C pointers

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C pointers

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#1 by (2 votes)

2

Well guys, I'm doing graph theory in C, and I've tried using dynamic allocation to create an array. Also, my code has a function to populate the array with 1 in the received indexes. However I have some error in the array allocation or in its padding.

The code hangs and returns 3221225477

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

struct digraph //Estrutura do grafo, V e A são contadores, matrix_adj é um ponteiro para um array de ponteiros
{
    int V , A;
    int **matrix_adj;
};

typedef struct digraph* Digraph;
int **matrix_alloc(int r,int c,int val);
Digraph graph_alloc(int vertex);
void insert(Digraph G, int v, int w);
void show(Digraph G);

int main(void)
{
    Digraph G;
    G = graph_alloc(6);
    insert(G, 1,2);
    insert(G, 1,3);
    insert(G, 2,4);
    insert(G, 3,4);
    insert(G, 4,5);
    insert(G, 5,6);
    show(G);
    return 0;

}

int **matrix_alloc(int r,int c,int val)
{
    int i, j;
    int **matrix = (int**)malloc(r*sizeof(int*));// aloca linhas
    for(i=0;i<r;i++)
    {
        matrix[i] = (int*)malloc(c*sizeof(int));//aloca colunas
        for(j=0;j<c;j++)
        {
            matrix[i][j] = val;
        }
    }
    return matrix;
}

Digraph graph_alloc(int vertex)
{
    Digraph G;
    G->V = vertex;
    G->A = 0;
    G->matrix_adj = matrix_alloc(vertex, vertex, 0);
    return G;
}

void insert(Digraph G,int v, int w)
{
    if(G->matrix_adj[v][w] == 0)
    {
        G->matrix_adj[v][w] = 1;
        G->matrix_adj[w][v] = 1;
        G->A++;
    }
}

void show(Digraph G)
{
    int v, w;
    for(v=0; v<G->V; v++)
    {
        for(w=0; w<G->V;w++)
        {
            printf("%d ", G->matrix_adj[v][w]);
        }
        printf("\n");
    }
    printf("\n");
    printf("Vertices: ", G->V);
    printf("Arestas:  ", G->A);
}

c ponteiro struct grafo

asked by anonymous 17.08.2018 / 22:07

1 answer

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score 2 · Accepted Answer

Attention to this type of typedefs :

typedef struct digraph* Digraph;

You should avoid this type of typedefs with pointers because they will eventually mask the types and give you the wrong idea, consequently causing you to make mistakes you would not normally make.

Note that in the graph_alloc creation function:

Digraph graph_alloc(int vertex)
{
    Digraph G;  // <---- aqui
    G->V = vertex;
    G->A = 0;
    G->matrix_adj = matrix_alloc(vertex, vertex, 0);
    return G;
}

You are using Digraph G as if it were a int or a normal type, but it is actually a pointer due to the typedef you did initially. It would have been just as done:

struct digraph *G;
G->V = vertex;

That's wrong. The problem is that is creating a pointer, says that in the given location V becomes vertex , but has not defined where G points . Because of this, the pointer is pointing to a random location in memory.

This represents undefined and potential segmentation fault behavior, which is the hang that you indicated.

It has several solutions. The simplest thing to do is to allocate with malloc :

Digraph G = malloc(sizeof(struct digraph));

See the code working on Ideone

I personally advise you to undo%% of the pointer and use only one for the base type to avoid having to always type typedef , like this:

typedef struct digraph digraph;