strcasecmp和strncasecmp

int strcasecmp(const char *s1, const char *s2);
int strncasecmp(const char *s1, const char *s2, size_t n);
忽略大小写进行比较，差别是strncasecmp只比较s1的前n个字符。
返回值：相等返回0，s1的长度小于s2返回负数，大于返回正数。
The strcasecmp() function compares the two strings s1 and s2, ignoring the case of the characters. It returns an integer less than, equal to,or greater than zero if s1 is found, respectively, to be less than, to match, or be greater than s2.