Segfault when giving a function a struct pointer

Question

I have a function that looks like this:

int lexWhitespace(TokenizerOutput* input) {

    printf("he");
    if (!(14 > input->toStillParse[0] > 8) && !(input->toStillParse[0] == 32)) {
        // checks if the first character in the toStillParse section of input is whitespace.
        return -1;
    } else {input->tokenizerOutput[0] = input->toStillParse[0];}



    for (int i = 1; ; i++) {
        if ((14 > input->toStillParse[0] > 8) || (input->toStillParse[0] == 32)) {
        // checks if the first character in the toStillParse section of input is whitespace.
            input->tokenizerOutput[i] = input->toStillParse[i];
        } else {return 0;}
    }
}

that takes in this struct:

struct TokenizerOutput {
    const char* toStillParse; // holds the text that still needs to be parsed.
    char* tokenizerOutput; // holds the text that was just output by tokenizer function.
};
typedef struct TokenizerOutput TokenizerOutput;

When I try to call it in the main function like this:

int main(void) {
    printf("hello\n");

    TokenizerOutput j = {"        k", " "};

    printf("%s\n", (&j)->toStillParse);

    lexWhitespace(&j);

    return 0;
}

I get a segfault. The segfault is occuring before the function lexWhitespace even runs anything because it does not print "he". I have no idea why this is happening. Any help would be greatly appreciated. I am using gcc 9.3.0.

String literals are not generally writable. You'll get the same error with `char *x = " "; x[0] = 'a';` — William Pursell, Jul 08 '21 at 04:11

score 1 · Accepted Answer · answered Jul 08 '21 at 03:58

There are some errors in this code.

First, this condition:

14 > input->toStillParse[0] > 8

This is probably unintentional. It is probably meant to be written as:

14 > input->toStillParse[0] && input->toStillParse[0] > 8

Second, this loop may never terminate:

for (int i = 1; ; i++) {
    if ((14 > input->toStillParse[0] > 8) || (input->toStillParse[0] == 32)) {
    // checks if the first character in the toStillParse section of input is whitespace.
        input->tokenizerOutput[i] = input->toStillParse[i];
    } else {return 0;}
}

Note that the character being compared, toStillParse[0], is the same character each loop iteration. So this loop will either exit immediately, or it will loop forever (and probably crash / segfault). It looks like the [0] should be [i]. Also note that the condition is probably written wrong.

In C, x > y > z is not the same thing as x > y && y > z. Whenever you see x > y > z, it is probably wrong (unless you're looking at IOCCC entries or something).

That's funny. Hadn't quite gotten to that part yet. Thanks! – Brad Oo Jul 08 '21 at 04:30 — Brad Oo, Jul 08 '21 at 04:30

Segfault when giving a function a struct pointer

1 Answers1