fseek.3

早期freebsd实现

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.\"     @(#)fseek.3	8.1 (Berkeley) 6/4/93
.\"
.Dd June 4, 1993
.Dt FSEEK 3
.Os
.Sh NAME
.Nm fgetpos ,
.Nm fseek ,
.Nm fsetpos ,
.Nm ftell ,
.Nm rewind
.Nd reposition a stream
.Sh SYNOPSIS
.Fd #include <stdio.h>
.Ft int
.Fn fseek "FILE *stream" "long offset" "int whence"
.Ft long
.Fn ftell "FILE *stream"
.Ft void
.Fn rewind "FILE *stream"
.Ft int
.Fn fgetpos "FILE *stream" "fpos_t *pos"
.Ft int
.Fn fsetpos "FILE *stream" "fpos_t *pos"
.Sh DESCRIPTION
The
.Fn fseek
function sets the file position indicator for the stream pointed
to by
.Fa stream .
The new position, measured in bytes, is obtained by adding
.Fa offset
bytes to the position specified by
.Fa whence .
If 
.Fa whence
is set to
.Dv SEEK_SET ,
.Dv SEEK_CUR ,
or
.Dv SEEK_END ,
the offset is relative to the
start of the file, the current position indicator, or end-of-file,
respectively.
A successful call to the
.Fn fseek
function clears the end-of-file indicator for the stream and undoes
any effects of the
.Xr ungetc 3
function on the same stream.
.Pp
The
.Fn ftell
function
obtains the current value of the file position indicator for the
stream pointed to by
.Fa stream .
.Pp
The
.Fn rewind
function sets the file position indicator for the stream pointed
to by
.Fa stream
to the beginning of the file.
It is equivalent to:
.Pp
.Dl (void)fseek(stream, 0L, SEEK_SET)
.Pp
except that the error indicator for the stream is also cleared
(see
.Xr clearerr 3 ) .
.Pp
The
.Fn fgetpos
and
.Fn fsetpos
functions
are alternate interfaces equivalent to
.Fn ftell
and
.Fn fseek
(with whence set to
.Dv SEEK_SET
), setting and storing the current value of
the file offset into or from the object referenced by 
.Fa pos .
On some
.Pq non- Ns Tn UNIX
systems an
.Dq Fa fpos_t
object may be a complex object
and these routines may be the only way to portably reposition a text stream.
.Sh RETURN VALUES
The
.Fn rewind
function
returns no value.
Upon successful completion,
.Fn fgetpos ,
.Fn fseek ,
.Fn fsetpos
return 0,
and
.Fn ftell
returns the current offset.
Otherwise, \-1 is returned and the global variable errno is set to
indicate the error.
.Sh ERRORS
.Bl -tag -width [EINVAL]
.It Bq Er EBADF
The
.Fa stream
specified
is not a seekable stream.
.It Bq Er EINVAL
The
.Fa whence
argument to 
.Fn fseek
was not
.Dv SEEK_SET ,
.Dv SEEK_END ,
or
.Dv SEEK_CUR .
.El
.Pp
The function
.Fn fgetpos ,
.Fn fseek ,
.Fn fsetpos ,
and 
.Fn ftell
may also fail and set
.Va errno
for any of the errors specified for the routines
.Xr fflush 3 ,
.Xr fstat 2 ,
.Xr lseek 2 ,
and 
.Xr malloc 3 .
.Sh SEE ALSO
.Xr lseek 2
.Sh STANDARDS
The
.Fn fgetpos ,
.Fn fsetpos ,
.Fn fseek ,
.Fn ftell ,
and
.Fn rewind
functions
conform to
.St -ansiC .