How to multiply 32-bit integers in c

问题

Execution of:

#define HIGH32(V64) ((uint32_t)((V64 >> 32)&0xffFFffFF))
#define LOW32(V64) ((uint32_t)(V64&0xffFFffFF))

uint32_t a = 0xffFFffFF;
uint32_t b = 0xffFFffFF;
uint64_t res = a * b;
printf("res = %08X %08X\n", HIGH32(res), LOW32(res));

Gives:

"res = 00000000 00000001"

But I expect: fffffffe00000001. What did I do wrong? Single assignment:

res = 0x0123456789ABCDEF;
printf("res = %08X %08X\n", HIGH32(res), LOW32(res));

Gives

res = 01234567 89ABCDEF

Environment:

$gcc --version
gcc (GCC) 4.8.3
Copyright (C) 2013 Free Software Foundation, Inc.

$ gcc -v
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/i686-pc-cygwin/4.8.3/lto-wrapper.exe
dest arch: i686-pc-cygwin

$ file a.exe
a.exe: PE32 executable (console) Intel 80386, for MS Windows

回答1:

You currently have:

uint64_t res = (uint32_t) a * (uint32_t) b;

You will want to promote the arguments to 64bit numbers before the multiplication. Therefore:

uint64_t res = (uint64_t) a * b;

来源：https://stackoverflow.com/questions/27203425/how-to-multiply-32-bit-integers-in-c