C - fgets segfault

Question

I have the following code:

int get_int(void) {
    char input[10];
    fgets(input, 10, stdin); // Segfault here
    return atoi(input);
}

It gives me a segfault where marked. I have absolutely no idea what the issue is, because I have the following code in a different program:

int main(void) {
    char card[17];
    printf("Number: ");
    fgets(card, 17, stdin);
    printf("%s\n", card_type(card));
    return 0;
}

And it works fine. I am 100% sure it isn't segfaulting on the atoi.

Is this reproducible by others, I'm on Linux amd64 using GCC 4.4.5. It compiled and output no warnings.

Since it was requested, here is the code that calls get_int:

void get_input(int *inputs) { // Stop cluttering up my main
    printf("M spotting F: ");
    inputs[0] = get_int();
    printf("F spotting M: ");
    inputs[1] = get_int();
    printf("F spotting F: ");
    inputs[2] = get_int();
    printf("M spotting M: ");
    inputs[3] = get_int();
}

The code that calls that is:

int main(void) {
    int *inputs[4];
    int *heights[4];
    get_input(*inputs);
    get_heights(*inputs, *heights);

    print_bars(*heights);

    printf("M4F  F4M  F4F  M4M\n");
}

And thus you have reached the top of the call stack.

I've tested it with the values: 10, 1, 400, 40000000, and 4adf — Corey Richardson, Feb 28 '11 at 01:00
Could not reproduce it... Maybe you should post more code of what comes with the first sample you gave (and that fails). — Matthieu, Feb 28 '11 at 01:05
Might this be some sort of null-termination issue (although I can't think of exactly why at the moment)? — user541686, Feb 28 '11 at 01:07

score 8 · Accepted Answer · answered Feb 28 '11 at 01:11

Your declarations for the input (and probably height) arrays should be int inputs[4]; You want an array of integers, not an array of pointers to integers. You then want to call the functions with just inputs and heights as the parameters.

What you are doing now is creating an array of 4 pointers to integers. And then calling the function with *inputs will pass only the first pointer in that array to the function get_int. The get_int function will attempt to use the first pointer to integer as an array, which is not the behavior you want.

I'll give it a shot. Yep it worked! I'm still very new to this and am not quite sure when and when not to use a pointer. I probably need to read more on pointers. Thank you, Kevin. — Corey Richardson, Feb 28 '11 at 01:18

C - fgets segfault

1 Answers1