reverse.c

#include <stdio.h>
#include <string.h>

void swap(char *a, char* b);
void reverse(char* s);

int main() {
	char s[] = "Hello, world!";

	printf("Before: %s\n", s);

	reverse(s);

	printf("After:  %s\n", s);

	return 0;
}

void swap(char* a, char* b) {
	char c = *a;
	*a = *b;
	*b = c;
}

void reverse(char* s) {
	if (!*s)
		return;

	char* head = s;
	char* tail = head + strlen(s) - 1;

	while (head <= tail) {
		swap(head, tail);
		head++;
		tail--;
	}
}