)

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

     #include <unistd.h>

     char *
     mktemp(char *template);

     int
     mkstemp(char *template);

     int
     mkstemps(char *template, int suffixlen);

     char *
     mkdtemp(char *template);

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

     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() and mkstemps() 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(), mkstemps() and mkdtemp() functions may set errno to one of
     the following values:

     [ENOTDIR]		The pathname portion of the template is not an exist-
			ing directory.
     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.  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 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()
     should be used instead, since it does not have the race condition.  If
     mkstemp() cannot be used, the filename created by mktemp() should be cre-
     ated 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.

     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 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.

FreeBSD 5.4		       February 11, 1998		   FreeBSD 5.4

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