Question

The following code causes a memory error in the insert_edge function (in the line p->next = g->edges[x] I guess) for large MAXV values. For smaller ones it works great. Where is the problem? How can I define the struct that it works?

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

#define MAX_LINE_SIZE (1024*128)


#define MAXV        23947347        /* maximum number of vertices */
#define NULLO              0        /* null pointer */


#define TRUE    1
#define FALSE   0

typedef int bool;

typedef struct edgenode {
    int y;              /* adjancency info */
    int weight;         /* edge weight, if any */
    struct edgenode *next;      /* next edge in list */
} edgenode;


typedef struct {
    edgenode *edges[MAXV+1];    /* adjacency info */
    int degree[MAXV+1];     /* outdegree of each vertex */
    int nvertices;          /* number of vertices in the graph */
    int nedges;         /* number of edges in the graph */
    int directed;           /* is the graph directed? */
} graph;

initialize_graph(graph *g, bool directed)
{
    int i;              /* counter */

    g -> nvertices = 0;
    g -> nedges = 0;
    g -> directed = directed;

    for (i=1; i<=MAXV; i++) 
            g->degree[i] = 0;
    for (i=1; i<=MAXV; i++) 
            g->edges[i] = NULL;
}

read_graph(graph *g, bool directed, const char *filename)
{

    FILE *f;
    char line[MAX_LINE_SIZE], buf[10];
    int format, rc;
    int edge; 
    int vertex_n;
    int vertex_m;

    char *token,*token2, *s;

    int v;

    int i;              /* counter */

    /* open file */
    f = fopen (filename, "r");
    if (f == NULL)
        return NULL;


    rc = sscanf (line, "%d %d %d", &(vertex_n), &(vertex_m), &(edge));

    initialize_graph(g, directed);

    for (i=1; i<=edge; i++) {
         s = fgets (line, MAX_LINE_SIZE, f); 

         token = strtok (line, " ");

         token2 = strtok (NULL, " ");

         int s = atoi(token);
         int t = atoi(token2);

         printf("%d, %d\n", start, ziel);

         insert_edge(g,s,t,directed);

 }

}

insert_edge(graph *g, int x, int y, bool directed)
{
    edgenode *p;            /* temporary pointer */

    p = malloc(sizeof(edgenode));   /* allocate storage for edgenode */

    p->weight = NULL;
    p->y = y;
    p->next = g->edges[x];
        g->edges[x];

    g->edges[x] = p;        /* insert at head of list */

    g->degree[x] ++;

    if (directed == FALSE)
        insert_edge(g,y,x,TRUE);
    else
        g->nedges ++;
 }

    main()
    {
        graph g;

        read_graph(&g,FALSE, "/path/graph_with_23947347_nodes.mtx");//  
    }

Answer 1

graph g;

graph is definitively too big to fit on the stack. Put it in the heap instead:

graph* g=malloc(sizeof(graph));

And follow Graham’s advice too. It may be too big even for the heap.