stdio/fseek.3

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  36 .\"     @(#)fseek.3     8.1 (Berkeley) 6/4/93
  37 .\" $FreeBSD: src/lib/libc/stdio/fseek.3,v 1.25 2004/03/20 08:38:27 tjr Exp $
  38 .\"
  39 .Dd March 19, 2004
  40 .Dt FSEEK 3
  41 .Os
  42 .Sh NAME
  43 .Nm fgetpos ,
  44 .Nm fseek ,
  45 .Nm fseeko ,
  46 .Nm fsetpos ,
  47 .Nm ftell ,
  48 .Nm ftello ,
  49 .Nm rewind
  50 .Nd reposition a stream
  51 .Sh LIBRARY
  52 .Lb libc
  53 .Sh SYNOPSIS
  54 .In stdio.h
  55 .Ft int
  56 .Fo fgetpos
  57 .Fa "FILE *restrict stream"
  58 .Fa "fpos_t *restrict pos"
  59 .Fc
  60 .Ft int
  61 .Fo fseek
  62 .Fa "FILE *stream"
  63 .Fa "long offset"
  64 .Fa "int whence"
  65 .Fc
  66 .Ft int
  67 .Fo fseeko
  68 .Fa "FILE *stream"
  69 .Fa "off_t offset"
  70 .Fa "int whence"
  71 .Fc
  72 .Ft int
  73 .Fo fsetpos
  74 .Fa "FILE *stream"
  75 .Fa "const fpos_t *pos"
  76 .Fc
  77 .Ft long
  78 .Fo ftell
  79 .Fa "FILE *stream"
  80 .Fc
  81 .Ft off_t
  82 .Fo ftello
  83 .Fa "FILE *stream"
  84 .Fc
  85 .Ft void
  86 .Fo rewind
  87 .Fa "FILE *stream"
  88 .Fc
  89 .Sh DESCRIPTION
  90 The
  91 .Fn fseek
  92 function sets the file position indicator for the stream pointed
  93 to by
  94 .Fa stream .
  95 The new position, measured in bytes, is obtained by adding
  96 .Fa offset
  97 bytes to the position specified by
  98 .Fa whence .
  99 If
 100 .Fa whence
 101 is set to
 102 .Dv SEEK_SET ,
 103 .Dv SEEK_CUR ,
 104 or
 105 .Dv SEEK_END ,
 106 the offset is relative to the
 107 start of the file, the current position indicator, or end-of-file,
 108 respectively.
 109 A successful call to the
 110 .Fn fseek
 111 function clears the end-of-file indicator for the stream and undoes
 112 any effects of the
 113 .Xr ungetc 3
 114 and
 115 .Xr ungetwc 3
 116 functions on the same stream.
 117 .Pp
 118 The
 119 .Fn ftell
 120 function
 121 obtains the current value of the file position indicator for the
 122 stream pointed to by
 123 .Fa stream .
 124 .Pp
 125 The
 126 .Fn rewind
 127 function sets the file position indicator for the stream pointed
 128 to by
 129 .Fa stream
 130 to the beginning of the file.
 131 It is equivalent to:
 132 .Pp
 133 .Dl (void)fseek(stream, 0L, SEEK_SET)
 134 .Pp
 135 except that the error indicator for the stream is also cleared
 136 (see
 137 .Xr clearerr 3 ) .
 138 .Pp
 139 Since
 140 .Fn rewind
 141 does not return a value,
 142 an application wishing to detect errors should clear
 143 .Va errno ,
 144 then call
 145 .Fn rewind ,
 146 and if
 147 .Va errno
 148 is non-zero, assume an error has occurred.
 149 .Pp
 150 The
 151 .Fn fseeko
 152 function is identical to
 153 .Fn fseek ,
 154 except it takes an
 155 .Fa off_t
 156 argument
 157 instead of a
 158 .Fa long .
 159 Likewise, the
 160 .Fn ftello
 161 function is identical to
 162 .Fn ftell ,
 163 except it returns an
 164 .Fa off_t .
