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...

Quote:

---

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.

Quote:

---

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

Quote:

---

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!