CPLEX MIP current node LP relaxation - c

Question

During a MIP optimization with CPLEX C APIs is it possible to retrieve the linear relaxation (dual variables, reduced costs, etc ...) of the current node (i.e. every n nodes)?
I've registered a callback function (CPXsetsolvecallbackfunc) in order to be notified each time a new node is available. In the callback I use CPXgetcallbackinfo to retrieve the node information and CPXgetcallbacknodelp to retrieve the linear relaxation but unfortunately the procedure CPXsolution returns that no solution exists and the MIP optimization exits.
Here is a sample code realized starting from an IBM example where environment and problem are assumed to be properly initialized.
struct noderange {
int startnode;
int endnode;
};
typedef struct noderange NODERANGE;
NODERANGE nodeswritten;
nodeswritten.startnode = -1;
nodeswritten.endnode = 2100000000;
status = CPXsetsolvecallbackfunc(environment, usersolve, &nodeswritten);
if(status) {goto TERMINATE;}
status = CPXmipopt(environment, problem);
if(status) {goto TERMINATE;}
where the usersolve procedure is
static int CPXPUBLIC usersolve(CPXCENVptr env, void *cbdata, int wherefrom, void *cbhandle, int *useraction_p) {
int status = 0;
int nodecount;
static int count = 0;
CPXLPptr nodelp;
NODERANGE *nodeswritten;
*useraction_p = CPX_CALLBACK_DEFAULT;
nodeswritten = (NODERANGE *)cbhandle;
/* Find out what node is being processed */
status = CPXgetcallbackinfo(env, cbdata, wherefrom, CPX_CALLBACK_INFO_NODE_COUNT, &nodecount);
if (status) goto TERMINATE;
if (nodecount >= nodeswritten->startnode && nodecount <= nodeswritten->endnode) {
/* Get pointer to LP subproblem, then write a SAV file. */
status = CPXgetcallbacknodelp(env, cbdata, wherefrom, &nodelp);
if (status) goto TERMINATE;
int rows = CPXgetnumcols(env, nodelp);
int cols = CPXgetnumrows(env, nodelp);
int lpstat;
double objval;
double* x = (double*)malloc(sizeof(double) * CPXgetnumcols(env, nodelp));
double* dj = (double*)malloc(sizeof(double) * CPXgetnumcols(env, nodelp));
double* pi = (double*)malloc(sizeof(double) * CPXgetnumrows(env, nodelp));
double* slack = (double*)malloc(sizeof(double) * CPXgetnumrows(env, nodelp));
status = CPXsolution(env, nodelp, &lpstat, &objval, x, pi, slack, dj);
printf("Solutionstatus = %d\n", lpstat);
if (status) { goto TERMINATE; } // <--- HERE it returns no solution exists
free(x);
free(dj);
free(pi);
free(slack);
printf("[%d]\trows = %d cols = %d\t sol stat = %d\t z = %f\n", nodecount, rows, cols, lpstat, objval);
if (nodecount == nodeswritten->endnode) status = 1;
count++;
}
TERMINATE:
return (status);
}

Answer 1

I found out the problem, the callback is called before the subproblem has been solved so CPXsolution returns that no solution exists.