Question

I copied some code that simply reads a file to a string and prints the string from an older program. It was working fine, so I decided to modify it a bit. The new program is

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

int main() {
    FILE *itemlist = fopen("itemlist", "r");
    char *currentstring, charbuffer[2];
    // char itemstart = 0;
    while (fgets(charbuffer, 2, itemlist)) {
        strcat(currentstring, charbuffer);
    }
    printf("%s", currentstring);
    return 0; 
}

And it works as expected. But when I uncomment the itemstart line, it gives a segmentation fault. I’m not even using it and as far as I’m concerned, initializing an char to 0 is not illegal. I thought it was an issue with types, then I changed it to a short and then to int and it was still giving a segfault.

But then I removed the = 0 part and it worked again. Then I decided to put it back, debug the binary with gdb, and the segfault was at strcat.

How is this possible?

Answer 1

currentstring is a dangling pointer, so strcat(currentstring, charbuffer); results in undefined behavior.

Probably uncommenting char itemstart = 0 initializes some memory to 0 and the access violation is made visible, however this is just a guess. Undefined behavior means anything can happen.

You should allocate memory for currentstring:

currentstring = malloc(10); //or whatever length you need