File pointer change inside a C function

Question

I'd like to ask why is it possible to change the pointer value of file in C function without passing reference to it, what I mean is:

void fun(FILE *f)
{
    fclose(f);
    f = fopen("newfile", "r");
}

int main(void)
{
    FILE *old = fopen("file", "r");
    char *msg = (char*)malloc(sizeof(char) * 100);
    fun(old);
    fscanf(old, "%s", msg);
    printf("%s", msg);
    free(msg);
    return 0;
}

Can anyone explain it to me? I was thought that pointers are being copied so I expected to get an error about closed file. Surprisingly I didn't get it.

Answer 1

Arguments in C is passed by value, and that means they are copied. So the functions never have the original values, just copies, and modifying a copy will of course not modify the original.

You can emulate pass by reference by using pointers, and in the case of passing a pointer by reference you of course have to pass a pointer to the pointer (by using the address-of operator &).

void fun(FILE **f)
{
    fclose(*f);
    *f = fopen("newfile", "r");
}

...

fun(&old);

Answer 2

It is copied, but it still points to the same file.