1 /* crc32.c -- compute the CRC-32 of a data stream
2 * Copyright (C) 1995-2006, 2010, 2011, 2012, 2016, 2018 Mark Adler
3 * For conditions of distribution and use, see copyright notice in zlib.h
5 * Thanks to Rodney Brown <rbrown64@csc.com.au> for his contribution of faster
6 * CRC methods: exclusive-oring 32 bits of data at a time, and pre-computing
7 * tables for updating the shift register in one step with three exclusive-ors
8 * instead of four steps with four exclusive-ors. This results in about a
9 * factor of two increase in speed on a Power PC G4 (PPC7455) using gcc -O3.
15 Note on the use of DYNAMIC_CRC_TABLE: there is no mutex or semaphore
16 protection on the static variables used to control the first-use generation
17 of the crc tables. Therefore, if you #define DYNAMIC_CRC_TABLE, you should
18 first call get_crc_table() to initialize the tables before allowing more than
19 one thread to use crc32().
21 DYNAMIC_CRC_TABLE and MAKECRCH can be #defined to write out crc32.h. A main()
22 routine is also produced, so that this one source file can be compiled to an
28 # ifndef DYNAMIC_CRC_TABLE
29 # define DYNAMIC_CRC_TABLE
30 # endif /* !DYNAMIC_CRC_TABLE */
33 #include "zutil.h" /* for STDC and FAR definitions */
35 /* Definitions for doing the crc four data bytes at a time. */
36 #if !defined(NOBYFOUR) && defined(Z_U4)
40 local unsigned long crc32_little OF((unsigned long,
41 const unsigned char FAR *, z_size_t));
42 local unsigned long crc32_big OF((unsigned long,
43 const unsigned char FAR *, z_size_t));
49 /* Local functions for crc concatenation */
50 #define GF2_DIM 32 /* dimension of GF(2) vectors (length of CRC) */
51 local z_crc_t gf2_matrix_times OF((const z_crc_t *mat, z_crc_t vec));
52 local uLong crc32_combine_ OF((uLong crc1, uLong crc2, z_off64_t len2));
54 /* ========================================================================= */
55 local z_crc_t gf2_matrix_times(mat, vec)
72 #ifdef DYNAMIC_CRC_TABLE
74 local volatile int crc_table_empty = 1;
75 local z_crc_t FAR crc_table[TBLS][256];
76 local z_crc_t FAR crc_comb[GF2_DIM][GF2_DIM];
77 local void make_crc_table OF((void));
78 local void gf2_matrix_square OF((z_crc_t *square, const z_crc_t *mat));
80 local void write_table OF((FILE *, const z_crc_t FAR *, int));
83 /* ========================================================================= */
84 local void gf2_matrix_square(square, mat)
90 for (n = 0; n < GF2_DIM; n++)
91 square[n] = gf2_matrix_times(mat, mat[n]);
95 Generate tables for a byte-wise 32-bit CRC calculation on the polynomial:
96 x^32+x^26+x^23+x^22+x^16+x^12+x^11+x^10+x^8+x^7+x^5+x^4+x^2+x+1.
98 Polynomials over GF(2) are represented in binary, one bit per coefficient,
99 with the lowest powers in the most significant bit. Then adding polynomials
100 is just exclusive-or, and multiplying a polynomial by x is a right shift by
101 one. If we call the above polynomial p, and represent a byte as the
102 polynomial q, also with the lowest power in the most significant bit (so the
103 byte 0xb1 is the polynomial x^7+x^3+x+1), then the CRC is (q*x^32) mod p,
104 where a mod b means the remainder after dividing a by b.
106 This calculation is done using the shift-register method of multiplying and
107 taking the remainder. The register is initialized to zero, and for each
108 incoming bit, x^32 is added mod p to the register if the bit is a one (where
109 x^32 mod p is p+x^32 = x^26+...+1), and the register is multiplied mod p by
110 x (which is shifting right by one and adding x^32 mod p if the bit shifted
111 out is a one). We start with the highest power (least significant bit) of
112 q and repeat for all eight bits of q.
114 The first table is simply the CRC of all possible eight bit values. This is
115 all the information needed to generate CRCs on data a byte at a time for all
116 combinations of CRC register values and incoming bytes. The remaining tables
117 allow for word-at-a-time CRC calculation for both big-endian and little-
118 endian machines, where a word is four bytes.
