stdio/mktemp.3

   1 .\" Copyright (c) 1989, 1991, 1993
   2 .\"     The Regents of the University of California.  All rights reserved.
   3 .\"
   4 .\" Redistribution and use in source and binary forms, with or without
   5 .\" modification, are permitted provided that the following conditions
   6 .\" are met:
   7 .\" 1. Redistributions of source code must retain the above copyright
   8 .\"    notice, this list of conditions and the following disclaimer.
   9 .\" 2. Redistributions in binary form must reproduce the above copyright
  10 .\"    notice, this list of conditions and the following disclaimer in the
  11 .\"    documentation and/or other materials provided with the distribution.
  12 .\" 3. All advertising materials mentioning features or use of this software
  13 .\"    must display the following acknowledgement:
  14 .\"     This product includes software developed by the University of
  15 .\"     California, Berkeley and its contributors.
  16 .\" 4. Neither the name of the University nor the names of its contributors
  17 .\"    may be used to endorse or promote products derived from this software
  18 .\"    without specific prior written permission.
  19 .\"
  20 .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
  21 .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  22 .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  23 .\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
  24 .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  25 .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  26 .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  27 .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  28 .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  29 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  30 .\" SUCH DAMAGE.
  31 .\"
  32 .\"     @(#)mktemp.3    8.1 (Berkeley) 6/4/93
  33 .\" $FreeBSD: src/lib/libc/stdio/mktemp.3,v 1.20 2004/02/20 04:08:28 green Exp $
  34 .\"
  35 .Dd February 11, 1998
  36 .Dt MKTEMP 3
  37 .Os
  38 .Sh NAME
  39 .Nm mkdtemp ,
  40 .Nm mkstemp ,
  41 .Nm mktemp ,
  42 .Nm mktemps
  43 .Nd make temporary file name (unique)
  44 .Sh LIBRARY
  45 .Lb libc
  46 .Sh SYNOPSIS
  47 .In unistd.h
  48 .Ft char *
  49 .Fo mkdtemp
  50 .Fa "char *template"
  51 .Fc
  52 .Ft int
  53 .Fo mkstemps
  54 .Fa "char *template"
  55 .Fa "int suffixlen"
  56 .Fc
  57 .In stdlib.h
  58 .Ft int
  59 .Fo mkstemp
  60 .Fa "char *template"
  61 .Fc
  62 .Ft char *
  63 .Fo mktemp
  64 .Fa "char *template"
  65 .Fc
  66 .Sh DESCRIPTION
  67 The
  68 .Fn mktemp
  69 function
  70 takes the given file name template and overwrites a portion of it
  71 to create a file name.
  72 This file name is guaranteed not to exist at the time of function invocation
  73 and is suitable for use
  74 by the application.
  75 The template may be any file name with some number of
  76 .Ql X Ns s
  77 appended
  78 to it, for example
  79 .Pa /tmp/temp.XXXXXX .
  80 The trailing
  81 .Ql X Ns s
  82 are replaced with a
  83 unique alphanumeric combination.
  84 The number of unique file names
  85 .Fn mktemp
  86 can return depends on the number of
  87 .Ql X Ns s
  88 provided; six
  89 .Ql X Ns s
  90 will
  91 result in
  92 .Fn mktemp
  93 selecting one of 56800235584 (62 ** 6) possible temporary file names.
  94 .Pp
  95 The
  96 .Fn mkstemp
  97 function
  98 makes the same replacement to the template and creates the template file,
  99 mode 0600, returning a file descriptor opened for reading and writing.
 100 This avoids the race between testing for a file's existence and opening it
 101 for use.
 102 .Pp
 103 The
 104 .Fn mkstemps
 105 function acts the same as
 106 .Fn mkstemp ,
 107 except it permits a suffix to exist in the template.
 108 The template should be of the form
 109 .Pa /tmp/tmpXXXXXXsuffix .
 110 The
 111 .Fn mkstemps
 112 function
 113 is told the length of the suffix string.
 114 .Pp
 115 The
 116 .Fn mkdtemp
 117 function makes the same replacement to the template as in
 118 .Fn mktemp
 119 and creates the template directory, mode 0700.
 120 .Sh RETURN VALUES
 121 The
 122 .Fn mktemp
 123 and
 124 .Fn mkdtemp
 125 functions return a pointer to the template on success and
 126 .Dv NULL
 127 on failure.