 165 .Pp
 166 The
 167 .Fn fgetpos
 168 and
 169 .Fn fsetpos
 170 functions
 171 are alternate interfaces for retrieving and setting the current position in
 172 the file, similar to
 173 .Fn ftell
 174 and
 175 .Fn fseek ,
 176 except that the current position is stored in an opaque object of
 177 type
 178 .Vt fpos_t
 179 pointed to by
 180 .Fa pos .
 181 These functions provide a portable way to seek to offsets larger than
 182 those that can be represented by a
 183 .Vt long int .
 184 They may also store additional state information in the
 185 .Vt fpos_t
 186 object to facilitate seeking within files containing multibyte
 187 characters with state-dependent encodings.
 188 Although
 189 .Vt fpos_t
 190 has traditionally been an integral type,
 191 applications cannot assume that it is;
 192 in particular, they must not perform arithmetic on objects
 193 of this type.
 194 .Pp
 195 If the stream is a wide character stream (see
 196 .Xr fwide 3 ) ,
 197 the position specified by the combination of
 198 .Fa offset
 199 and
 200 .Fa whence
 201 must contain the first byte of a multibyte sequence.
 202 .Sh RETURN VALUES
 203 The
 204 .Fn rewind
 205 function
 206 returns no value.
 207 .Pp
 208 .Rv -std fgetpos fseek fseeko fsetpos
 209 .Pp
 210 Upon successful completion,
 211 .Fn ftell
 212 and
 213 .Fn ftello
 214 return the current offset.
 215 Otherwise, \-1 is returned and the global variable
 216 .Va errno
 217 is set to indicate the error.
 218 .Sh ERRORS
 219 .Bl -tag -width Er
 220 .It Bq Er EBADF
 221 The
 222 .Fa stream
 223 argument
 224 is not a seekable stream.
 225 .It Bq Er EINVAL
 226 The
 227 .Fa whence
 228 argument is invalid or
 229 the resulting file-position
 230 indicator would be set to a negative value.
 231 .It Bq Er EOVERFLOW
 232 The resulting file offset would be a value which
 233 cannot be represented correctly in an object of type
 234 .Fa off_t
 235 for
 236 .Fn fseeko
 237 and
 238 .Fn ftello
 239 or
 240 .Fa long
 241 for
 242 .Fn fseek
 243 and
 244 .Fn ftell .
 245 .It Bq Er ESPIPE
 246 The file descriptor underlying stream is associated with a pipe or FIFO
 247 or file-position indicator value is unspecified
 248 (see
 249 .Xr ungetc 3 ) .
 250 .El
 251 .Pp
 252 The functions
 253 .Fn fgetpos ,
 254 .Fn fseek ,
 255 .Fn fseeko ,
 256 .Fn fsetpos ,
 257 .Fn ftell ,
 258 and
 259 .Fn ftello
 260 may also fail and set
 261 .Va errno
 262 for any of the errors specified for the routines
 263 .Xr fflush 3 ,
 264 .Xr fstat 2 ,
 265 .Xr lseek 2 ,
 266 and
 267 .Xr malloc 3 .
 268 .Sh LEGACY SYNOPSIS
 269 .Fd #include <stdio.h>
 270 .Fd #include <sys/types.h>
 271 .Pp
 272 .Ft int
 273 .br
 274 .Fo fseeko
 275 .Fa "FILE *stream"
 276 .Fa "off_t offset"
 277 .Fa "int whence"
 278 .Fc ;
 279 .Pp
 280 The include file
 281 .In sys/types.h
 282 supplies the definition for
 283 .Vt off_t .
 284 .Sh SEE ALSO
 285 .Xr lseek 2 ,
 286 .Xr clearerr 3 ,
 287 .Xr fwide 3 ,
 288 .Xr ungetc 3 ,
 289 .Xr ungetwc 3 ,
 290 .Xr compat 5
 291 .Sh STANDARDS
 292 The
 293 .Fn fgetpos ,
 294 .Fn fsetpos ,
 295 .Fn fseek ,
 296 .Fn ftell ,
 297 and
 298 .Fn rewind
 299 functions
 300 conform to
 301 .St -isoC .
 302 .Pp
 303 The
 304 .Fn fseeko
 305 and
 306 .Fn ftello
 307 functions conform to
 308 .St -p1003.1-2001 .