About Flexible Array Members(FAM)

About Flexible Array Members(FAM)
My latest column in LFY is about Flexible Array Members (FAM) - see http://www.lfymag.com/currentissue.asp?id=13 (the article is not available online yet.

For those who dont know that FAM is: FAM is about the C99 feature where the last member of a struct can be an incomplete one. We can allocate memory dynamically for that structure with additional storage for that struct to treat the last element as a flexible lenght array. We remember the size of the allocated array in a member of the struct itself.

So far so good. An LFY reader - Basavaraj Dengi - asked me this intersting question: "when we can use a pointer and keep the things simple,
why use flexible array member?" An interesting question, indeed!

It appears that using FAM member and a pointer are equivalent. For example

struct varlength1 {
int len;
int *data;
};

struct varlength2 {
int len;
int data[];
};

Just think about it - what are the differences here? varlength1 has a *data, and we can allocate the struct statically. Then we need to allocate data with some dynamically allocated memory. What about the struct that is allocate dynamically? We logically do two allocations: one for the struct itself and another for the pointer member inside.

However with varlenght2, we can allocate it statically like this:

struct varlength2 {
int len;
int data[];
} varray { 3, { 1, 2, 3 } };

(This is a GCC feature and I am not sure if its C99), so it is very convenient. Coming to dynamic allocation of varlenght2, we do one dynamic allocation and allocate extra space for the data so that the end of the structure is treated as part of the array. How about doing free of the data? The compiler will obviously give an error, and so we need to do a free once: and that is for the struct.

As we can see, FAM is a convenience feature. Assume that we're writing a program for reading a file which is written according to a simple fileformat (bitmap format comes to my mind, but since I dont remember that format off-my-head, I am not sure if its a very good example for explaining the use of FAM). Now, we read the file header, which is typically a fixed lenght header. The header gives info on the rest of the file, which let us assume, is a simple list of integers. Now, for implementing it, FAM comes handy: as we read the header, we can allocate the struct for that file format once and use that FAM as if we allocated it statically. Freeing that struct is also easy. With a pointer member it is a little more work, but I dont deny it, it will also work.

Get me right: I am not saying using FAM is good and recommended. What I am saying is that FAM is a convenience feature and sometimes we find it as a handy feature to use. Yes, I can hear some of you still murmuring with disapproving my explanation. I see their point: C programmers can live without FAM, just like the K&R C days. But as I said, its just a "convenience" feature. If you want, take it and use it, or you can live with the plain old pointers, and keep going: you make the choice!