understanding strlen function in C

Question

I am learning C. And, I see this function find length of a string.

size_t strlen(const char *str) 
{ 
 size_t len = 0U; 
 while(*(str++)) ++len; return len; 
}

Now, when does the loop exit? I am confused, since str++, always increases the pointer.

Answer 1

while(*(str++)) ++len;

is same as:

while(*str) {
 ++len;
 ++str;
}

is same as:

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

So now you see when str points to the null char at the end of the string, the test condition fails and you stop looping.

Answer 2

1. C strings are terminated by the NUL character which has the value of 0
2. 0 is false in C and anything else is true.

So we keep incrementing the pointer into the string and the length until we find a NUL and then return.