Structs with pointer

Hi guys, I'm new around here. I'm currently doing my internship, and work daily with C. My question is, why is code below crashing?

Code:

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

struct bpf_insn {
	short	code;
	char 	jt;
	char 	jf;
    int      k;
};


struct bpf_program {
    int en;
	struct bpf_insn *bf_insn;
};


int main()
{

    struct bpf_insn *bf_insn;
    bf_insn->code   =12;
    bf_insn->jt     =11;
    bf_insn->jf     =10;
    bf_insn->k      =9;

    printf("bf_insn->jt is %d",bf_insn->jt);
    struct bpf_program *program;

    return 0;
}

The error apears when I make a pointer of type bpf_program.
a)why is this?
b)How can i fix this?

All what I want, is a pointer from type bpf_program, wich directs me to my counter and my pointer(wich point to the data)(this is the other struct)

Thanks,

Libpgeak

Code:

struct bpf_insn *bf_insn;

This just means that you have a pointer to a bpf_insn. There is no bpf_insn yet. The pointer is dangling around pointing a nothing in particular.

1. Initialize your pointer to NULL. That won't fix your problem, but at least it's pointing to a defined value, not at random garbage.
2. look up malloc or calloc to actually get a real bpf_insn to point to.

Well, i tested some more and i found out that:

-struct bpf_program program; gives no error
struct bpf_program *program; gives the error.

At this point, i realy don't get it...

Originally Posted by Libpgeak

Well, i tested some more and i found out that:

-struct bpf_program program; gives no error
struct bpf_program *program; gives the error.

At this point, i realy don't get it...

Simply having a pointer to struct bpf_program ain't gonna cut it, because the pointer has to hold the address of an object of that type else it contains garbage. So if you insist on using a pointer, then you'd need to create an object of that type too.

Originally Posted by Libpgeak

Well, i tested some more and i found out that:

-struct bpf_program program; gives no error
struct bpf_program *program; gives the error.

At this point, i realy don't get it...

I'm assuming that "-" is a typo.

The first one is a struct bpf_program. But you can't use indirect notation with that (->), you would use "bf_insn.code", et al.

The second one is a pointer to a struct bpf_program, but as nvoigt points out, the struct it points to does not exist, so when you try to assign it values, you have a problem.

Code:

struct bpf_program *program = malloc(sizeof(struct bpf_program));

Presuming malloc() succeeded, "program" is a pointer to a struct bpf_program that actually has an appropriate amount of memory assigned to it, and so you can put values there.

You should get a beginner C book and read the first half a dozen chapters or something, it will make your life much easier.

@LIPGEAK
In the main() function

Code:

struct bpf_insn *bf_insn;

This is just a pointer to a structure (It can hold address of a structure), but no actual structure has been created by you yet.
The first thing you need to do is:
- Make a structure (i.e. Memory must be allocated for the structure), which can be done in the following ways:

- Using a dynamic memory allocation by using malloc() function
- Use a statement

Code:

struct bpf_insn x;

Now x itself is a structure.
Now either you can use x to access the members of the structure using the dot (.) operator
eg: x.code=12;
Or either you can now make ur pointer which u defines point to this structure and then use indirection (->) operator.
eg:x->code=12;

Hope this helps
Regards
Sumit

Originally Posted by Libpgeak

Hi guys, I'm new around here. I'm currently doing my internship, and work daily with C. My question is, why is code below crashing?

Code:

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

struct bpf_insn {
	short	code;
	char 	jt;
	char 	jf;
    int      k;
};


struct bpf_program {
    int en;
	struct bpf_insn *bf_insn;
};


int main()
{

    struct bpf_insn *bf_insn;
    bf_insn->code   =12;
    bf_insn->jt     =11;
    bf_insn->jf     =10;
    bf_insn->k      =9;

    printf("bf_insn->jt is %d",bf_insn->jt);
    struct bpf_program *program;

    return 0;
}

The error apears when I make a pointer of type bpf_program.
a)why is this?
b)How can i fix this?

All what I want, is a pointer from type bpf_program, wich directs me to my counter and my pointer(wich point to the data)(this is the other struct)

Thanks,

Libpgeak

Tnx guys! sumit180288 you were right, the error was in the first struct. Thnx!