Thread: flexible array member not at end of struct

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.

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

>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.

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 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;
}