Modifying string in function C

Question

Let's say I want to modify char array using function. I am always seeing people using malloc, calloc, or pointers to modify int, char, or 2D arrays. Am I right, if I say, that string can be returned from function only if I use malloc, create that array pointer and return him? Then why not getting/altering string, by passing it to function parameter? Isn't my demonstration, which is using char array in parameter easier, than allocating/freeing? Is my concept wrong, or why am I never seeing people passing arrays to function? I am only seeing codes with passing like "char *array", not "char array[]", using malloc etc, when I see this method of altering char array easy. Am I missing something?

#include <stdio.h>

void change(char array[]){
    array[0]='K';
}

int main(){

    char array[]="HEY";
    printf("%s\n", array);
    change(array);
    printf("%s\n",array );

    return 0;
}

Did this not provide enough info? https://stackoverflow.com/questions/10186765/what-is-the-difference-between-char-array-vs-char-pointer-in-c — Random Davis, Mar 19 '19 at 23:32
not all strings are mutable, so not all strings can be changed. That is why the usual method is to `malloc()` enough memory and produce the changed string inside the resulting allocated memory — user3629249, Mar 21 '19 at 18:56

score 3 · Accepted Answer · answered Mar 20 '19 at 03:24

If you only need to change existing characters in the string, and the string will be in a variable, and you don't mind the side-effect of your original string being modified, then your solution may be acceptable and indeed easier. But:

What if you want to get a modified string, but also want to retain the original? To avoid destroying an arbitrary-sized original, you need to malloc space, make a copy, and modify that.
And what if you want to extend the string? If your change is to add " YOU" to the string, it can't modify the original because there's no space for it--it'll cause a buffer overflow, since there's only 4 bytes allocated for "HEY" (three letters plus the null terminator). Again, the solution involves mallocing space to work with.

Functions that make changes using your technique typically need a size or length parameter to avoid overflowing the array and causing a crash and a potential security risk. But although that avoids the overflow, there's still the question of what happens if there's not enough space: Silently drop some data? Pass back a flag or special value to indicate there wasn't enough space, and expect the caller to handle it? In the long run, it ends up easier to write it right the first time, and malloc/calloc the space and deal with having to free it up later and all that.

Got the point, thank you. Anyway if I get char array or pointer to array in parameter, I should be able to get it's length by calling function strlen(), so another parameter "length" is not needed. Is it correct? — victory, Mar 20 '19 at 13:28
It depends. `strlen()` counts *the bytes in the string up until a null*--i.e., the length of the current data in the string. It doesn't tell you the capacity of the array those bytes are in. If you're extracting data from the string, or only modifying bytes inside it, you're good. If you're expanding the string, the function needs to be told the capacity to use this approach safely. — Perette, Mar 20 '19 at 13:39

Modifying string in function C

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