Trying to change the pointer stops the program from execution

Question

I'm practicing on Code Wars and I've encountered a strange problem. Omitting some details: the task is to replace each digit by its complement to 9, and shift each letter by a given number. To do so I've made this loop, which I believe should change each character based on its ANSI code representation:

char *str = "ABC 123!"; // example
int n = 1; // example

for (char *c = str; *c; c++) {
    printf("Before: *c = %d\n", *c);
    if (*c > 47 && *c < 58) {
        *c = 105 - *c;
        // replace digit
    }
    if (*c > 64 && *c < 91) {
        *c = (*c - 64 + number) % 26 + 64;
        // replace letter
    }
    printf("After: *c = %d\n", *c);
}

I've made various log statements in various places from which I can tell that some condition checking fails and I don't enter the if block, while the other succeeds and I do enter the if block, but as soon as I run into the *c = ...; statement, the test crashes (also tried running the same code in a separate IDE, but the execution still suddenly stops).

Why would this happen?

Post the full code, please. I suspect `str` is string literal.... — Eugene Sh., Feb 21 '18 at 18:12
Well, our suspicions are true. String literals are not modifiable. Attempting to write them will lead to undefined behavior. — Eugene Sh., Feb 21 '18 at 18:15
@EugeneSh.:Just a small question - does the standard say so? (*String literals are not modifiable*)? I guess no. (I didn't find it when I checked last time - but the behavior of attempting to modify it is undefined). :) — user2736738, Feb 21 '18 at 18:25
@coderredoc They are not modifiable in a sense that modifying them is illegal (undefined). It's like your private property is your private, until it is stolen :) — Eugene Sh., Feb 21 '18 at 18:27

score 1 · Accepted Answer · answered Feb 21 '18 at 18:17

Attempting to modify the string literal has undefined behavior. In your case the program stopped because you tried to modify the non-modifiable. (Generally it is placed in .rodata section - read only).However, modifying a mutable array of char directly, or through a pointer is not undefined behavior, even though its initializer is a literal string. The following is fine:

char str[] = "ABC 123!";

This is all you need to get the correct behavior out of your code. Also one of the reasons we don't write code in binary is make things human readable. Here instead of using 47,58 use '0' or '9' etc. It's more readable.

Trying to change the pointer stops the program from execution

1 Answers1