两个几乎完全相同的程序，但输出不同

Question

Hi I have made a very simple program that should work but it don't:

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <string.h>

int main(int argc, char *argv[]) {
    int usedNumbers[256];
    memset(usedNumbers,0,256);

    srand(time(NULL));

    for(int i=0; i<256; ++i){
        while(1){
            int r = rand()%256;
            if( !usedNumbers[r] ){
                usedNumbers[r] = 1;
                break;
            }
            printf("Test: %03d -> %03d\n", i, r);
        }
    }

    return 0;
}

The idea of the program is to print numbers from 0 to 255 on the screen in the random order but the program stops on 84th number on 32 computers and 144th number on 64 bit computers. If i only move the "int usedNumbers[256];" above the function like that:

#include <string.h>

int usedNumbers[256];

int main(int argc, char *argv[]) {

Program works as it supposed to. Why is it that? I am using the newest GNU/GCC compiler and C11 standard.

Answer 1

The usedNumbers inside main is a local variable and these are not zero-initialized (ie they can contain garbage). Since you only use memset(..., 256) , only the first 256 bytes are zero-initialized, and the rest (eg half or three quarters of the array -- or more, depending on the size of int ) is not.

The usedNumbers outside main is a global variable, however, and these are completely zero-initialized, even without memset . So there, you really have an empty array with no garbage in it, and that is why that works as expected.

So do:

    memset(usedNumbers, 0, sizeof(usedNumbers));

and both versions should produce the same, expected result.