120 local void make_crc_table()
124 z_crc_t poly; /* polynomial exclusive-or pattern */
125 /* terms of polynomial defining this crc (except x^32): */
126 static volatile int first = 1; /* flag to limit concurrent making */
127 static const unsigned char p[] = {0,1,2,4,5,7,8,10,11,12,16,22,23,26};
129 /* See if another task is already doing this (not thread-safe, but better
130 than nothing -- significantly reduces duration of vulnerability in
131 case the advice about DYNAMIC_CRC_TABLE is ignored) */
135 /* make exclusive-or pattern from polynomial (0xedb88320UL) */
137 for (n = 0; n < (int)(sizeof(p)/sizeof(unsigned char)); n++)
138 poly |= (z_crc_t)1 << (31 - p[n]);
140 /* generate a crc for every 8-bit value */
141 for (n = 0; n < 256; n++) {
143 for (k = 0; k < 8; k++)
144 c = c & 1 ? poly ^ (c >> 1) : c >> 1;
149 /* generate crc for each value followed by one, two, and three zeros,
150 and then the byte reversal of those as well as the first table */
151 for (n = 0; n < 256; n++) {
153 crc_table[4][n] = ZSWAP32(c);
154 for (k = 1; k < 4; k++) {
155 c = crc_table[0][c & 0xff] ^ (c >> 8);
157 crc_table[k + 4][n] = ZSWAP32(c);
162 /* generate zero operators table for crc32_combine() */
164 /* generate the operator to apply a single zero bit to a CRC -- the
165 first row adds the polynomial if the low bit is a 1, and the
166 remaining rows shift the CRC right one bit */
168 crc_comb[k][0] = 0xedb88320UL; /* CRC-32 polynomial */
170 for (n = 1; n < GF2_DIM; n++) {
171 crc_comb[k][n] = row;
175 /* generate operators that apply 2, 4, and 8 zeros to a CRC, putting
176 the last one, the operator for one zero byte, at the 0 position */
177 gf2_matrix_square(crc_comb[k + 1], crc_comb[k]);
178 gf2_matrix_square(crc_comb[k + 2], crc_comb[k + 1]);
179 gf2_matrix_square(crc_comb[0], crc_comb[k + 2]);
181 /* generate operators for applying 2^n zero bytes to a CRC, filling out
182 the remainder of the table -- the operators repeat after GF2_DIM
183 values of n, so the table only needs GF2_DIM entries, regardless of
184 the size of the length being processed */
185 for (n = 1; n < k; n++)
186 gf2_matrix_square(crc_comb[n], crc_comb[n - 1]);
188 /* mark tables as complete, in case someone else is waiting */
191 else { /* not first */
192 /* wait for the other guy to finish (not efficient, but rare) */
193 while (crc_table_empty)
200 out = fopen("crc32.h", "w");
201 if (out == NULL) return;
203 /* write out CRC table to crc32.h */
204 fprintf(out, "/* crc32.h -- tables for rapid CRC calculation\n");
205 fprintf(out, " * Generated automatically by crc32.c\n */\n\n");
206 fprintf(out, "local const z_crc_t FAR ");
207 fprintf(out, "crc_table[%d][256] =\n{\n {\n", TBLS);
208 write_table(out, crc_table[0], 256);
210 fprintf(out, "#ifdef BYFOUR\n");
211 for (k = 1; k < 8; k++) {
212 fprintf(out, " },\n {\n");
213 write_table(out, crc_table[k], 256);
215 fprintf(out, "#endif\n");
217 fprintf(out, " }\n};\n");
219 /* write out zero operator table to crc32.h */
220 fprintf(out, "\nlocal const z_crc_t FAR ");
221 fprintf(out, "crc_comb[%d][%d] =\n{\n {\n", GF2_DIM, GF2_DIM);
222 write_table(out, crc_comb[0], GF2_DIM);
223 for (k = 1; k < GF2_DIM; k++) {
224 fprintf(out, " },\n {\n");
225 write_table(out, crc_comb[k], GF2_DIM);
227 fprintf(out, " }\n};\n");
230 #endif /* MAKECRCH */
234 local void write_table(out, table, k)
236 const z_crc_t FAR *table;
241 for (n = 0; n < k; n++)
242 fprintf(out, "%s0x%08lxUL%s", n % 5 ? "" : " ",
243 (unsigned long)(table[n]),
244 n == k - 1 ? "\n" : (n % 5 == 4 ? ",\n" : ", "));
252 #endif /* MAKECRCH */
254 #else /* !DYNAMIC_CRC_TABLE */
255 /* ========================================================================
256 * Tables of CRC-32s of all single-byte values, made by make_crc_table(),
257 * and tables of zero operator matrices for crc32_combine().
