#include "mathlink.h"

/* read data from the link represented by id */

void f(MLENV env, long id)
{
    MLINK lp;

    lp = MLFromLinkID(env, id);
    if(lp == (MLINK)0)
        { /* unable to get the link from the id number */ }

    /* determine the type of data on lp */
    switch(MLGetNext(lp))
    {
        /* ... */
    }

    /* ... */
}