stdio/FreeBSD/fopen.3

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  32 .\"     @(#)fopen.3     8.1 (Berkeley) 6/4/93
  33 .\" $FreeBSD$
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  35 .Dd January 30, 2013
  36 .Dt FOPEN 3
  37 .Os
  38 .Sh NAME
  39 .Nm fopen ,
  40 .Nm fdopen ,
  41 .Nm freopen ,
  42 .Nm fmemopen
  43 .Nd stream open functions
  44 .Sh LIBRARY
  45 .Lb libc
  46 .Sh SYNOPSIS
  47 .In stdio.h
  48 .Ft FILE *
  49 .Fn fopen "const char * restrict path" "const char * restrict mode"
  50 .Ft FILE *
  51 .Fn fdopen "int fildes" "const char *mode"
  52 .Ft FILE *
  53 .Fn freopen "const char *path" "const char *mode" "FILE *stream"
  54 .Ft FILE *
  55 .Fn fmemopen "void *restrict *buf" "size_t size" "const char * restrict mode"
  56 .Sh DESCRIPTION
  57 The
  58 .Fn fopen
  59 function
  60 opens the file whose name is the string pointed to by
  61 .Fa path
  62 and associates a stream with it.
  63 .Pp
  64 The argument
  65 .Fa mode
  66 points to a string beginning with one of the following letters:
  67 .Bl -tag -width indent
  68 .It Dq Li r
  69 Open for reading.
  70 The stream is positioned at the beginning of the file.
  71 Fail if the file does not exist.
  72 .It Dq Li w
  73 Open for writing.
  74 The stream is positioned at the beginning of the file.
  75 Create the file if it does not exist.
  76 .It Dq Li a
  77 Open for writing.
  78 The stream is positioned at the end of the file.
  79 Subsequent writes to the file will always end up at the then current
  80 end of file, irrespective of any intervening
  81 .Xr fseek 3
  82 or similar.
  83 Create the file if it does not exist.
  84 .El
  85 .Pp
  86 An optional
  87 .Dq Li +
  88 following
  89 .Dq Li r ,
  90 .Dq Li w ,
  91 or
  92 .Dq Li a
  93 opens the file for both reading and writing.
  94 An optional
  95 .Dq Li x
  96 following
  97 .Dq Li w
  98 or
  99 .Dq Li w+
 100 causes the
 101 .Fn fopen
 102 call to fail if the file already exists.
 103 An optional
 104 .Dq Li e
 105 following the above
 106 causes the
 107 .Fn fopen
 108 call to set the
 109 .Dv FD_CLOEXEC
 110 flag on the underlying file descriptor.
 111 .Pp
 112 The
 113 .Fa mode
 114 string can also include the letter
 115 .Dq Li b
 116 after either the
 117 .Dq Li +
 118 or the first letter.
 119 This is strictly for compatibility with
 120 .St -isoC
 121 and has effect only for
 122 .Fn fmemopen
 123 ; otherwise
 124 .Dq Li b
 125 is ignored.
 126 .Pp
 127 Any created files will have mode
 128 .Do Dv S_IRUSR
 129 \&|
 130 .Dv S_IWUSR
 131 \&|
 132 .Dv S_IRGRP
 133 \&|
 134 .Dv S_IWGRP
 135 \&|
 136 .Dv S_IROTH
 137 \&|
 138 .Dv S_IWOTH Dc
 139 .Pq Li 0666 ,
 140 as modified by the process'
 141 umask value (see
 142 .Xr umask 2 ) .
 143 .Pp
 144 Reads and writes may be intermixed on read/write streams in any order; however,
 145 a file positioning function must be called when switching between output and
 146 input, unless an input operation encounters end-of-file.
 147 .Pp
 148 The
 149 .Fn fdopen
 150 function associates a stream with the existing file descriptor,
 151 .Fa fildes .
 152 The mode
 153 of the stream must be compatible with the mode of the file descriptor.
 154 The
 155 .Dq Li x
 156 mode option is ignored.
 157 If the
 158 .Dq Li e
 159 mode option is present, the
 160 .Dv FD_CLOEXEC
 161 flag is set, otherwise it remains unchanged.
 162 When the stream is closed via
 163 .Xr fclose 3 ,
 164 .Fa fildes
 165 is closed also.
 166 .Pp
 167 The
 168 .Fn freopen
 169 function
 170 opens the file whose name is the string pointed to by
 171 .Fa path
 172 and associates the stream pointed to by
 173 .Fa stream
 174 with it.
 175 The original stream (if it exists) is closed.
 176 The
 177 .Fa mode
 178 argument is used just as in the
 179 .Fn fopen
 180 function.
