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.Dd November 13, 2018
.Dt GZSEEK 3
.Os
.
.Sh NAME
.Nm gzseek ,
.Nm gzrewind ,
.Nm gztell
.Nd seek compressed file
.
.Sh LIBRARY
.Lb libz
.
.Sh SYNOPSIS
.In zlib.h
.Ft z_off_t
.Fn gzseek "gzFile file" "z_off_t offset" "int whence"
.Ft int
.Fn gzrewind "gzFile file"
.Ft z_off_t
.Fn gztell "gzFile file"
.
.Sh DESCRIPTION
Sets the starting position
for the next
.Xr gzread 3
or
.Xr gzwrite 3
on the given compressed file.
The
.Fa offset
represents a number of bytes
in the uncompressed data stream.
The
.Fa whence
parameter
is defined as in
.Xr lseek 2 ;
the value
.Dv SEEK_END
is not supported.
.
.Pp
If the file is opened for reading,
this function is emulated
but can be extremely slow.
If the file is opened for writing,
only forward seeks are supported;
.Fn gzseek
then compresses a sequence of zeroes
up to the new starting position.
.
.Pp
.Fn gzrewind
rewinds the given file.
This function is supported
only for reading.
.
.Pp
.Fn gzrewind file
is equivalent to
.Li (int) Ns Fn gzseek file 0L SEEK_SET .
.
.Pp
.Fn gztell
returns the starting position
for the next
.Xr gzread 3
or
.Xr gzwrite 3
on the given compressed file.
This position represents a number of bytes
in the uncompressed data stream,
and is zero when starting,
even if appending or reading
a gzip stream from the middle of a file using
.Xr gzdopen 3 .
.
.Pp
.Fn gztell file
is equivalent to
.Fn gzseek file 0L SEEK_CUR .
.
.Sh RETURN VALUES
.Fn gzseek
returns the resulting offset location
as measured in bytes
from the beginning of the uncompressed stream,
or -1 in case of error,
in particular if the file
is opened for writing
and the new starting position
would be before the current position.
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