Why are side effects on given arguments only possible if they are dynamically allocated in C?

Question

NOTE: THIS QUESTION TURNS OUT TO ALREADY BE ANSWERED HERE → Is it possible to modify a string of char in C?

Assume we want to create side effects this way:

void sideEffect(char *str) {
  str++;
  *str = 'X';
}

Why are side effects on dynamically allocated memory allowed through functions?:

int main(int argc, char **argv) {
  char *test = malloc(4 * sizeof(char));
  strcpy(test, "abc");
  printf("before: %s\n", test); // before: abc
  sideEffect(test);
  printf("after: %s\n", test); // after: aXc
  free(test);
}

And why are side effects on statically allocated memory not allowed through functions?:

int main(int argc, char **argv) {
  char *test = "abc";
  printf("before: %s\n", test); // before: abc
  sideEffect(test); // creates segmentation fault :(
  printf("after: %s\n", test);
}

https://stackoverflow.com/questions/1011455/is-it-possible-to-modify-a-string-of-char-in-c — Rinkesh P, Nov 11 '22 at 10:58
`char *test = "abc";` -> `char test[] = "abc";`. String literals are constant. — Retired Ninja, Nov 11 '22 at 10:58
If you're using `gcc` you can get a warning for this if you add `-Wwrite-strings` to your compile flags. https://godbolt.org/z/s8TYKT3so — Retired Ninja, Nov 11 '22 at 11:02
@RetiredNinja Woah this is brilliant!! This flag will now save me so much head ache - thank you so so so much! — sueszli, Nov 11 '22 at 11:19

score 2 · Accepted Answer · answered Nov 11 '22 at 11:05

Why are side effects on given arguments only possible if they are dynamically allocated in C?

No, you can modify the object (including char arrays) passed by a pointer to the function. It does not matter how those objects were created or allocated.

In your example the problem is:

char *test = "ABC";

test references string literal "ABC" and string literals cannot be modified in C language (more precisely it is an Undefined Behaviour). Change to char test[] = "abc";

int main(int argc, char **argv) {
  char test[] = "abc";
  printf("before: %s\n", test); 
  sideEffect(test); 
  printf("after: %s\n", test);
}

Oh - now that clears everything up for me . Thank you! – sueszli Nov 11 '22 at 11:06 — sueszli, Nov 11 '22 at 11:06

Why are side effects on given arguments only possible if they are dynamically allocated in C?

1 Answers1