 181 .Pp
 182 If the
 183 .Fa path
 184 argument is
 185 .Dv NULL ,
 186 .Fn freopen
 187 attempts to re-open the file associated with
 188 .Fa stream
 189 with a new mode.
 190 The new mode must be compatible with the mode that the stream was originally
 191 opened with:
 192 Streams open for reading can only be re-opened for reading,
 193 streams open for writing can only be re-opened for writing,
 194 and streams open for reading and writing can be re-opened in any mode.
 195 The
 196 .Dq Li x
 197 mode option is not meaningful in this context.
 198 .Pp
 199 The primary use of the
 200 .Fn freopen
 201 function
 202 is to change the file associated with a
 203 standard text stream
 204 .Dv ( stderr , stdin ,
 205 or
 206 .Dv stdout ) .
 207 .Pp
 208 The
 209 .Fn fmemopen
 210 function
 211 associates the buffer given by the
 212 .Fa buf
 213 and
 214 .Fa size
 215 arguments with a stream.
 216 The
 217 .Fa buf
 218 argument is either a null pointer or a pointer to a buffer that
 219 is at least
 220 .Fa size
 221 bytes long.
 222 If a null pointer is specified as the
 223 .Fa buf
 224 argument,
 225 .Fn fmemopen
 226 allocates
 227 .Fa size
 228 bytes of memory, and this allocation is automatically freed when the stream is
 229 closed.
 230 If a non-null pointer is specified, the caller retains ownership of
 231 the buffer and is responsible for disposing of it after the stream has been
 232 closed.
 233 Buffers can be opened in text-mode (default) or binary-mode
 234 (if
 235 .Dq Li b
 236 is present in the second or third position of the
 237 .Fa mode
 238 argument). Buffers opened in text-mode make sure that writes are terminated with
 239 a NULL byte, if the last write hasn't filled up the whole buffer. Buffers
 240 opened in binary-mode never append a NULL byte.
 241 .Sh RETURN VALUES
 242 Upon successful completion
 243 .Fn fopen ,
 244 .Fn fdopen ,
 245 .Fn freopen
 246 and
 247 .Fn fmemopen
 248 return a
 249 .Tn FILE
 250 pointer.
 251 Otherwise,
 252 .Dv NULL
 253 is returned and the global variable
 254 .Va errno
 255 is set to indicate the error.
 256 .Sh ERRORS
 257 .Bl -tag -width Er
 258 .It Bq Er EINVAL
 259 The
 260 .Fa mode
 261 argument
 262 to
 263 .Fn fopen ,
 264 .Fn fdopen ,
 265 .Fn freopen ,
 266 or
 267 .Fn fmemopen
 268 was invalid.
 269 .El
 270 .Pp
 271 The
 272 .Fn fopen ,
 273 .Fn fdopen ,
 274 .Fn freopen
 275 and
 276 .Fn fmemopen
 277 functions
 278 may also fail and set
 279 .Va errno
 280 for any of the errors specified for the routine
 281 .Xr malloc 3 .
 282 .Pp
 283 The
 284 .Fn fopen
 285 function
 286 may also fail and set
 287 .Va errno
 288 for any of the errors specified for the routine
 289 .Xr open 2 .
 290 .Pp
 291 The
 292 .Fn fdopen
 293 function
 294 may also fail and set
 295 .Va errno
 296 for any of the errors specified for the routine
 297 .Xr fcntl 2 .
 298 .Pp
 299 The
 300 .Fn freopen
 301 function
 302 may also fail and set
 303 .Va errno
 304 for any of the errors specified for the routines
 305 .Xr open 2 ,
 306 .Xr fclose 3
 307 and
 308 .Xr fflush 3 .
 309 .Pp
 310 The
 311 .Fn fmemopen
 312 function
 313 may also fail and set
 314 .Va errno
 315 if the
 316 .Fa size
 317 argument is 0.
 318 .Sh SEE ALSO
 319 .Xr open 2 ,
 320 .Xr fclose 3 ,
 321 .Xr fileno 3 ,
 322 .Xr fseek 3 ,
 323 .Xr funopen 3
 324 .Sh STANDARDS
 325 The
 326 .Fn fopen
 327 and
 328 .Fn freopen
 329 functions
 330 conform to
 331 .St -isoC ,
 332 with the exception of the
 333 .Dq Li x
 334 mode option which conforms to
 335 .St -isoC-2011 .
 336 The
 337 .Fn fdopen
 338 function
 339 conforms to
 340 .St -p1003.1-88 .
 341 The
 342 .Dq Li e
 343 mode option does not conform to any standard
 344 but is also supported by glibc.
 345 The
 346 .Fn fmemopen
 347 function
 348 conforms to
 349 .St -p1003.1-2008 .
 350 The
 351 .Dq Li b
 352 mode does not conform to any standard
 353 but is also supported by glibc.