260 #endif /* DYNAMIC_CRC_TABLE */
262 /* =========================================================================
263 * This function can be used by asm versions of crc32()
265 const z_crc_t FAR * ZEXPORT get_crc_table()
267 #ifdef DYNAMIC_CRC_TABLE
270 #endif /* DYNAMIC_CRC_TABLE */
271 return (const z_crc_t FAR *)crc_table;
274 /* ========================================================================= */
275 #define DO1 crc = crc_table[0][((int)crc ^ (*buf++)) & 0xff] ^ (crc >> 8)
276 #define DO8 DO1; DO1; DO1; DO1; DO1; DO1; DO1; DO1
278 /* ========================================================================= */
279 unsigned long ZEXPORT crc32_z(crc, buf, len)
281 const unsigned char FAR *buf;
284 if (buf == Z_NULL) return 0UL;
286 #ifdef DYNAMIC_CRC_TABLE
289 #endif /* DYNAMIC_CRC_TABLE */
292 if (sizeof(void *) == sizeof(z_size_t)) {
296 if (*((unsigned char *)(&endian)))
297 return crc32_little(crc, buf, len);
299 return crc32_big(crc, buf, len);
302 crc = crc ^ 0xffffffffUL;
310 return crc ^ 0xffffffffUL;
313 /* ========================================================================= */
314 unsigned long ZEXPORT crc32(crc, buf, len)
316 const unsigned char FAR *buf;
319 return crc32_z(crc, buf, len);
325 This BYFOUR code accesses the passed unsigned char * buffer with a 32-bit
326 integer pointer type. This violates the strict aliasing rule, where a
327 compiler can assume, for optimization purposes, that two pointers to
328 fundamentally different types won't ever point to the same memory. This can
329 manifest as a problem only if one of the pointers is written to. This code
330 only reads from those pointers. So long as this code remains isolated in
331 this compilation unit, there won't be a problem. For this reason, this code
332 should not be copied and pasted into a compilation unit in which other code
333 writes to the buffer that is passed to these routines.
336 /* ========================================================================= */
337 #define DOLIT4 c ^= *buf4++; \
338 c = crc_table[3][c & 0xff] ^ crc_table[2][(c >> 8) & 0xff] ^ \
339 crc_table[1][(c >> 16) & 0xff] ^ crc_table[0][c >> 24]
340 #define DOLIT32 DOLIT4; DOLIT4; DOLIT4; DOLIT4; DOLIT4; DOLIT4; DOLIT4; DOLIT4
342 /* ========================================================================= */
343 local unsigned long crc32_little(crc, buf, len)
345 const unsigned char FAR *buf;
349 register const z_crc_t FAR *buf4;
353 while (len && ((z_size_t)buf & 3)) {
354 c = crc_table[0][(c ^ *buf++) & 0xff] ^ (c >> 8);
358 buf4 = (const z_crc_t FAR *)(const void FAR *)buf;
367 buf = (const unsigned char FAR *)buf4;
370 c = crc_table[0][(c ^ *buf++) & 0xff] ^ (c >> 8);
373 return (unsigned long)c;
376 /* ========================================================================= */
377 #define DOBIG4 c ^= *buf4++; \
378 c = crc_table[4][c & 0xff] ^ crc_table[5][(c >> 8) & 0xff] ^ \
379 crc_table[6][(c >> 16) & 0xff] ^ crc_table[7][c >> 24]
380 #define DOBIG32 DOBIG4; DOBIG4; DOBIG4; DOBIG4; DOBIG4; DOBIG4; DOBIG4; DOBIG4
382 /* ========================================================================= */
383 local unsigned long crc32_big(crc, buf, len)
385 const unsigned char FAR *buf;
389 register const z_crc_t FAR *buf4;
391 c = ZSWAP32((z_crc_t)crc);
393 while (len && ((z_size_t)buf & 3)) {
394 c = crc_table[4][(c >> 24) ^ *buf++] ^ (c << 8);
398 buf4 = (const z_crc_t FAR *)(const void FAR *)buf;
407 buf = (const unsigned char FAR *)buf4;
410 c = crc_table[4][(c >> 24) ^ *buf++] ^ (c << 8);
413 return (unsigned long)(ZSWAP32(c));
418 /* ========================================================================= */
419 local uLong crc32_combine_(crc1, crc2, len2)
426 #ifdef DYNAMIC_CRC_TABLE
429 #endif /* DYNAMIC_CRC_TABLE */
432 /* operator for 2^n zeros repeats every GF2_DIM n values */
433 for (n = 0; len2; n = (n + 1) % GF2_DIM, len2 >>= 1)
435 crc1 = gf2_matrix_times(crc_comb[n], crc1);
439 /* ========================================================================= */
440 uLong ZEXPORT crc32_combine(crc1, crc2, len2)
445 return crc32_combine_(crc1, crc2, len2);
448 uLong ZEXPORT crc32_combine64(crc1, crc2, len2)
453 return crc32_combine_(crc1, crc2, len2);