reverse a string in C

void reverse(char *buf)
{
    char *start = buf;
    char *end = buf + strlen(buf) - 1;

    while(start < end)
        swap(start++, end--);
}

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

void swap(char *s1, char *s2)
{
    char t = *s1;
    *s1 = *s2;
    *s2 = t;
}

void swap2(char *s1, char *s2)
{
    *s1 ^= *s2;
    *s2 ^= *s1;
    *s1 ^= *s2;
}

void reverse(char *buf)
{
    char *start = buf;
    char *end = buf + strlen(buf) - 1;

    while(start < end)
        swap(start++, end--);
}

int main()
{
    char buf[] = "program to reverse a string";

    printf("before: %s\n", buf);
    reverse(buf);
    printf("after : %s\n", buf);

    return 0;
}

output:

before: program to reverse a string
after : gnirts a esrever ot margorp