Compile a program using mhash

Question

I am trying to use lessfs and learning how it uses mhash to produce its cryptographic fingerprints, so I am taking a look at mhash to see how it handles the hashing algorithms, so I am trying to run some of the examples provided in the program, but I am running into complications and errors

The Mhash example that I was trying to solve is found here: http://mhash.sourceforge.net/mhash.3.html (or below)

#include <mhash.h>
 #include <stdio.h>

 int main()
 {

        char password[] = "Jefe";
        int keylen = 4;
        char data[] = "what do ya want for nothing?";
        int datalen = 28;
        MHASH td;
        unsigned char *mac;
        int j;


        td = mhash_hmac_init(MHASH_MD5, password, keylen,
                            mhash_get_hash_pblock(MHASH_MD5));

        mhash(td, data, datalen);
        mac = mhash_hmac_end(td);

 /* 
  * The output should be 0x750c783e6ab0b503eaa86e310a5db738
  * according to RFC 2104.
  */

        printf("0x");
        for (j = 0; j < mhash_get_block_size(MHASH_MD5); j++) {
                printf("%.2x", mac[j]);
        }
        printf("\n");


        exit(0);
 }

But I get the following errors:

mhash.c.text+0x6c): undefined reference to `mhash_get_hash_pblock'
mhash.c.text+0x82): undefined reference to `mhash_hmac_init'
mhash.c.text+0x9c): undefined reference to `mhash'
mhash.c.text+0xa8): undefined reference to `mhash_hmac_end'
mhash.c.text+0xf9): undefined reference to `mhash_get_block_size'
collect2: error: ld returned 1 exit status

Answer 1

This is a linker error — ld is the linker program on Unix systems. The linker is complaining because you're using library functions (mhash_get_hash_pblock, etc.) but you didn't provide a definition for them.

The preprocessor directive #include <mhash.h> declares functions (and types, etc.) from the mhash library. That's good enough to compile your program (produce a .o file) but not to link it (to produce an executable): you also need to define these functions.

Add -lmhash at the end of your compilation command line. This instructs the linker that it can look for functions in the library libmhash.a on its search path; at run time, the functions will be loaded from libmhash.so on the search path. Note that libraries must come on the command line after they're used: the linker builds up a link of required functions, which need to be provided by a subsequent argument.

gcc -o myprogram myprogram.c -lmhash