flexible array member not at end of struct

in a struct, I would have an array of size [][3], where [] is undefined. But it won't work :

example

Code:

---

struct foo {
    int bar[][3];
};

---

I get :

Code:

---

11: error: flexible array member not at end of struct

---

I can't use

Code:

---

int *bar[3];

---

cause then I have bar[3][].

Anyone ?... Anyone ?

The size of the array must be known at compile time. So, either go with a fixed array size:
int foo[10][3];
or go with an array of pointers and malloc the memory later on.

OK, so what would the malloc line look like ?

Code:

---

myFoo.bar=malloc(3*size*sizeof(int));

---

or should I make two mallocs (one for *(myFoo.bar) and one for myFoo.bar) ? though I don't know how to write the first one.

pointers and memory management are still fuzzy to me

OK, I'm advancing here. I have my struct now like this

Code:

---

struct foo {
      int **bar;
      int **bar2;
}

---

and it works in most cases.

the problem is I want to use a foo as a global variable. But I can't if I don't give the entries to the struct. and bar2 is not easy stuff. I have a big function to calculate bar2's entries. I can't just write them down.

Any ideas on how to handle this ?

You want to make a variable of type struct foo, global or foo itself? just declare it outside of any functions (and above them)

Some light reading for you
http://pw2.netcom.com/~tjensen/ptr/pointers.htm
:)

Which compiler are you using?

Code:

---

struct foo {
    int a;
    int bar[][3];
};

---

compiles it just fine with
gcc -std=c99 prog.c



Bear in mind that it is wrong to have a struct consisting only of a flexible array
Commenting out the int a; causes gcc to report
flexible array member in otherwise empty struct

I toyed around for a while with what you guys told me, understood some important I am sillyI am sillyI am sillyI am silly, learned a few tricks, and finally, when I thought everything was going to work I have some new stupid problems.

first of all :

Quote:

---

compiles it just fine with
gcc -std=c99 prog.c

---

doesn't work, I get the same goddamn errors. I guess it the BSD kernel or something, security issues maybe.

Next, i still have my struct but this time I'm trying to mallocate it.

Code:

---

struct foo {
      int **bar;
}

---

Everything compiles nicely with this, but when I try to declare a foo type global variable in the main program with this line :

Code:

---

struct foo *myFoo = malloc(12*sizeof(int));

---

I get :

Code:

---

error: initializer element is not constant

---

I put (12*sizeof(int)) thinking of bar being 4x3...

>error: initializer element is not constant
You can't call malloc outside of a function. Global initializers must be constant.

I think this is what you're trying to do:

Code:

---

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

struct foo
{
        int (* bar)[3]; /* Declares a pointer to an int array[3] */
};

struct foo myFoo;

int main(void)
{
        int i, j;
        int k = 0;

        /* Allocate memory for myFoo.bar */
        myFoo.bar = malloc(4 * sizeof(*myFoo.bar));

        /* Check success of memory allocation */
        if (myFoo.bar == NULL)
        {
                fprintf(stderr, "Out of memory\n");
                return EXIT_FAILURE;
        }

        /* Sample usage of myFoo.bar */

        for (i = 0;i < 4;i++)
                for (j = 0;j < 3;j++)
                        myFoo.bar[i][j] = k++;

        for (i = 0;i < 4;i++)
                for (j = 0;j < 3;j++)
                        printf("myFoo.bar[%d][%d] = %d\n", i, j, myFoo.bar[i][j]);


        /* Use myFoo in the rest of your program as needed. */

        getchar();
        return EXIT_SUCCESS;
}

---

Note especially the method of declaring a pointer to an int array[3]. This is covered in more detail in "Chapter 8: Pointers to Arrays" at the site Hammer linked to.

YES ! PERFECT ! THANK YOU !

I guess you guys spent a whole lot of time coding in C ;)

I've got to get used to thinking like this.

Well, thanks a lot everyone.

not so perfect,

I can only fill the first row of bar. when I start putting some stuff in myFoo.bar[1][0], I get a Bus Error.

Can anyone explain ?


actually your code works but mine (the real one, not the example) doesn't. I'll check this a little further.

I'd changed the .c but not the .h

Some days I really loathe myself.

I dunno if this is what you're trying to do - you keep changing between arrays and pointers.

Code:

---

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

struct foo {
    char name[20];  // 20
    int a;          // 4
    int bar[][3];  // 12*n
};

int main()
{
    struct foo *jobber;
    size_t  alloc;
    int    i, j;

    printf( "%u\n", sizeof *jobber );
    printf( "%u\n", sizeof jobber->a );
    printf( "%u\n", sizeof jobber->bar[0] );
   
    // put a bar[10][3] at the end of the structure
    alloc = sizeof *jobber + sizeof jobber->bar[0] * 10;
    printf( "allocating %u bytes\n", alloc );

    jobber = malloc( alloc );
    strcpy( jobber->name, "hello" );
    jobber->a = 123;
    for ( i = 0 ; i < 10 ; i++ ) {
        for ( j = 0 ; j < 3 ; j++ ) {
            jobber->bar[i][j] = i * j;
        }
    }
    return 0;
}

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