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 1.19 2001/10/01 16:08:59 ru Exp $
  38 .\"
  39 .Dd March 5, 1999
  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 *stream" "fpos_t *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 function on the same stream.
  96 .Pp
  97 The
  98 .Fn ftell
  99 function
 100 obtains the current value of the file position indicator for the
 101 stream pointed to by
 102 .Fa stream .
 103 .Pp
 104 The
 105 .Fn rewind
 106 function sets the file position indicator for the stream pointed
 107 to by
 108 .Fa stream
 109 to the beginning of the file.
 110 It is equivalent to:
 111 .Pp
 112 .Dl (void)fseek(stream, 0L, SEEK_SET)
 113 .Pp
 114 except that the error indicator for the stream is also cleared
 115 (see
 116 .Xr clearerr 3 ) .
 117 .Pp
 118 Since
 119 .Fn rewind
 120 does not return a value,
 121 an application wishing to detect errors should clear
 122 .Va errno ,
 123 then call
 124 .Fn rewind ,
 125 and if
 126 .Va errno
 127 is non-zero, assume an error has occurred.
 128 .Pp
 129 The
 130 .Fn fseeko
 131 function is identical to
 132 .Fn fseek ,
 133 except it takes an
 134 .Fa off_t
 135 argument
 136 instead of a
 137 .Fa long .
 138 Likewise, the
 139 .Fn ftello
 140 function is identical to
 141 .Fn ftell ,
 142 except it returns an
 143 .Fa off_t .
 144 .Pp
 145 The
 146 .Fn fgetpos
 147 and
 148 .Fn fsetpos
 149 functions
 150 are alternate interfaces equivalent to
 151 .Fn ftell
 152 and
 153 .Fn fseek
 154 (with whence set to
 155 .Dv SEEK_SET ) ,
 156 setting and storing the current value of
 157 the file offset into or from the object referenced by
 158 .Fa pos .
 159 On some
 160 .Pq non- Ns Tn UNIX
 161 systems an
 162 .Dq Fa fpos_t
 163 object may be a complex object
 164 and these routines may be the only way to portably reposition a text stream.
 165 .Sh RETURN VALUES
 166 The
 167 .Fn rewind
 168 function
 169 returns no value.
 170 .Pp
 171 .Rv -std fgetpos fseek fseeko fsetpos
 172 .Pp
 173 Upon successful completion,
 174 .Fn ftell
 175 and
 176 .Fn ftello
 177 return the current offset.
 178 Otherwise, \-1 is returned and the global variable
 179 .Va errno
 180 is set to indicate the error.
 181 .Sh ERRORS
 182 .Bl -tag -width Er
 183 .It Bq Er EBADF
 184 The
 185 .Fa stream
 186 specified
 187 is not a seekable stream.
 188 .It Bq Er EINVAL
 189 The
 190 .Fa whence
 191 argument is invalid or
 192 the resulting file-position
 193 indicator would be set to a negative value.
 194 .It Bq Er EOVERFLOW
 195 The resulting file offset would be a value which
 196 cannot be represented correctly in an object of type
 197 .Fa off_t
 198 for
 199 .Fn fseeko
 200 and
 201 .Fn ftello
 202 or
 203 .Fa long
 204 for
 205 .Fn fseek
 206 and
 207 .Fn ftell .
 208 .It Bq Er ESPIPE
 209 The file descriptor underlying stream is associated with a pipe or FIFO
 210 or file-position indicator value is unspecified
 211 (see
 212 .Xr ungetc 3 ) .
 213 .El
 214 .Pp
 215 The functions
 216 .Fn fgetpos ,
 217 .Fn fseek ,
 218 .Fn fseeko ,
 219 .Fn fsetpos ,
 220 .Fn ftell ,
 221 and
 222 .Fn ftello
 223 may also fail and set
 224 .Va errno
 225 for any of the errors specified for the routines
 226 .Xr fflush 3 ,
 227 .Xr fstat 2 ,
 228 .Xr lseek 2 ,
 229 and
 230 .Xr malloc 3 .
 231 .Sh SEE ALSO
 232 .Xr lseek 2 ,
 233 .Xr clearerr 3 ,
 234 .Xr ungetc 3
 235 .Sh STANDARDS
 236 The
 237 .Fn fgetpos ,
 238 .Fn fsetpos ,
 239 .Fn fseek ,
 240 .Fn ftell ,
 241 and
 242 .Fn rewind
 243 functions
 244 conform to
 245 .St -isoC .
 246 .Pp
 247 The
 248 .Fn fseeko
 249 and
 250 .Fn ftello
 251 functions conform to
 252 .St -susv2 .