reverse.c

#include <assert.h>
#include <string.h>

void reverse(char *str) {
    size_t len = strlen(str);
    size_t half = len >> 1;

    for (int i = 0; i < half; i++) {
        int ei = (len - 1) - i;
        char begin = str[i];
        char end = str[ei];

        str[i] = end;
        str[ei] = begin;
    }

    return;
}

int main(void) {
    char str[] = "some";
    reverse(str);
    assert(strcmp(str, "emos") == 0);

    char str2[] = "some shit";
    reverse(str2);
    assert(strcmp(str2, "tihs emos") == 0);

    return 0;
}