 128 The
 129 .Fn mkstemp
 130 and
 131 .Fn mkstemps
 132 functions
 133 return \-1 if no suitable file could be created.
 134 If either call fails an error code is placed in the global variable
 135 .Va errno .
 136 .Sh ERRORS
 137 The
 138 .Fn mkstemp ,
 139 .Fn mkstemps
 140 and
 141 .Fn mkdtemp
 142 functions
 143 may set
 144 .Va errno
 145 to one of the following values:
 146 .Bl -tag -width Er
 147 .It Bq Er ENOTDIR
 148 The pathname portion of the template is not an existing directory.
 149 .El
 150 .Pp
 151 The
 152 .Fn mkstemp ,
 153 .Fn mkstemps ,
 154 and
 155 .Fn mkdtemp
 156 functions
 157 may also set
 158 .Va errno
 159 to any value specified by the
 160 .Xr stat 2
 161 function.
 162 .Pp
 163 The
 164 .Fn mkstemp
 165 and
 166 .Fn mkstemps
 167 functions
 168 may also set
 169 .Va errno
 170 to any value specified by the
 171 .Xr open 2
 172 function.
 173 .Pp
 174 The
 175 .Fn mkdtemp
 176 function
 177 may also set
 178 .Va errno
 179 to any value specified by the
 180 .Xr mkdir 2
 181 function.
 182 .Sh NOTES
 183 A common problem that results in a core dump is that the programmer
 184 passes in a read-only string to
 185 .Fn mktemp ,
 186 .Fn mkstemp ,
 187 .Fn mkstemps ,
 188 or
 189 .Fn mkdtemp .
 190 This is common with programs that were developed before
 191 .St -isoC
 192 compilers were common.
 193 For example, calling
 194 .Fn mkstemp
 195 with an argument of
 196 .Qq /tmp/tempfile.XXXXXX
 197 will result in a core dump due to
 198 .Fn mkstemp
 199 attempting to modify the string constant that was given.
 200 If the program in question makes heavy use of that type
 201 of function call, you do have the option of compiling the program
 202 so that it will store string constants in a writable segment of memory.
 203 See
 204 .Xr gcc 1
 205 for more information.
 206 .Sh BUGS
 207 This family of functions produces filenames which can be guessed,
 208 though the risk is minimized when large numbers of
 209 .Ql X Ns s
 210 are used to
 211 increase the number of possible temporary filenames.
 212 This makes the race in
 213 .Fn mktemp ,
 214 between testing for a file's existence (in the
 215 .Fn mktemp
 216 function call)
 217 and opening it for use
 218 (later in the user application)
 219 particularly dangerous from a security perspective.
 220 Whenever it is possible,
 221 .Fn mkstemp
 222 should be used instead, since it does not have the race condition.
 223 If
 224 .Fn mkstemp
 225 cannot be used, the filename created by
 226 .Fn mktemp
 227 should be created using the
 228 .Dv O_EXCL
 229 flag to
 230 .Xr open 2
 231 and the return status of the call should be tested for failure.
 232 This will ensure that the program does not continue blindly
 233 in the event that an attacker has already created the file
 234 with the intention of manipulating or reading its contents.
 235 .Pp
 236 The implementation of these functions calls
 237 .Xr arc4random 3 ,
 238 which is not reentrant.
 239 You must provide your own locking around this and other consumers of the
 240 .Xr arc4random 3
 241 API.
 242 .Sh LEGACY SYNOPSIS
 243 .Fd #include <unistd.h>
 244 .Pp
 245 The include file
 246 .In unistd.h
 247 is necessary and sufficient for all functions.
 248 .Sh SEE ALSO
 249 .Xr chmod 2 ,
 250 .Xr getpid 2 ,
 251 .Xr mkdir 2 ,
 252 .Xr open 2 ,
 253 .Xr stat 2 ,
 254 .Xr compat 5
 255 .Sh HISTORY
 256 A
 257 .Fn mktemp
 258 function appeared in
 259 .At v7 .
 260 The
 261 .Fn mkstemp
 262 function appeared in
 263 .Bx 4.4 .
 264 The
 265 .Fn mkdtemp
 266 function first appeared in
 267 .Ox 2.2 ,
 268 and later in
 269 .Fx 3.2 .
 270 The
 271 .Fn mkstemps
 272 function first appeared in
 273 .Ox 2.4 ,
 274 and later in
 275 .Fx 3.4 .