Requesting help on problem regarding using "^=" on strings

[kuasimikua]

Hi all, I am having trouble understanding the answer for this exercise in KNKING c programming a modern approach book

make a function
(d) strrev(s) -- Reverses the characters in s (except the null character); returns s.

char *strrev(char *s)
{
    char *t = s, *p;
    p = s + strlen(s) - 1;
    while (t < p)
    {
        *t ^= *p;
        *p ^= *t;
        *t ^= *p;
        t++;
        p--;
    }
    return s; }

I understand t is a pointer to the first character in the string, and p is the last pointer. I know that ^ is a bitwise exclusive OR operator, but i have no clue what the first 3 lines inside the while loop do, and how they swap the character around. Everything else I understand.

Thank you all for any help that can be provided. Cheers

[Salem]

Question 20.15c

Use a temporary variable in your code.
It's a lot clearer to basically anybody what your intention is.

[flp1969]

XOR has 3 basic uses: To invert bits based in a mask; To get the result of different bits in both operands or as a caryless add.
This kind of code is a trick to swap values without using a temporary variable, but it isn't a good one. Take a look at this two optimized (by the compiler, using -O2 option) functions f() and g():

; void f( int *a, int *b )
; { int t; t = *a; *a = *b; *b = t; }
f:
  mov eax,[rdi]
  mov edx,[rsi]
  mov [rdi],edx
  mov [rsi],eax
  ret

; void g( int *a, int *b )
; { *a ^= *b; *b ^= *a; *a ^= *b; }
g:
  mov eax,[rdi]
  xor eax,[rsi]
  mov [rdi],eax
  xor eax,[rsi]
  mov [rsi],eax
  xor [rdi],eax
  ret

[kuasimikua]

Thank you for the responses. Cheers a lot !