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