Weird segfault on variable assignment [closed]

Question

I have a weird segfault with the following program:

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

void bugtest(char **output)
{
    *output = malloc(10);
    if (! *output)
    {
        fprintf(stderr, "malloc() failed\n");
        return;
    }
    memset(*output, 0, 10);

    for (char i = 0; i < 6; i += 2)
    {
        *output[i / 2] = i;
    }
}

int main (int argc, char *argv[]) 
{
    char *output = NULL;
    bugtest(&output);

    printf("%s\n", output);
    return 0;
}

*output is not NULL but it crashes at *output[i / 2] = i;. Does anyone know why?

Answer 1

*output[i / 2] Operator precedence. Should have been (*output)[i / 2].

Instead of dragging that ugly pointer-to-pointer across your whole function (and causing bugs because you do), then do this:

void bugtest(char **output)
{
    char* result = malloc(10);
    if (result == NULL)
    {
        fprintf(stderr, "malloc() failed\n");
        return;
    }
    memset(result, 0, 10);

    for (char i = 0; i < 6; i += 2)
    {
        result[i / 2] = i;
    }
    
    *output = result;
}

Unrelated to your bug, it doesn't make any sense to print the symbol values 0 to 6 as a string here: printf("%s\n", output);. Symbol 0, the null terminator, is particularly troublesome since it will just stop the printing. Perhaps you meant to print integer values?