Question

I am trying to make a graph this program is just in initial phases where I
declared an array of structure pointers which holds address of vertices
*vertices[20] is an array which whose elements are all addresses of type struct node * which will contain the address of node of graphs.

#include<stdio.h>
#include<stdlib.h>
struct node {
    int data;
    struct node *links[10];
};
struct node *create_node(int);
void create_vertices(struct node **);
int main()
{
    struct node *vertices[20];
    int d, i, choice;
        *vertices[0]=(struct node *)malloc(sizeof(struct node)*20);
        create_vertices (&vertices);
}

struct node *create_node(int data)
{
    int i;
    struct node *temp = (struct node *)malloc(sizeof(struct node));
    temp->data = data;
    for (i = 0; i < 10; i++)
        temp->links[i] = NULL;
    return temp;
}

void create_vertices (struct node **v)
{
 int i,choice;
 i=0;
    printf("enter choice\n");
    scanf("%d", &choice);
    while (choice == 1) {
        printf("enter data\n");
        scanf("%d", &d);
        vertices[i] = create_node(d);
        i++;
        printf("enter choice\n");
        scanf("%d", &choice);
    }
}

Compiling the above code gives me following errors

bfs.c: In function ‘main’:
bfs.c:13:21: error: incompatible types when assigning to type ‘struct node’ from type ‘struct node *’
bfs.c:14:9: warning: passing argument 1 of ‘create_vertices’ from incompatible pointer type [enabled by default]
bfs.c:8:6: note: expected ‘struct node **’ but argument is of type ‘struct node * (*)[20]’
bfs.c: In function ‘create_vertices’:
bfs.c:35:16: error: ‘d’ undeclared (first use in this function)
bfs.c:35:16: note: each undeclared identifier is reported only once for each function it appears in
bfs.c:36:3: error: ‘vertices’ undeclared (first use in this function)

the program says an error in following line

    struct node *vertices[20];
*vertices[0]=(struct node *)malloc(sizeof(struct node)*20);

what is the harm in this kind of declaration.
I declare an array of pointers of type struct node * should I give memory also to them.

Answer 1

in main:

create_vertices (&vertices);

should be:

create_vertices (vertices);

Also: it would be more safe to let the create_vertices() function know what the size of the array is. The i index should not be allowed to exceed this size.

UPDATE: in main, remove the line:

*vertices[0]=(struct node *)malloc(sizeof(struct node)*20);

Vertices[] is an array of 20 (uninitialised) pointers in automatic storage (“on the stack”).

The create_vertices() function will give the pointers a value. (by calling malloc() and assigning the result to vertices[i])