How to create a md5 hash of a string in C?

Question

I've found some md5 code that consists of the following prototypes...

I've been trying to find out where I have to put the string I want to hash, what functions I need to call, and where to find the string once it has been hashed. I'm confused with regards to what the uint32 buf[4] and uint32 bits[2] are in the struct.

struct MD5Context {
    uint32 buf[4];
    uint32 bits[2];
    unsigned char in[64];
};

/*
 * Start MD5 accumulation.  Set bit count to 0 and buffer to mysterious
 * initialization constants.
 */
void MD5Init(struct MD5Context *context);

/*
 * Update context to reflect the concatenation of another buffer full
 * of bytes.
 */
void MD5Update(struct MD5Context *context, unsigned char const *buf, unsigned len);

/*
 * Final wrapup - pad to 64-byte boundary with the bit pattern 
 * 1 0* (64-bit count of bits processed, MSB-first)
 */
void MD5Final(unsigned char digest[16], struct MD5Context *context);

/*
 * The core of the MD5 algorithm, this alters an existing MD5 hash to
 * reflect the addition of 16 longwords of new data.  MD5Update blocks
 * the data and converts bytes into longwords for this routine.
 */
void MD5Transform(uint32 buf[4], uint32 const in[16]);

Answer 1

I don't know this particular library, but I've used very similar calls. So this is my best guess:

unsigned char digest[16];
const char* string = "Hello World";
struct MD5Context context;
MD5Init(&context);
MD5Update(&context, string, strlen(string));
MD5Final(digest, &context);

This will give you back an integer representation of the hash. You can then turn this into a hex representation if you want to pass it around as a string.

char md5string[33];
for(int i = 0; i < 16; ++i)
    sprintf(&md5string[i*2], "%02x", (unsigned int)digest[i]);