C中带指针的字符串串联

Question

So this is the standard string concatenation code in C:

char *stringcat(char *dest, const char *src){
    char *save=dest;
    while(*save !='\0'){
        save++;
    }
    while(*src!='\0'){
        *save=*src;
        save++;
        src++;
    }
    *save='\0';
    return dest;
}

My question is why when we replace the first while loop with the following:

while(*save++){};

It does not work, but, when replaced with:

while(*++save){};

It does work. In the first two instances, save points to the null terminator at the end of dest at the end, which is then overwritten by the first character in src. However, in the third instance, it seems like save will be pointing to the character after the null terminator, which is weird.

Answer 1

while(*save++) 

First dereference save and compare that value to 0, then increment save.

while(*save !='\0'){
    save++;
}

First dereference save, compare to 0 and only increment if non-zero. See the difference?

Answer 2

If you make it while (*save++) {} , the operation you are repeating is: load byte, increment pointer, check whether byte was zero. Therefore, what will happen at the end of the string is: load null byte, increment pointer, check whether byte was zero, see it was, exit loop. So save will be pointing just after the null byte. But you want to start copying the second string on top of that null byte, not after it.

(Is it possible that you have the meanings of ++save and save++ interchanged in your head? If so, here's a useful mnemonic: ++save means increment, then load the value; save++ means load the value, then increment. So the order in which the variable name and the ++ appear corresponds to the order of operations.)