simple problem with pointers

Jo_Nak · 06-17-2003, 03:57 PM

This seems like a simple problem, but I can't figure it out. In the .c file, i declared a global structure and global pointers to the structure this way:

typedef struct{
int fd;
...
} video_t;

video_t *vid0, *vid1, *vid2, *vid3;

then, i want to initialise it in my first function. This is what i did:

memset( vid0, 0, sizeof(video_t) );
vid0->fd = open( "dev/video0", O_RDONLY );
if (vid0->fd == -1) return;

It works when i don't use a structure and use directly the things in it. But I absolutly need a structure.

Is this the way to initialise the pointer and to access it?

thx a lot

Jo_Nak

Dark_Helmet · 06-17-2003, 04:28 PM

You're initializing the memory the pointers are indicating, but you aren't initializing the pointers themselves. You need to assign a value to the pointer before dereferencing it. Like this:

Code:

vid0 = (video_t *)malloc(sizeof(video_t));

At least, that's how I'm comfortable doing it.

THEN, you can initialize the memory it points at with memset.

Hko · 06-17-2003, 04:46 PM

You declared pointers to point to instances op the struct, but you have not made instances to point to...

Code:

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

typedef struct{
            int fd;
} video_t;

video_t *vid0, *vid1, *vid2, *vid3;

/* make 4 instances of the struct */
video_t v0, v1, v2, v3;

int main()
{
        /* make the pointers point to the instances */
        vid0 = &v0;
        vid1 = &v1;
        vid2 = &v2;
        vid3 = &v3;

        /* you can now do a memset on them: (only one here: vid0) */
        memset( vid0, 0, sizeof(video_t) );
        return 0;
}

Instead of declaring normal variable of type video_t and then assign the pointers to them, you can also create them dynamically with malloc() which returns the pointer you will want to store in the pointer variables:

Code:

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

typedef struct{
            int fd;
} video_t;

video_t *vid0, *vid1, *vid2, *vid3;

int main()
{       
        /* make an instance (only one here for the */
        /* example) on the heap with malloc() */
        /* which returns a pointer to the struct    */
        /* floating somewhere in memory:            */
        vid0 = (video_t *) malloc(sizeof(video_t));

        memset( vid0, 0, sizeof(video_t) );

        /* You need to free the memory allocated    */
        /* by malloc() explicitly.                  */
        free(vid0);

        return 0;
}