The th_read() function reads the next block from the tar archive
associated with the TAR handle t. It then sets the
current tar header associated with t to the contents of the block
read.
The th_write() function writes the contents of the current
tar header associated with t to the tar archive associated
with t.
RETURN VALUE
On successful completion, th_read() and th_write() will
return 0. On failure, they will return -1 and set errno to an
appropriate value.
On EOF, th_read() will return 1.
ERRORS
th_read() and th_write() will fail if:
EINVAL
Less than T_BLOCKSIZE blocks were read or written.