stdio/FreeBSD/mktemp.3

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  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 $
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  35 .Dd February 11, 1998
  36 .Dt MKTEMP 3
  37 .Os
  38 .Sh NAME
  39 .Nm mktemp
  40 .Nd make temporary file name (unique)
  41 .Sh LIBRARY
  42 .Lb libc
  43 .Sh SYNOPSIS
  44 .In unistd.h
  45 .Ft char *
  46 .Fn mktemp "char *template"
  47 .Ft int
  48 .Fn mkstemp "char *template"
  49 .Ft int
  50 .Fn mkstemps "char *template" "int suffixlen"
  51 .Ft char *
  52 .Fn mkdtemp "char *template"
  53 .Sh DESCRIPTION
  54 The
  55 .Fn mktemp
  56 function
  57 takes the given file name template and overwrites a portion of it
  58 to create a file name.
  59 This file name is guaranteed not to exist at the time of function invocation
  60 and is suitable for use
  61 by the application.
  62 The template may be any file name with some number of
  63 .Ql X Ns s
  64 appended
  65 to it, for example
  66 .Pa /tmp/temp.XXXXXX .
  67 The trailing
  68 .Ql X Ns s
  69 are replaced with a
  70 unique alphanumeric combination.
  71 The number of unique file names
  72 .Fn mktemp
  73 can return depends on the number of
  74 .Ql X Ns s
  75 provided; six
  76 .Ql X Ns s
  77 will
  78 result in
  79 .Fn mktemp
  80 selecting one of 56800235584 (62 ** 6) possible temporary file names.
  81 .Pp
  82 The
  83 .Fn mkstemp
  84 function
  85 makes the same replacement to the template and creates the template file,
  86 mode 0600, returning a file descriptor opened for reading and writing.
  87 This avoids the race between testing for a file's existence and opening it
  88 for use.
  89 .Pp
  90 The
  91 .Fn mkstemps
  92 function acts the same as
  93 .Fn mkstemp ,
  94 except it permits a suffix to exist in the template.
  95 The template should be of the form
  96 .Pa /tmp/tmpXXXXXXsuffix .
  97 The
  98 .Fn mkstemps
  99 function
 100 is told the length of the suffix string.
 101 .Pp
 102 The
 103 .Fn mkdtemp
 104 function makes the same replacement to the template as in
 105 .Fn mktemp
 106 and creates the template directory, mode 0700.
 107 .Sh RETURN VALUES
 108 The
 109 .Fn mktemp
 110 and
 111 .Fn mkdtemp
 112 functions return a pointer to the template on success and
 113 .Dv NULL
 114 on failure.
 115 The
 116 .Fn mkstemp
 117 and
 118 .Fn mkstemps
 119 functions
 120 return \-1 if no suitable file could be created.
 121 If either call fails an error code is placed in the global variable
 122 .Va errno .
 123 .Sh ERRORS
 124 The
 125 .Fn mkstemp ,
 126 .Fn mkstemps
 127 and
 128 .Fn mkdtemp
 129 functions
 130 may set
 131 .Va errno
 132 to one of the following values:
 133 .Bl -tag -width Er
 134 .It Bq Er ENOTDIR
 135 The pathname portion of the template is not an existing directory.
 136 .El
 137 .Pp
 138 The
 139 .Fn mkstemp ,
 140 .Fn mkstemps
 141 and
 142 .Fn mkdtemp
 143 functions
 144 may also set
 145 .Va errno
 146 to any value specified by the
 147 .Xr stat 2
 148 function.
 149 .Pp
 150 The
 151 .Fn mkstemp
 152 and
 153 .Fn mkstemps
 154 functions
 155 may also set
 156 .Va errno
 157 to any value specified by the
 158 .Xr open 2
 159 function.
 160 .Pp
 161 The
 162 .Fn mkdtemp
 163 function
 164 may also set
 165 .Va errno
 166 to any value specified by the
 167 .Xr mkdir 2
 168 function.
 169 .Sh NOTES
 170 A common problem that results in a core dump is that the programmer
 171 passes in a read-only string to
 172 .Fn mktemp ,
 173 .Fn mkstemp ,
 174 .Fn mkstemps
 175 or
 176 .Fn mkdtemp .
 177 This is common with programs that were developed before
 178 .St -isoC
 179 compilers were common.
 180 For example, calling
 181 .Fn mkstemp
 182 with an argument of
 183 .Qq /tmp/tempfile.XXXXXX
 184 will result in a core dump due to
 185 .Fn mkstemp
 186 attempting to modify the string constant that was given.
 187 If the program in question makes heavy use of that type
 188 of function call, you do have the option of compiling the program
 189 so that it will store string constants in a writable segment of memory.
 190 See
 191 .Xr gcc 1
 192 for more information.
 193 .Sh BUGS
 194 This family of functions produces filenames which can be guessed,
 195 though the risk is minimized when large numbers of
 196 .Ql X Ns s
 197 are used to
 198 increase the number of possible temporary filenames.
 199 This makes the race in
 200 .Fn mktemp ,
 201 between testing for a file's existence (in the
 202 .Fn mktemp
 203 function call)
 204 and opening it for use
 205 (later in the user application)
 206 particularly dangerous from a security perspective.
 207 Whenever it is possible,
 208 .Fn mkstemp
 209 should be used instead, since it does not have the race condition.
 210 If
 211 .Fn mkstemp
 212 cannot be used, the filename created by
 213 .Fn mktemp
 214 should be created using the
 215 .Dv O_EXCL
 216 flag to
 217 .Xr open 2
 218 and the return status of the call should be tested for failure.
 219 This will ensure that the program does not continue blindly
 220 in the event that an attacker has already created the file
 221 with the intention of manipulating or reading its contents.
 222 .Pp
 223 The implementation of these functions calls
 224 .Xr arc4random 3 ,
 225 which is not reentrant.
 226 You must provide your own locking around this and other consumers of the
 227 .Xr arc4random 3
 228 API.
 229 .Sh SEE ALSO
 230 .Xr chmod 2 ,
 231 .Xr getpid 2 ,
 232 .Xr mkdir 2 ,
 233 .Xr open 2 ,
 234 .Xr stat 2
 235 .Sh HISTORY
 236 A
 237 .Fn mktemp
 238 function appeared in
 239 .At v7 .
 240 The
 241 .Fn mkstemp
 242 function appeared in
 243 .Bx 4.4 .
 244 The
 245 .Fn mkdtemp
 246 function first appeared in
 247 .Ox 2.2 ,
 248 and later in
 249 .Fx 3.2 .
 250 The
 251 .Fn mkstemps
 252 function first appeared in
 253 .Ox 2.4 ,
 254 and later in
 255 .Fx 3.4 .