strcmp

if (strcmp(str1, str2) < 0) { /* is str1 < str2 */
    /* do something */
}

s1 is less than s2 when?

first i chars are the same but s1[i+1] char is less than s2[i+1]. for example s1 = "abca" s2 = "abcd" ⟹ s1 < s2.
s1 is the same string as s2 but truncated.

This follows the ascii character set.