How to multiply 32-bit integers in c

c, cygwin, gcc, multiplication

Solution

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;

Problem

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 ```

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