The gztell() function shall return the starting position for the next read or write operation on compressed file stream file. This position represents the number of bytes from the beginning of file in the uncompressed data.
gztell() is equivalent to
gzseek(file, 0L, SEEK_CUR)
gztell() shall return the current offset in the file expressed as a byte position in the uncompressed data stream. On error, gztell() shall return -1, and may set the error value for file accordingly.
On error, gztell() shall return
indicating that file is
NULL, or does not represent
an open compressed file stream.