252 lines
6.1 KiB
Groff
252 lines
6.1 KiB
Groff
.\" 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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.\" 4. 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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.\" $FreeBSD$
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.\"
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.Dd February 11, 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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.Nd make temporary file name (unique)
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In unistd.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 int
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.Fn mkstemps "char *template" "int suffixlen"
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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 guaranteed not to exist at the time of function invocation
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and is 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.XXXXXX .
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The trailing
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.Ql X Ns s
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are replaced with a
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unique alphanumeric 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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selecting one of 56800235584 (62 ** 6) possible temporary file names.
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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 mkstemps
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function acts the same as
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.Fn mkstemp ,
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except it permits a suffix to exist in the template.
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The template should be of the form
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.Pa /tmp/tmpXXXXXXsuffix .
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The
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.Fn mkstemps
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function
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is told the length of the suffix string.
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.Pp
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The
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.Fn mkdtemp
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function makes the same replacement to the template as in
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.Fn mktemp
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and creates the template directory, mode 0700.
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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 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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and
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.Fn mkstemps
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functions
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return \-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 ERRORS
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The
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.Fn mkstemp ,
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.Fn mkstemps
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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 mkstemp ,
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.Fn mkstemps
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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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and
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.Fn mkstemps
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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 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 NOTES
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A common problem that results in a core dump is that the programmer
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passes in a read-only string to
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.Fn mktemp ,
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.Fn mkstemp ,
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.Fn mkstemps
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or
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.Fn mkdtemp .
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This is common with programs that were developed before
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.St -isoC
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compilers were common.
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For example, calling
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.Fn mkstemp
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with an argument of
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.Qq /tmp/tempfile.XXXXXX
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will result in a core dump due to
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.Fn mkstemp
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attempting to modify the string constant that was given.
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If the program in question makes heavy use of that type
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of function call, you do have the option of compiling the program
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so that it will store string constants in a writable segment of memory.
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See
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.Xr gcc 1
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for more information.
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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 mkdir 2 ,
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.Xr open 2 ,
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.Xr stat 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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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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The
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.Fn mkdtemp
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function first appeared in
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.Ox 2.2 ,
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and later in
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.Fx 3.2 .
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The
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.Fn mkstemps
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function first appeared in
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.Ox 2.4 ,
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and later in
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.Fx 3.4 .
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.Sh BUGS
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This family of functions produces filenames which can be guessed,
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though the risk is minimized when large numbers of
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.Ql X Ns s
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are used to
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increase the number of possible temporary filenames.
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This makes the race in
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.Fn mktemp ,
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between testing for a file's existence (in the
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.Fn mktemp
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function call)
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and opening it for use
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(later in the user application)
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particularly dangerous from a security perspective.
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Whenever it is possible,
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.Fn mkstemp
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should be used instead, since it does not have the race condition.
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If
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.Fn mkstemp
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cannot be used, the filename created by
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.Fn mktemp
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should be created using the
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.Dv O_EXCL
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flag to
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.Xr open 2
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and the return status of the call should be tested for failure.
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This will ensure that the program does not continue blindly
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in the event that an attacker has already created the file
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with the intention of manipulating or reading its contents.
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.Pp
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The implementation of these functions calls
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.Xr arc4random 3 ,
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which is not reentrant.
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You must provide your own locking around this and other consumers of the
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.Xr arc4random 3
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API.
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