Segmentation Fault when access to a pointer of struct in struct

Question

I have an issue with pointers to struct that have members that are pointers to struct too.

Browsing the suggested similar questions I found out this:

Accessing elements within a pointer of a struct inside another pointer to a struct

where people suggest to pay attention to allocation memory of the structure.

I think this is done correctly in my code.

typedef struct {
  int id_vec;
  float *vec_value;
} Vector;

typedef struct cluster{
  int id_cluster;
  float *centroid;
  Vector *patternInCluster; 
} Cluster;

int main(void){
  Cluster *cluster_ptr= malloc(3 * sizeof(Cluster));
  if (cluster_ptr==NULL){
    printf("NULL");
  }
  cluster_ptr->patternInCluster=malloc(2 * sizeof(Vector *));
  if (cluster_ptr->patternInCluster==NULL){
    printf("NULL");
    cluster_ptr->patternInCluster=NULL;
  }

  float p1[3]={0.0f,1.0f,2.0f};
  Vector *somePattern=malloc(2 * sizeof(Vector));
  somePattern[0].id_vec=1;
  somePattern[0].vec_value=p1;
  somePattern[1].id_vec=2;
  somePattern[1].vec_value=p1;
}

Then I expect that this statement works:

cluster_ptr[1].patternInCluster[1]=somePattern[1];

But it compiles and produces a Segmentation Fault.

Unexpectedly, the following statement doesn't get errors:

cluster_ptr[0].patternInCluster[1]=somePattern[1];

and a test show me correct result(somePattern[1] id and value)

I tried to debug with gdb but I only can see this:

Program received signal SIGSEGV, Segmentation fault. 0x00005555555547fe in main () at test_struct.c:36 36 cluster_ptr[1].patternInCluster[1]=somePattern[1];

Am I missing some allocation mistakes?

Answer 1

It's because you're not populating things fully.

This line

cluster_ptr->patternInCluster=malloc(2 * sizeof(Vector *));

is the same as saying

cluster_ptr[0].patternInCluster=malloc(2 * sizeof(Vector *));

and really given that cluster_ptr has been allocated as 3 Cluster it would be more clearer in your code to do the latter.

Since cluster_ptr[1].patternInCluster hasn't been given a value, trying to dereference it will lead to undefined behaviour but more likely will result in a segmentation fault.

Answer 2

You do not allocate enough memory:

cluster_ptr->patternInCluster=malloc(2 * sizeof(Vector *));

With patternInCluster being of type Vector * , you should allocate memory to hold elements of type Vector , not Vector* .

cluster_ptr->patternInCluster=malloc(2 * sizeof(Vector));

Answer 3

Your problem is NOT accessing the pointer inside the struct. Your problem is how you are using malloc() .

When you have one pointer, you malloc only once:

int *pointer = (int* )malloc(sizeof(int));
*pointer = 1;
printf("*pointer:%d\n", *pointer);

When you have pointer-to-pointer, you malloc() once for the **pointer_to_pointer , but you also have to malloc() once for the *pointer_to_pointer :

int** pointer_to_pointer = (int** )malloc(sizeof(int*));
*pointer_to_pointer = (int* )malloc(sizeof(int));
**pointer_to_pointer = 2;
printf("**pointer:%d\n", **pointer_to_pointer);

And if you have more than one pointer, at the location pointed to by **pointer_to_pointer , you need a for loop to assign memory to every one of those *pointer_to_pointer s.

for (unsigned int i = 0; i < 3; i++)
{
    *(pointer_to_pointer + i*sizeof(int)) = (int* )malloc(sizeof(int));
}
**(pointer_to_pointer + sizeof(int)) = 3;
**(pointer_to_pointer + 2UL*sizeof(int)) = 4;
printf("**(pointer_to_pointer + sizeof(int):%d\n", **(pointer_to_pointer + sizeof(int)));
printf("**(pointer_to_pointer + 2UL*sizeof(int):%d\n", **(pointer_to_pointer + 2UL*sizeof(int)));

You are mistaken to think that Cluster *cluster_ptr= malloc(3 * sizeof(Cluster)); will automatically/magically assign memory for Cluster[0] and Cluster[1] and Cluster[2] .

Your statement actually assigns memory only for Cluster[0] , but big enough for 3 Cluster s.

So the the modified code will look like this:

#include <string.h>
#include <stdio.h>
#include <malloc.h>

typedef struct {
    int id_vec;
    float *vec_value;
} Vector;

typedef struct cluster{
    int id_cluster;
    float *centroid;
    Vector **patternInCluster;
} Cluster;

int main(void){

    Cluster **cluster_ptr = (Cluster **)malloc(sizeof(Cluster*));
    for (long unsigned int i = 0; i < 3; i++) {
        cluster_ptr[i] = (Cluster *)malloc(sizeof(Cluster));
        if (cluster_ptr[i]==NULL){
            printf("NULL");
        }

        cluster_ptr[i]->patternInCluster = (Vector **) malloc(sizeof(Vector*));
        for (long unsigned int j = 0; j < 3; j++) {
            (*cluster_ptr)->patternInCluster[j] = (Vector *) malloc(sizeof(Vector));
            if ((*cluster_ptr)->patternInCluster[j]==NULL){
                printf("NULL");
                (*cluster_ptr)->patternInCluster[j]=NULL;
            }
        }
    }



    float p1[3]={0.0f,1.0f,2.0f};
    Vector *somePattern= (Vector *) malloc(sizeof(Vector));
    somePattern[0].id_vec=1;
    somePattern[0].vec_value=p1;
    somePattern[1].id_vec=2;
    somePattern[1].vec_value=p1;

    cluster_ptr[1]->patternInCluster[1] = &somePattern[0];
    cluster_ptr[0]->patternInCluster[1] = &somePattern[1];
    cluster_ptr[1]->patternInCluster[0] = &somePattern[1];
    cluster_ptr[2]->patternInCluster[1] = &somePattern[0];

    printf("%d\n", cluster_ptr[1]->patternInCluster[1]->id_vec);
    printf("%d\n", cluster_ptr[0]->patternInCluster[1]->id_vec);
    printf("%d\n", cluster_ptr[1]->patternInCluster[0]->id_vec);
    printf("%d\n", cluster_ptr[2]->patternInCluster[1]->id_vec);

    return 0;
}

• On my system, I just compiled and it builds and runs error-free.