tmpfile(3) - NetBSD Manual Pages

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TMPFILE(3)              NetBSD Library Functions Manual             TMPFILE(3)


NAME

     tempnam, tmpfile, tmpnam -- temporary file routines


LIBRARY

     Standard C Library (libc, -lc)


SYNOPSIS

     #include <stdio.h>

     FILE *
     tmpfile(void);

     char *
     tmpnam(char *str);

     char *
     tempnam(const char *tmpdir, const char *prefix);


DESCRIPTION

     The tmpfile() function returns a pointer to a stream associated with a
     file descriptor returned by the routine mkstemp(3).  The created file is
     unlinked before tmpfile() returns, causing the file to be automatically
     deleted when the last reference to it is closed.  The file is opened with
     the access value `w+'.

     The tmpnam() function returns a pointer to a file name, in the P_tmpdir
     directory, which did not reference an existing file at some indeterminate
     point in the past.  P_tmpdir is defined in the include file <stdio.h>.
     If the argument s is non-NULL, the file name is copied to the buffer it
     references.  Otherwise, the file name is copied to a static buffer.  In
     either case, tmpnam() returns a pointer to the file name.

     The buffer referenced by s is expected to be at least L_tmpnam bytes in
     length.  L_tmpnam is defined in the include file <stdio.h>.

     The tempnam() function is similar to tmpnam(), but provides the ability
     to specify the directory which will contain the temporary file and the
     file name prefix.

     The environment variable TMPDIR (if set), the argument tmpdir (if
     non-NULL), the directory P_tmpdir, and the directory /tmp are tried, in
     the listed order, as directories in which to store the temporary file.

     The argument prefix, if non-NULL, is used to specify a file name prefix,
     which will be the first part of the created file name.  tempnam() allo-
     cates memory in which to store the file name; the returned pointer may be
     used as a subsequent argument to free(3).


RETURN VALUES

     The tmpfile() function returns a pointer to an open file stream on suc-
     cess, and a NULL pointer on error.

     The tmpnam() and tempnam() functions return a pointer to a file name on
     success, and a NULL pointer on error.


ERRORS

     The tmpfile() function may fail and set the global variable errno for any
     of the errors specified for the library functions fdopen(3) or
     mkstemp(3).

     The tmpnam() function may fail and set errno for any of the errors speci-
     fied for the library function mktemp(3).

     The tempnam() function may fail and set errno for any of the errors spec-
     ified for the library functions malloc(3) or mktemp(3).


SEE ALSO

     mkstemp(3), mktemp(3)


STANDARDS

     The tmpfile() and tmpnam() functions conform to ANSI X3.159-1989
     (``ANSI C89'').


BUGS

     These interfaces are provided for AT&T System V UNIX and ANSI compatibil-
     ity only.  The mkstemp(3) interface is strongly preferred.


SECURITY CONSIDERATIONS

     There are four important problems with these interfaces (as well as with
     the historic mktemp(3) interface).  First, there is an obvious race
     between file name selection and file creation and deletion: the program
     is typically written to call tmpnam(), tempnam(), or mktemp(3).  Subse-
     quently, the program calls open(2) or fopen(3) and erroneously opens a
     file (or symbolic link, or fifo or other device) that the attacker has
     placed in the expected file location.  Hence mkstemp(3) is recommended,
     since it atomically creates the file.

     Second, most historic implementations provide only a limited number of
     possible temporary file names (usually 26) before file names will start
     being recycled.  Third, the AT&T System V UNIX implementations of these
     functions (and of mktemp(3)) use the access(2) system call to determine
     whether or not the temporary file may be created.  This has obvious rami-
     fications for setuid or setgid programs, complicating the portable use of
     these interfaces in such programs.  Finally, there is no specification of
     the permissions with which the temporary files are created.

     This implementation of tmpfile() does not have these flaws, and that of
     tmpnam() and tempnam() only have the first limitation, but portable soft-
     ware cannot depend on that.  In particular, the tmpfile() interface
     should not be used in software expected to be used on other systems if
     there is any possibility that the user does not wish the temporary file
     to be publicly readable and writable.

     A link-time warning will be issued if tmpnam() or tempnam() is used, and
     advises the use of mkstemp() instead.

NetBSD 5.2                       June 18, 2005                      NetBSD 5.2