variable sized structure

[ramkrishna1]

Hi All,

Consider the following piece of code:

struct foo1
{
unsigned char tag;
unsigned char length;
unsigned char value[];
};

struct foo2
{
unsigned char tag;
unsigned char length;
unsigned char value[];
};

struct foo3
{
unsigned char tag;
unsigned char length;
unsigned char value[];
};

struct foo4
{
unsigned char tag;
unsigned char length;
unsigned char value[];
};


struct abc
{

struct foo1 foo_1;
struct foo2 foo_2;
struct foo3 foo_3;
struct foo4 foo_4;

};

In 'C' struct foo1,foo2,foo3, foo4 cannot be members of struct abc as they declared with empty index. Is there any another method which solves the above scnerio to dynamically fill data into arrays in the structures.

[JosAH]

You could change those unsigned char values[] arrays to simple unsigned char* values
pointers and add a variable size array at the end of your abc struct. You do need
to initialize your pointers in the modified abc struct:

struct abc* p= malloc( ... );
p->foo_1.value= p->data;
p->foo_2.value= foo_1.value+foo_1.length;
p->foo_3.value= foo_2.value+foo_2.length;
p->foo_4.value= foo_3.value+foo_3,length;

I personally find it a bit 'mechanical' but that's what you get with those variable
length structs ;-)

kind regards,

Jos

[ramkrishna1]

hi Jos,
Thanks for ur reply.

we cannot use malloc() for our processor(AVR Series from atmel).is there any alternate way to allocate memory dynamically?.

Best Regards
Ramkrishna

You could change those unsigned char values[] arrays to simple unsigned char* values
pointers and add a variable size array at the end of your abc struct. You do need
to initialize your pointers in the modified abc struct:

struct abc* p= malloc( ... );
p->foo_1.value= p->data;
p->foo_2.value= foo_1.value+foo_1.length;
p->foo_3.value= foo_2.value+foo_2.length;
p->foo_4.value= foo_3.value+foo_3,length;

I personally find it a bit 'mechanical' but that's what you get with those variable
length structs ;-)

kind regards,

Jos

[JosAH]

hi Jos,
Thanks for ur reply.

we cannot use malloc() for our processor(AVR Series from atmel).is there any alternate way to allocate memory dynamically?.

Best Regards
Ramkrishna

If you use those little Atmels (an Atmega?) you are on your own, i.e. you can
make that pointer p (see my previous reply) point to anything you like. You
have to manage your own memory.

kind regards,

Jos