307 lines
7.2 KiB
Groff
307 lines
7.2 KiB
Groff
.\" $NetBSD: mktemp.3,v 1.22 2003/08/07 16:43:28 agc Exp $
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.\"
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.\" Copyright (c) 1989, 1991, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)mktemp.3 8.1 (Berkeley) 6/4/93
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.\"
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.Dd July 28, 1998
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.Dt MKTEMP 3
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.Os
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.Sh NAME
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.Nm mktemp ,
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.Nm mkstemp ,
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.Nm mkdtemp
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.Nd make unique temporary file or directory name
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In stdlib.h
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.Ft char *
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.Fn mktemp "char *template"
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.Ft int
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.Fn mkstemp "char *template"
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.Ft char *
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.Fn mkdtemp "char *template"
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.Sh DESCRIPTION
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The
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.Fn mktemp
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function
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takes the given file name template and overwrites a portion of it
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to create a file name.
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This file name is unique and suitable for use
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by the application.
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The template may be any file name with some number of
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.Ql X Ns s
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appended
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to it, for example
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.Pa /tmp/temp.XXXX .
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The trailing
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.Ql X Ns s
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are replaced with the current process number and/or a
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unique letter combination.
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The number of unique file names
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.Fn mktemp
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can return depends on the number of
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.Ql X Ns s
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provided; six
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.Ql X Ns s
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will
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result in
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.Fn mktemp
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testing roughly 26 ** 6 combinations.
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At least 6
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.So Li X
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.Sc Ns s
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should be used, though 10 is much better.
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.Pp
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The
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.Fn mkstemp
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function
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makes the same replacement to the template and creates the template file,
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mode 0600, returning a file descriptor opened for reading and writing.
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This avoids the race between testing for a file's existence and opening it
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for use.
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.Pp
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The
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.Fn mkdtemp
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function
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is similar to
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.Fn mkstemp ,
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but it creates a mode 0700 directory instead and returns the path.
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.Pp
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Please note that the permissions of the file or directory being created are
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subject to the restrictions imposed by the
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.Xr umask 2
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system call.
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It may thus happen that the created file is unreadable and/or unwritable.
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.Sh RETURN VALUES
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The
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.Fn mktemp
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and
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.Fn mkdtemp
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functions
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return a pointer to the template on success and
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.Dv NULL
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on failure.
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The
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.Fn mkstemp
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function
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returns \-1 if no suitable file could be created.
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If either call fails an error code is placed in the global variable
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.Va errno .
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.Sh EXAMPLES
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Quite often a programmer will want to replace a use of
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.Fn mktemp
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with
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.Fn mkstemp ,
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usually to avoid the problems described above.
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Doing this correctly requires a good understanding of the code in question.
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.Pp
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For instance, code of this form:
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.Bd -literal -offset indent
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char sfn[15] = "";
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FILE *sfp;
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strlcpy(sfn, "/tmp/ed.XXXXXX", sizeof sfn);
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if (mktemp(sfn) == NULL || (sfp = fopen(sfn, "w+")) == NULL) {
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fprintf(stderr, "%s: %s\en", sfn, strerror(errno));
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return (NULL);
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}
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return (sfp);
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.Ed
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.Pp
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should be rewritten like this:
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.Bd -literal -offset indent
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char sfn[15] = "";
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FILE *sfp;
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int fd = -1;
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strlcpy(sfn, "/tmp/ed.XXXXXX", sizeof sfn);
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if ((fd = mkstemp(sfn)) == -1 ||
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(sfp = fdopen(fd, "w+")) == NULL) {
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if (fd != -1) {
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unlink(sfn);
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close(fd);
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}
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fprintf(stderr, "%s: %s\en", sfn, strerror(errno));
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return (NULL);
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}
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return (sfp);
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.Ed
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.Pp
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Often one will find code which uses
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.Fn mktemp
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very early on, perhaps to globally initialize the template nicely, but the
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code which calls
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.Xr open 2
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or
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.Xr fopen 3
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on that filename will occur much later.
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(In almost all cases, the use of
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.Xr fopen 3
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will mean that the flags
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.Dv O_CREAT
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.Dv O_EXCL
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are not given to
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.Xr open 2 ,
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and thus a symbolic link race becomes possible, hence making
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necessary the use of
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.Xr fdopen 3
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as seen above).
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Furthermore, one must be careful about code which opens, closes, and then
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re-opens the file in question.
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Finally, one must ensure that upon error the temporary file is
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removed correctly.
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.Pp
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There are also cases where modifying the code to use
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.Fn mktemp ,
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in concert with
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.Xr open 2
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using the flags
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.Dv O_CREAT
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.Dv O_EXCL ,
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is better, as long as the code retries a new template if
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.Xr open 2
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fails with an
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.Va errno
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of
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.Er EEXIST .
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.Sh ERRORS
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The
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.Fn mktemp ,
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.Fn mkstemp
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and
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.Fn mkdtemp
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functions
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may set
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.Va errno
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to one of the following values:
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.Bl -tag -width Er
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.It Bq Er ENOTDIR
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The pathname portion of the template is not an existing directory.
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.El
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.Pp
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The
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.Fn mktemp ,
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.Fn mkstemp
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and
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.Fn mkdtemp
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functions
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may also set
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.Va errno
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to any value specified by the
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.Xr stat 2
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function.
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.Pp
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The
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.Fn mkstemp
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function
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may also set
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.Va errno
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to any value specified by the
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.Xr open 2
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function.
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.Pp
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The
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.Fn mkdtemp
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function
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may also set
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.Va errno
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to any value specified by the
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.Xr mkdir 2
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function.
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.Sh SEE ALSO
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.Xr chmod 2 ,
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.Xr getpid 2 ,
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.Xr open 2 ,
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.Xr stat 2 ,
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.Xr umask 2
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.Sh HISTORY
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A
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.Fn mktemp
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function appeared in
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.At v7 .
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.Pp
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The
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.Fn mkstemp
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function appeared in
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.Bx 4.4 .
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.Pp
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The
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.Fn mkdtemp
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function appeared in
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.Nx 1.4 .
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.Sh BUGS
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For
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.Fn mktemp
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there is an obvious race between file name selection and file
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creation and deletion: the program is typically written to call
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.Xr tmpnam 3 ,
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.Xr tempnam 3 ,
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or
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.Fn mktemp .
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Subsequently, the program calls
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.Xr open 2
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or
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.Xr fopen 3
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and erroneously opens a file (or symbolic link, fifo or other
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device) that the attacker has created in the expected file location.
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Hence
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.Fn mkstemp
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is recommended, since it atomically creates the file.
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An attacker can guess the filenames produced by
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.Fn mktemp .
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Whenever it is possible,
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.Fn mkstemp
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or
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.Fn mkdtemp
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should be used instead.
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.Pp
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For this reason,
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.Xr ld 1
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will output a warning message whenever it links code that uses
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.Fn mktemp .
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.Pp
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The
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.Fn mkdtemp
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function is nonstandard and should not be used if portability is required.
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.Sh SECURITY CONSIDERATIONS
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The use of
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.Fn mktemp
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should generally be avoided, as a hostile process can exploit a race
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condition in the time between the generation of a temporary filename by
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.Fn mktemp
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and the invoker's use of the temporary name.
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A link-time warning will be issued advising the use of
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.Fn mkstemp
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or
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.Fn mkdtemp
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instead.
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