+/* Check header checksum */
+/* The standard BSD tar sources create the checksum by adding up the
+ bytes in the header as type char. I think the type char was unsigned
+ on the PDP-11, but it's signed on the Next and Sun. It looks like the
+ sources to BSD tar were never changed to compute the checksum
+ correctly, so both the Sun and Next add the bytes of the header as
+ signed chars. This doesn't cause a problem until you get a file with
+ a name containing characters with the high bit set. So tar_checksum
+ computes two checksums -- signed and unsigned. */
+
+enum read_header
+tar_checksum (union block *header, bool silent)
+{
+ size_t i;
+ int unsigned_sum = 0; /* the POSIX one :-) */
+ int signed_sum = 0; /* the Sun one :-( */
+ int recorded_sum;
+ uintmax_t parsed_sum;
+ char *p;
+
+ p = header->buffer;
+ for (i = sizeof *header; i-- != 0;)
+ {
+ unsigned_sum += (unsigned char) *p;
+ signed_sum += (signed char) (*p++);
+ }
+
+ if (unsigned_sum == 0)
+ return HEADER_ZERO_BLOCK;
+
+ /* Adjust checksum to count the "chksum" field as blanks. */
+
+ for (i = sizeof header->header.chksum; i-- != 0;)
+ {
+ unsigned_sum -= (unsigned char) header->header.chksum[i];
+ signed_sum -= (signed char) (header->header.chksum[i]);
+ }
+ unsigned_sum += ' ' * sizeof header->header.chksum;
+ signed_sum += ' ' * sizeof header->header.chksum;
+
+ parsed_sum = from_header (header->header.chksum,
+ sizeof header->header.chksum, 0,
+ (uintmax_t) 0,
+ (uintmax_t) TYPE_MAXIMUM (int), true, silent);
+ if (parsed_sum == (uintmax_t) -1)
+ return HEADER_FAILURE;
+
+ recorded_sum = parsed_sum;
+
+ if (unsigned_sum != recorded_sum && signed_sum != recorded_sum)
+ return HEADER_FAILURE;
+
+ return HEADER_SUCCESS;
+}
+