How are md5.h: pg_md5_hash() function and below functions working?

[Ali Koca]

I can't understand functions in md5.h, these are seemingly little bit 
weird. Such as:

        /* Utilities common to all the MD5 implementations,
        as of md5_common.c */
        extern bool pg_md5_hash(const void *buff, size_t len, char *hexsum);
        extern bool pg_md5_binary(const void *buff, size_t len, void *outbuf);
        extern bool pg_md5_encrypt(const char *passwd, const char *salt,
                                                   size_t salt_len, char *buf);