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FreeBSD 11.0 - man page for mktemp (freebsd section 3)

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

NAME
     mktemp -- make temporary file name (unique)

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <stdlib.h>

     char *
     mktemp(char *template);

     int
     mkstemp(char *template);

     int
     mkostemp(char *template, int oflags);

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

     char *
     mkdtemp(char *template);

     #include <unistd.h>

     int
     mkstemps(char *template, int suffixlen);

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 invo-
     cation 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() select-
     ing 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 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, O_EXLOCK,
     O_SYNC and O_CLOEXEC.

     The mkstemps() and mkostemps() functions act the same as mkstemp() and mkostemp() respec-
     tively, 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 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(), 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() functions may set errno to
     one of the following values:

     [ENOTDIR]		The pathname portion of the template is not an existing directory.

     The mkostemp() and mkostemps() functions may also set errno to the following value:

     [EINVAL]		The oflags argument is invalid.

     The mkstemp(), mkostemp(), mkstemps(), mkostemps() and mkdtemp() functions 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.

     The mkdtemp(), mkstemp() and mktemp() function prototypes are also available from
     <unistd.h>.

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

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() function 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 FreeBSD 10.0.

BUGS
     This family of functions produces filenames which can be guessed, though the risk is mini-
     mized when large numbers of 'Xs' are used to increase the number of possible temporary file-
     names.  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
     dangerous 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
     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.

BSD					  August 8, 2013				      BSD
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