C Function implementation - with Pointer vs without Pointer

Question

I've just started to work with C, and never had to deal with pointers in previous languages I used, so I was wondering what method is better if just modifying a string.

pointerstring vs normal.

Also if you want to provide more information about when to use pointers that would be great. I was shocked when I found out that the function "normal" would even modify the string passed, and update in the main function without a return value.

#include <stdio.h>

void pointerstring(char *s);
void normal(char s[]);

int main() {
    char string[20];
    pointerstring(string);
    printf("\nPointer: %s\n",string);
    normal(string);
    printf("Normal: %s\n",string);
}

void pointerstring(char *s) {
    sprintf(s,"Hello");
}
void normal(char s[]) {
    sprintf(s,"World");
}

Output:

Pointer: Hello
Normal: World

Answer 1

In a function declaration, char [] and char * are equivalent. Function parameters with outer-level array type are transformed to the equivalent pointer type; this affects calling code and the function body itself.

Because of this, it's better to use the char * syntax as otherwise you could be confused and attempt e.g. to take the sizeof of an outer-level fixed-length array type parameter:

void foo(char s[10]) {
    printf("%z\n", sizeof(s));  // prints 4 (or 8), not 10
}

When you pass a parameter declared as a pointer to a function (and the pointer parameter is not declared const), you are explicitly giving the function permission to modify the object or array the pointer points to.

Answer 2

One of the problems in C is that arrays are second-class citizens. In almost all useful circumstances, among them when passing them to a function, arrays decay to pointers (thereby losing their size information).

Therefore, it makes no difference whether you take an array as T* arg or T arg[] — the latter is a mere synonym for the former. Both are pointers to the first character of the string variable defined in main(), so both have access to the original data and can modify it.

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Note: C always passes arguments per copy. This is also true in this case. However, when you pass a pointer (or an array decaying to a pointer), what is copied is the address, so that the object referred to is accessible through two different copies of its address.