C Programming/string.h/strncmp
strncmp() is a standard-library function that compares two strings up to a specified length. The function is declared in string.h, and is compatible with the following prototype:
int strncmp (const char * s1, const char * s2, size_t num);
The function compares the strings using a lexicographical ordering, but never examines more than num characters. It returns zero if the strings are equal, or if their first num characters are equal; otherwise, it returns a negative integer if s1 is less than s2, and a positive integer if s1 is greater than s2. A simple implementation might be as follows:
int strncmp (const char * const s1, const char * const s2, const size_t num)
{
const unsigned char * const us1 = (const unsigned char *) s1;
const unsigned char * const us2 = (const unsigned char *) s2;
for(size_t i = (size_t)0; i < num; ++i)
{
if(us1[i] < us2[i])
return -1;
else if(us1[i] > us2[i])
return 1;
else if(! us1[i]) /* null byte -- end of string */
return 0;
}
return 0;
}
In practice, however, real implementations are significantly optimized.