GLPK minimum flow problem

example.c

/*
Simple example of minimum cost flow problem in GLPK

To build and run:
    $ gcc -o example -lglpk example.c
    $ ./example
    ret = 0; sol =    25
    flow =     5

*/

#include <stdio.h>
#include <stdlib.h>
#include <glpk.h>

typedef struct { double rhs; } v_data;
typedef struct { double low, cap, x, rc; int cost; } a_data;

#define node(v) ((v_data *)((v)->data))
#define arc(a)  ((a_data *)((a)->data))

int main(void) {
    glp_graph *G;
    glp_vertex *v;
    glp_arc *a;
    // offsets of node data
    int v_rhs = sizeof(double)*0;
    // offsets for arc data
    int a_low = sizeof(double)*0;
    int a_cap = sizeof(double)*1;
    int a_cost = sizeof(double)*2;
    int a_x = sizeof(double)*2;
    int a_rc = sizeof(double)*3;    

    int crash = 0;    
    double sol;
    int ret;
    
    G = glp_create_graph(sizeof(v_data), sizeof(a_data));
    
    glp_add_vertices(G, 2);
    a = glp_add_arc(G, 1, 2);
    
    node(G->v[1])->rhs = 5; // input 5 units to the network
    node(G->v[2])->rhs = -5; // output 5 units from the network
    
    arc(a)->cap = 100;
    arc(a)->cost = 2;  // MUST be integer, even though stored in a double
    
    ret = glp_mincost_relax4(G, v_rhs, a_low, a_cap, a_cost, crash, &sol, a_x, a_rc);
    printf("ret = %d; sol = %5g\n", ret, sol);
    printf("flow = %5g\n", arc(a)->x);
    
    glp_delete_graph(G);
    return 0;
}