crypt() not returning standard MD5 Hash?

I'm trying to use the crypt() function found in <crypt.h> in C, and when I attempt to hash a string in MD5 for example, it returns something different than a standard MD5 hash would produce from a tool such as md5sum. Example/Proof:

hash.c:

#include <stdio.h>
#include <crypt.h>

#define MD5 "$1$"

int main() {

        const char string[] = "helloworld";
        char * hash = crypt(string, MD5);

        printf("%s\n",hash);

        return 0;
}

I compile with gcc -o hash hash.c -lcrypt and run:

./hash
$1$$edK86ZB1Vvaz2eneY.itb.

From my knowledge of UNIX systems, the format of the output of crypt matches the password format of UNIX users whereas the first part is the hash ID/type, the second part is the salt and the third is the actual hash - $id$salt$hash. Yet the ACTUAL standard MD5 hash of helloworld is fc5e038d38a57032085441e7fe7010b0. Is there any way to produce this hash using crypt()? Many thanks!

Solution

The string returned by the glibc implementation of crypt() is not just a simple hash of the salt + password encoded into base 16.

If you look at the source code of the MD5 version, you'll see among other things a loop that takes a previous MD5 hash and hashes that, for one thousand repetitions. Then it's turned into a string with repeated use of a function named __b64_from_24bit() that encodes 3 bytes of the final MD5 hash at a time (not in linear order) into what I assume is base 64.

This is why what you see looks nothing like the output of md5sum or the like.