How do you reverse a string in place in C or C++?

Question

How do you reverse a string in C or C++ without requiring a separate buffer to hold the reversed string?

Answer 1

I like Evgeny's K&R answer. However, it is nice to see a version using pointers. Otherwise, it's essentially the same:

#include 
#include 
#include 

char *reverse(char *str) {
    if( str == NULL || !(*str) ) return NULL;
    int i, j = strlen(str)-1;
    char *sallocd;
    sallocd = malloc(sizeof(char) * (j+1));
    for(i=0; j>=0; i++, j--) {
        *(sallocd+i) = *(str+j);
    }
    return sallocd;
}

int main(void) {
    char *s = "a man a plan a canal panama";
    char *sret = reverse(s);
    printf("%s\n", reverse(sret));
    free(sret);
    return 0;
}