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git.ipfire.org Git - thirdparty/mdadm.git/blob - crc32.c
1 /* crc32.c -- compute the CRC-32 of a data stream
2 * Copyright (C) 1995-2003 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 about a factor
9 * 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().
24 # ifndef DYNAMIC_CRC_TABLE
25 # define DYNAMIC_CRC_TABLE
26 # endif /* !DYNAMIC_CRC_TABLE */
29 /* #include "zutil.h" / * for STDC and FAR definitions */
32 #define Z_NULL ((void*)0)
35 typedef long ptrdiff_t;
40 /* Find a four-byte integer type for crc32_little() and crc32_big(). */
42 # ifdef STDC /* need ANSI C limits.h to determine sizes */
45 # if (UINT_MAX == 0xffffffffUL)
46 typedef unsigned int u4
;
48 # if (ULONG_MAX == 0xffffffffUL)
49 typedef unsigned long u4
;
51 # if (USHRT_MAX == 0xffffffffUL)
52 typedef unsigned short u4
;
54 # undef BYFOUR /* can't find a four-byte integer type! */
59 #endif /* !NOBYFOUR */
61 /* Definitions for doing the crc four data bytes at a time. */
63 # define REV(w) (((w)>>24)+(((w)>>8)&0xff00)+ \
64 (((w)&0xff00)<<8)+(((w)&0xff)<<24))
65 local
unsigned long crc32_little
OF((unsigned long,
66 const unsigned char FAR
*, unsigned));
67 local
unsigned long crc32_big
OF((unsigned long,
68 const unsigned char FAR
*, unsigned));
74 #ifdef DYNAMIC_CRC_TABLE
76 local
volatile int crc_table_empty
= 1;
77 local
unsigned long FAR crc_table
[TBLS
][256];
78 local
void make_crc_table
OF((void));
80 local
void write_table
OF((FILE *, const unsigned long FAR
*));
84 Generate tables for a byte-wise 32-bit CRC calculation on the polynomial:
85 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.
87 Polynomials over GF(2) are represented in binary, one bit per coefficient,
88 with the lowest powers in the most significant bit. Then adding polynomials
89 is just exclusive-or, and multiplying a polynomial by x is a right shift by
90 one. If we call the above polynomial p, and represent a byte as the
91 polynomial q, also with the lowest power in the most significant bit (so the
92 byte 0xb1 is the polynomial x^7+x^3+x+1), then the CRC is (q*x^32) mod p,
93 where a mod b means the remainder after dividing a by b.
95 This calculation is done using the shift-register method of multiplying and
96 taking the remainder. The register is initialized to zero, and for each
97 incoming bit, x^32 is added mod p to the register if the bit is a one (where
98 x^32 mod p is p+x^32 = x^26+...+1), and the register is multiplied mod p by
99 x (which is shifting right by one and adding x^32 mod p if the bit shifted
100 out is a one). We start with the highest power (least significant bit) of
101 q and repeat for all eight bits of q.
103 The first table is simply the CRC of all possible eight bit values. This is
104 all the information needed to generate CRCs on data a byte at a time for all
105 combinations of CRC register values and incoming bytes. The remaining tables
106 allow for word-at-a-time CRC calculation for both big-endian and little-
107 endian machines, where a word is four bytes.
109 local
void make_crc_table()
113 unsigned long poly
; /* polynomial exclusive-or pattern */
114 /* terms of polynomial defining this crc (except x^32): */
115 static volatile int first
= 1; /* flag to limit concurrent making */
116 static const unsigned char p
[] = {0,1,2,4,5,7,8,10,11,12,16,22,23,26};
118 /* See if another task is already doing this (not thread-safe, but better
119 than nothing -- significantly reduces duration of vulnerability in
120 case the advice about DYNAMIC_CRC_TABLE is ignored) */
124 /* make exclusive-or pattern from polynomial (0xedb88320UL) */
126 for (n
= 0; n
< sizeof(p
)/sizeof(unsigned char); n
++)
127 poly
|= 1UL << (31 - p
[n
]);
129 /* generate a crc for every 8-bit value */
130 for (n
= 0; n
< 256; n
++) {
131 c
= (unsigned long)n
;
132 for (k
= 0; k
< 8; k
++)
133 c
= c
& 1 ? poly
^ (c
>> 1) : c
>> 1;
138 /* generate crc for each value followed by one, two, and three zeros,
139 and then the byte reversal of those as well as the first table */
140 for (n
= 0; n
< 256; n
++) {
142 crc_table
[4][n
] = REV(c
);
143 for (k
= 1; k
< 4; k
++) {
144 c
= crc_table
[0][c
& 0xff] ^ (c
>> 8);
146 crc_table
[k
+ 4][n
] = REV(c
);
153 else { /* not first */
154 /* wait for the other guy to finish (not efficient, but rare) */
155 while (crc_table_empty
)
160 /* write out CRC tables to crc32.h */
164 out
= fopen("crc32.h", "w");
165 if (out
== NULL
) return;
166 fprintf(out
, "/* crc32.h -- tables for rapid CRC calculation\n");
167 fprintf(out
, " * Generated automatically by crc32.c\n */\n\n");
168 fprintf(out
, "local const unsigned long FAR ");
169 fprintf(out
, "crc_table[TBLS][256] =\n{\n {\n");
170 write_table(out
, crc_table
[0]);
172 fprintf(out
, "#ifdef BYFOUR\n");
173 for (k
= 1; k
< 8; k
++) {
174 fprintf(out
, " },\n {\n");
175 write_table(out
, crc_table
[k
]);
177 fprintf(out
, "#endif\n");
179 fprintf(out
, " }\n};\n");
182 #endif /* MAKECRCH */
186 local
void write_table(out
, table
)
188 const unsigned long FAR
*table
;
192 for (n
= 0; n
< 256; n
++)
193 fprintf(out
, "%s0x%08lxUL%s", n
% 5 ? "" : " ", table
[n
],
194 n
== 255 ? "\n" : (n
% 5 == 4 ? ",\n" : ", "));
196 #endif /* MAKECRCH */
198 #else /* !DYNAMIC_CRC_TABLE */
199 /* ========================================================================
200 * Tables of CRC-32s of all single-byte values, made by make_crc_table().
