stdio/mktemp.3

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  32 .\"     @(#)mktemp.3    8.1 (Berkeley) 6/4/93
  33 .\" $FreeBSD: src/lib/libc/stdio/mktemp.3,v 1.17 2001/10/01 16:08:59 ru 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 .Fn mkstemps
  98 is told the length of the suffix string.
  99 .Pp
 100 The
 101 .Fn mkdtemp
 102 function makes the same replacement to the template as in
 103 .Xr mktemp 3
 104 and creates the template directory, mode 0700.
 105 .Sh RETURN VALUES
 106 The
 107 .Fn mktemp
 108 and
 109 .Fn mkdtemp
 110 functions return a pointer to the template on success and
 111 .Dv NULL
 112 on failure.
 113 The
 114 .Fn mkstemp
 115 and
 116 .Fn mkstemps
 117 functions
 118 return \-1 if no suitable file could be created.
 119 If either call fails an error code is placed in the global variable
 120 .Va errno .
 121 .Sh ERRORS
 122 The
 123 .Fn mkstemp ,
 124 .Fn mkstemps
 125 and
 126 .Fn mkdtemp
 127 functions
 128 may set
 129 .Va errno
 130 to one of the following values:
 131 .Bl -tag -width Er
 132 .It Bq Er ENOTDIR
 133 The pathname portion of the template is not an existing directory.
 134 .El
 135 .Pp
 136 The
 137 .Fn mkstemp ,
 138 .Fn mkstemps
 139 and
 140 .Fn mkdtemp
 141 functions
 142 may also set
 143 .Va errno
 144 to any value specified by the
 145 .Xr stat 2
 146 function.
 147 .Pp
 148 The
 149 .Fn mkstemp
 150 and
 151 .Fn mkstemps
 152 functions
 153 may also set
 154 .Va errno
 155 to any value specified by the
 156 .Xr open 2
 157 function.
 158 .Pp
 159 The
 160 .Fn mkdtemp
 161 function
 162 may also set
 163 .Va errno
 164 to any value specified by the
 165 .Xr mkdir 2
 166 function.
 167 .Sh NOTES
 168 A common problem that results in a core dump is that the programmer
 169 passes in a read-only string to
 170 .Fn mktemp ,
 171 .Fn mkstemp ,
 172 .Fn mkstemps
 173 or
 174 .Fn mkdtemp .
 175 This is common with programs that were developed before
 176 .St -isoC
 177 compilers were common.
 178 For example, calling
 179 .Fn mkstemp
 180 with an argument of
 181 .Qq /tmp/tempfile.XXXXXX
 182 will result in a core dump due to
 183 .Fn mkstemp
 184 attempting to modify the string constant that was given.
 185 If the program in question makes heavy use of that type
 186 of function call, you do have the option of compiling the program
 187 so that it will store string constants in a writable segment of memory.
 188 See
 189 .Xr gcc 1
 190 for more information.
 191 .Sh BUGS
 192 This family of functions produces filenames which can be guessed,
 193 though the risk is minimized when large numbers of
 194 .Ql X Ns s
 195 are used to
 196 increase the number of possible temporary filenames.
 197 This makes the race in
 198 .Fn mktemp ,
 199 between testing for a file's existence (in the
 200 .Fn mktemp
 201 function call)
 202 and opening it for use
 203 (later in the user application)
 204 particularly dangerous from a security perspective.
 205 Whenever it is possible,
 206 .Fn mkstemp
 207 should be used instead, since it does not have the race condition.
 208 If
 209 .Fn mkstemp
 210 cannot be used, the filename created by
 211 .Fn mktemp
 212 should be created using the
 213 .Dv O_EXCL
 214 flag to
 215 .Xr open 2
 216 and the return status of the call should be tested for failure.
 217 This will ensure that the program does not continue blindly
 218 in the event that an attacker has already created the file
 219 with the intention of manipulating or reading its contents.
 220 .Sh SEE ALSO
 221 .Xr chmod 2 ,
 222 .Xr getpid 2 ,
 223 .Xr mkdir 2 ,
 224 .Xr open 2 ,
 225 .Xr stat 2
 226 .Sh HISTORY
 227 A
 228 .Fn mktemp
 229 function appeared in
 230 .At v7 .
 231 The
 232 .Fn mkstemp
 233 function appeared in
 234 .Bx 4.4 .
 235 The
 236 .Fn mkdtemp
 237 function first appeared in
 238 .Ox 2.2 ,
 239 and later in
 240 .Fx 3.2 .
 241 The
 242 .Fn mkstemps
 243 function first appeared in
 244 .Ox 2.4 ,
 245 and later in
 246 .Fx 3.4 .