projects / unix-history / blame
| Commit | Line | Data |
|---|---|---|
| 93a804a7 SL |
1 | /* ns_cksum.c 1.1 86/01/23 */ |
| 2 | ||
| 3 | /* | |
| 4 | * ns_cksum.c - Xerox Internet Datagram protocol checksum | |
| 5 | */ | |
| 6 | #include "types.h" | |
| 7 | #include "mbuf.h" | |
| 8 | ||
| 9 | /* | |
| 10 | * Perform (slowly) the Xerox Internet checksum algorithm on a | |
| 11 | * chain of mbufs. This means we add all the 16-bits words, | |
| 12 | * shifting the sum after each 16-bit add. Ones-complement | |
| 13 | * arithmetic is required, so we fold the carry bits after | |
| 14 | * each 16-bit add as well. | |
| 15 | * If the result is the *no-checksum* value 0xffff, return zero instead. | |
| 16 | * | |
| 17 | * Chris Torek <chris@maryland> | |
| 18 | * James O'Toole <james@maryland> | |
| 19 | */ | |
| 20 | u_short | |
| 21 | ns_cksum(m, len) | |
| 22 | register struct mbuf *m; | |
| 23 | register int len; | |
| 24 | { | |
| 25 | register int cksum = 0; | |
| 26 | register int shift = 8; | |
| 27 | register u_char *p; | |
| 28 | register int mlen; | |
| 29 | ||
| 30 | if (len & 1) | |
| 31 | printf("ns_cksum: odd length\n"); | |
| 32 | while (m && len) { | |
| 33 | p = mtod(m, u_char *); | |
| 34 | mlen = m->m_len; | |
| 35 | if ((len -= mlen) < 0) | |
| 36 | mlen += len, len = 0; | |
| 37 | while (--mlen >= 0) { | |
| 38 | cksum += *p++ << shift; | |
| 39 | if ((shift = 8 - shift) != 0) { | |
| 40 | cksum <<= 1; | |
| 41 | cksum = (cksum & 0xffff) + (cksum >> 16); | |
| 42 | } | |
| 43 | } | |
| 44 | m = m->m_next; | |
| 45 | } | |
| 46 | #ifdef notdef | |
| 47 | if (len) | |
| 48 | printf("ns_cksum: out of data\n"); | |
| 49 | #endif | |
| 50 | return (cksum == 0xffff ? 0 : cksum); | |
| 51 | } |