203 #endif /* DYNAMIC_CRC_TABLE */
205 /* =========================================================================
206 * This function can be used by asm versions of crc32()
208 const unsigned long FAR
* ZEXPORT
get_crc_table(void)
210 #ifdef DYNAMIC_CRC_TABLE
213 #endif /* DYNAMIC_CRC_TABLE */
214 return (const unsigned long FAR
*)crc_table
;
217 /* ========================================================================= */
218 #define DO1 crc = crc_table[0][((int)crc ^ (*buf++)) & 0xff] ^ (crc >> 8)
219 #define DO8 DO1; DO1; DO1; DO1; DO1; DO1; DO1; DO1
221 /* ========================================================================= */
222 unsigned long ZEXPORT
crc32(
224 const unsigned char FAR
*buf
,
227 if (buf
== Z_NULL
) return 0UL;
229 #ifdef DYNAMIC_CRC_TABLE
232 #endif /* DYNAMIC_CRC_TABLE */
235 if (sizeof(void *) == sizeof(ptrdiff_t)) {
239 if (*((unsigned char *)(&endian
)))
240 return crc32_little(crc
, buf
, len
);
242 return crc32_big(crc
, buf
, len
);
245 /* crc = crc ^ 0xffffffffUL;*/
253 return crc
/* ^ 0xffffffffUL*/;
258 /* ========================================================================= */
259 #define DOLIT4 c ^= *buf4++; \
260 c = crc_table[3][c & 0xff] ^ crc_table[2][(c >> 8) & 0xff] ^ \
261 crc_table[1][(c >> 16) & 0xff] ^ crc_table[0][c >> 24]
262 #define DOLIT32 DOLIT4; DOLIT4; DOLIT4; DOLIT4; DOLIT4; DOLIT4; DOLIT4; DOLIT4
264 /* ========================================================================= */
265 local
unsigned long crc32_little(crc
, buf
, len
)
267 const unsigned char FAR
*buf
;
271 register const u4 FAR
*buf4
;
275 while (len
&& ((ptrdiff_t)buf
& 3)) {
276 c
= crc_table
[0][(c
^ *buf
++) & 0xff] ^ (c
>> 8);
280 buf4
= (const u4 FAR
*)buf
;
289 buf
= (const unsigned char FAR
*)buf4
;
292 c
= crc_table
[0][(c
^ *buf
++) & 0xff] ^ (c
>> 8);
295 return (unsigned long)c
;
298 /* ========================================================================= */
299 #define DOBIG4 c ^= *++buf4; \
300 c = crc_table[4][c & 0xff] ^ crc_table[5][(c >> 8) & 0xff] ^ \
301 crc_table[6][(c >> 16) & 0xff] ^ crc_table[7][c >> 24]
302 #define DOBIG32 DOBIG4; DOBIG4; DOBIG4; DOBIG4; DOBIG4; DOBIG4; DOBIG4; DOBIG4
304 /* ========================================================================= */
305 local
unsigned long crc32_big(crc
, buf
, len
)
307 const unsigned char FAR
*buf
;
311 register const u4 FAR
*buf4
;
315 while (len
&& ((ptrdiff_t)buf
& 3)) {
316 c
= crc_table
[4][(c
>> 24) ^ *buf
++] ^ (c
<< 8);
320 buf4
= (const u4 FAR
*)buf
;
331 buf
= (const unsigned char FAR
*)buf4
;
334 c
= crc_table
[4][(c
>> 24) ^ *buf
++] ^ (c
<< 8);
337 return (unsigned long)(REV(c
));