.\" $OpenBSD: tmpnam.3,v 1.17 2012/09/07 05:37:06 jmc Exp $ .\" .\" Copyright (c) 1988, 1991, 1993 .\" The Regents of the University of California. All rights reserved. .\" .\" This code is derived from software contributed to Berkeley by .\" the American National Standards Committee X3, on Information .\" Processing Systems. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" 3. Neither the name of the University nor the names of its contributors .\" may be used to endorse or promote products derived from this software .\" without specific prior written permission. .\" .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE .\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .Dd $Mdocdate: September 7 2012 $ .Dt TMPFILE 3 .Os .Sh NAME .Nm tempnam , .Nm tmpfile , .Nm tmpnam .Nd temporary file routines .Sh SYNOPSIS .Fd #include .Ft FILE * .Fn tmpfile void .Ft char * .Fn tmpnam "char *str" .Ft char * .Fn tempnam "const char *tmpdir" "const char *prefix" .Sh DESCRIPTION The .Fn tmpfile function returns a pointer to a stream associated with a file descriptor returned by the routine .Xr mkstemp 3 . The created file is unlinked before .Fn tmpfile returns, causing the file to be automatically deleted when the last reference to it is closed. Since .Xr mkstemp 3 creates the file with mode .Dv S_IRUSR \*(Ba S_IWUSR , after the unlink, .Xr fchown 2 and .Xr umask 2 are used to set the file mode to the expected value. The file is opened with the access value .Ql w+ . .Pp The .Fn tmpnam function returns a pointer to a file name, in the .Dv P_tmpdir directory, which did not reference an existing file at some indeterminate point in the past. .Dv P_tmpdir is defined in the include file .Aq Pa stdio.h . If the argument .Fa str 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, .Fn tmpnam returns a pointer to the file name. .Pp The buffer referenced by .Fa str is expected to be at least .Dv L_tmpnam bytes in length. .Dv L_tmpnam is defined in the include file .Aq Pa stdio.h . .Pp The .Fn tempnam function is similar to .Fn tmpnam , but provides the ability to specify the directory which will contain the temporary file and the file name prefix. .Pp The environment variable .Ev TMPDIR (if set), the argument .Fa tmpdir (if non-null), the directory .Dv P_tmpdir , and the directory .Pa /tmp are tried, in the listed order, as directories in which to store the temporary file. .Pp The argument .Fa prefix , if non-null, is used to specify a file name prefix, which will be the first part of the created file name. .Fn tempnam allocates memory in which to store the file name; the returned pointer may be used as a subsequent argument to .Xr free 3 . .Sh RETURN VALUES The .Fn tmpfile function returns a pointer to an open file stream on success, and a null pointer on error. .Pp The .Fn tmpnam and .Fn tempnam functions return a pointer to a file name on success, and a null pointer on error. .Sh ENVIRONMENT .Bl -tag -width Ds .It Ev TMPDIR .Pf [ Fn tempnam only] If set, the directory in which the temporary file is stored. .Ev TMPDIR is ignored for processes for which .Xr issetugid 2 is true. .El .Sh ERRORS The .Fn tmpfile function may fail and set the global variable .Va errno for any of the errors specified for the library functions .Xr fdopen 3 or .Xr mkstemp 3 . .Pp The .Fn tmpnam function may fail and set .Va errno for any of the errors specified for the library function .Xr mktemp 3 . .Pp The .Fn tempnam function may fail and set .Va errno for any of the errors specified for the library functions .Xr malloc 3 or .Xr mktemp 3 . .Sh SEE ALSO .Xr issetugid 2 , .Xr mkstemp 3 , .Xr mktemp 3 .Sh STANDARDS The .Fn tmpfile and .Fn tmpnam functions conform to .St -ansiC . .Sh BUGS .Fn tmpnam and .Fn tempnam are provided for System V and .Tn ANSI compatibility only. These interfaces are typically not used in safe ways. The .Xr mkstemp 3 interface is strongly preferred. .Pp There are four important problems with these interfaces (as well as with the historic .Xr 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 .Fn tmpnam , .Fn tempnam , or .Xr mktemp 3 . Subsequently, the program calls .Xr open 2 or .Xr 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 .Xr mkstemp 3 is recommended, since it atomically creates the file. .Pp 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 System V implementations of these functions (and of .Xr mktemp 3 ) use the .Xr access 2 function to determine whether or not the temporary file may be created. This has obvious ramifications for daemons or setuid/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. .Pp This implementation does not have these flaws, but portable software cannot depend on that. .Pp For these reasons, .Xr ld 1 will output a warning message whenever it links code that uses the functions .Fn tmpnam or .Fn tempnam .