mchapman/zlib
1
0
Fork 0
Code Issues Pull Requests Projects Releases Wiki Activity

zlib 1.2.2

Browse Source
This commit is contained in:
Mark Adler
2011-09-09 23:24:02 -07:00
parent 7a6955760b
commit 79fbcdc939
55 changed files with 2178 additions and 205 deletions
Download Patch File Download Diff File Expand all files Collapse all files

88
crc32.c
View File

@@ -12,11 +12,11 @@
/* @(#) $Id$ */
/*
Note on the use of DYNAMIC_CRC_TABLE: there is no mutex or semaphore protection
on the static variables used to control the first-use generation of the crc
tables. Therefore if you #define DYNAMIC_CRC_TABLE, you should first call
get_crc_table() to initialize the tables before allowing more than on thread
to use crc32().
Note on the use of DYNAMIC_CRC_TABLE: there is no mutex or semaphore
protection on the static variables used to control the first-use generation
of the crc tables. Therefore, if you #define DYNAMIC_CRC_TABLE, you should
first call get_crc_table() to initialize the tables before allowing more than
one thread to use crc32().
*/
#ifdef MAKECRCH
@@ -103,51 +103,51 @@ local void make_crc_table()
{
unsigned long c;
int n, k;
unsigned long poly; /* polynomial exclusive-or pattern */
unsigned long poly; /* polynomial exclusive-or pattern */
/* terms of polynomial defining this crc (except x^32): */
static volatile int first = 1; /* flag to limit concurrent making */
static volatile int first = 1; /* flag to limit concurrent making */
static const unsigned char p[] = {0,1,2,4,5,7,8,10,11,12,16,22,23,26};
/* See if another task is already doing this (not thread-safe, but better
than nothing -- significantly reduces duration of vulnerability in
case the advice about DYNAMIC_CRC_TABLE is ignored) */
if (first) {
first = 0;
/* See if another task is already doing this (not thread-safe, but better
than nothing -- significantly reduces duration of vulnerability in
case the advice about DYNAMIC_CRC_TABLE is ignored) */
if (first) {
first = 0;
/* make exclusive-or pattern from polynomial (0xedb88320UL) */
poly = 0UL;
for (n = 0; n < sizeof(p)/sizeof(unsigned char); n++)
poly |= 1UL << (31 - p[n]);
/* make exclusive-or pattern from polynomial (0xedb88320UL) */
poly = 0UL;
for (n = 0; n < sizeof(p)/sizeof(unsigned char); n++)
poly |= 1UL << (31 - p[n]);
/* generate a crc for every 8-bit value */
for (n = 0; n < 256; n++) {
c = (unsigned long)n;
for (k = 0; k < 8; k++)
c = c & 1 ? poly ^ (c >> 1) : c >> 1;
crc_table[0][n] = c;
}
/* generate a crc for every 8-bit value */
for (n = 0; n < 256; n++) {
c = (unsigned long)n;
for (k = 0; k < 8; k++)
c = c & 1 ? poly ^ (c >> 1) : c >> 1;
crc_table[0][n] = c;
}
#ifdef BYFOUR
/* generate crc for each value followed by one, two, and three zeros, and
then the byte reversal of those as well as the first table */
for (n = 0; n < 256; n++) {
c = crc_table[0][n];
crc_table[4][n] = REV(c);
for (k = 1; k < 4; k++) {
c = crc_table[0][c & 0xff] ^ (c >> 8);
crc_table[k][n] = c;
crc_table[k + 4][n] = REV(c);
}
}
/* generate crc for each value followed by one, two, and three zeros,
and then the byte reversal of those as well as the first table */
for (n = 0; n < 256; n++) {
c = crc_table[0][n];
crc_table[4][n] = REV(c);
for (k = 1; k < 4; k++) {
c = crc_table[0][c & 0xff] ^ (c >> 8);
crc_table[k][n] = c;
crc_table[k + 4][n] = REV(c);
}
}
#endif /* BYFOUR */
crc_table_empty = 0;
}
else { /* not first */
/* wait for the other guy to finish (not exactly efficient, but rare) */
while (crc_table_empty)
;
}
crc_table_empty = 0;
}
else { /* not first */
/* wait for the other guy to finish (not efficient, but rare) */
while (crc_table_empty)
;
}
#ifdef MAKECRCH
/* write out CRC tables to crc32.h */
@@ -201,10 +201,10 @@ local void write_table(out, table)
const unsigned long FAR * ZEXPORT get_crc_table()
{
#ifdef DYNAMIC_CRC_TABLE
if (crc_table_empty)
make_crc_table();
if (crc_table_empty)
make_crc_table();
#endif /* DYNAMIC_CRC_TABLE */
return (const unsigned long FAR *)crc_table;
return (const unsigned long FAR *)crc_table;
}
/* ========================================================================= */