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

NAME
     mkdtemp, mkstemp, mkstemps, mktemp -- make temporary file name (unique)

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <unistd.h>

     char *
     mkdtemp(char *template);

     int
     mkstemps(char *template, int suffixlen);

     #include <stdlib.h>

     int
     mkstemp(char *template);

     char *
     mktemp(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 suit-able suitable
     able 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 combi-nation. combination.
     nation.  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 tem-plate. template.
     plate.  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 tem-plate template
     plate 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 existing directory.

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

     The mkstemp() and mkstemps() functions may also set errno to any value specified by the open(2) func-tion. function.
     tion.

     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.  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 seg-ment segment
     ment of memory.  See gcc(1) for more information.

LEGACY SYNOPSIS
     #include <unistd.h>

     The include file <unistd.h> is necessary and sufficient for all functions.

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

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.

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 dangerous 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 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 program
     gram 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                            February 11, 1998                           BSD

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