libc/stdio/fseek.3

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.\"     @(#)fseek.3	8.1 (Berkeley) 6/4/93
.\"
.Dd September 11, 2021
.Dt FSEEK 3
.Os
.Sh NAME
.Nm fgetpos ,
.Nm fseek ,
.Nm fseeko ,
.Nm fsetpos ,
.Nm ftell ,
.Nm ftello ,
.Nm rewind
.Nd reposition a stream
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In stdio.h
.Ft int
.Fn fseek "FILE *stream" "long int offset" "int whence"
.Ft int
.Fn fseeko "FILE *stream" "off_t offset" "int whence"
.Ft long int
.Fn ftell "FILE *stream"
.Ft off_t
.Fn ftello "FILE *stream"
.Ft void
.Fn rewind "FILE *stream"
.Ft int
.Fn fgetpos "FILE * restrict stream" "fpos_t * restrict pos"
.Ft int
.Fn fsetpos "FILE * restrict stream" "const fpos_t * restrict 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 fseeko
function is identical to the
.Fn fseek
function except that the
.Fa offset
argument is of type
.Fa off_t .
.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 ftello
function is identical to the
.Fn ftell
function except that the return value is of type
.Fa off_t .
.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
In this implementations,
.Dq Fa fpos_t
is a complex object that represents both the position and the parse
state of the stream, making these routines the only way to portably
reposition a text stream.
The
.Ar pos
argument of
.Fn fsetpos
must always be initialized by
a call to
.Fn fgetpos .
.Sh RETURN VALUES
The
.Fn rewind
function
returns no value.
.Pp
Upon successful completion,
.Fn fgetpos ,
.Fn fseek ,
.Fn fseeko ,
and
.Fn fsetpos
return 0,
whereas the functions
.Fn ftell
and
.Fn ftello
return the current offset.
On failure,
.Fn fseek ,
.Fn fseeko ,
.Fn ftell ,
and
.Fn ftello
return \-1, while
.Fn fgetpos
and
.Fn fsetpos
return a nonzero value.
.Pp
On error all functions set the global variable
.Va errno
to indicate the error.
Since the
.Fn rewind
function does not return an error code, applications need to clear
.Va errno
before calling it in order to detect errors.
.Sh ERRORS
.Bl -tag -width Er
.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 .
.It Bq Er EOVERFLOW
For
.Fn ftell ,
the current file offset cannot be represented correctly in an object of type
.Fa long .
.El
.Pp
The function
.Fn fgetpos ,
.Fn fseek ,
.Fn fseeko ,
.Fn fsetpos ,
.Fn ftell ,
.Fn ftello ,
and
.Fn rewind
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 ,
.Xr clearerr 3 ,
.Xr ungetc 3
.Sh STANDARDS
The
.Fn fgetpos ,
.Fn fsetpos ,
.Fn fseek ,
.Fn ftell ,
and
.Fn rewind
functions
conform to
.St -ansiC .
The
.Fn fseeko
and
.Fn ftello
functions conform to
.St -xsh5 .
.Sh BUGS
The
.Fn fgetpos
and
.Fn fsetpos
functions don't store/set shift states of the stream in this implementation.