c - Why is *p++ different from *p += 1?

Question

Consider:

void foo1(char **p) { *p++; }
void foo2(char **p) { *p += 1; }

and

char *s = "abcd";
char *a = s; 
foo1(&a); 
printf("%s", a); //abcd

but if I use foo2() instead of:

char *a = s; 
foo2(&a); 
printf("%s", a); //bcd

Can someone explain it?

Answer 1

The key is the precedence of the += and the ++ operator. The ++ has a higher precedence than the += (in fact, assignment operators have the second lowest precedence in C), so the operation

*p++

means dereference the pointer, then increment the pointer itself by 1 (as usually, according to the rules of pointer arithmetic, it's not necessarily one byte, but rather sizeof(*p) regarding the resulting address). On the other hand,

*p += 1

means increment the value pointed to by the pointer by one (and do nothing with the pointer itself).