程序代码：

#include <stdio.h>

void reverse_string()
{
    char *str = "abcd";    
    char *last_char;
    for(last_char = str; *last_char != '\0'; last_char++);
    last_char--;
    while(str < last_char)
    {
        char temp;
        temp = *str;
        *str++ = *last_char;
        *last_char-- = temp;
        
    }
    printf("%s\n",str);
}
int main()
{
    reverse_string();
    return 0;
}