A reminder about variable size structures

Variable-size structures are helpful, because you can allocate a structure containing an array of an unkown (at compile time) size at the end. For example, a “nuda_conf” structure containing informations about an MD Array and a “dev_info” structure per disks. The number of disks is unknown at compile time, so the only solution is to use a kalloc/vmalloc. The classical solution is to do :

struct dev_info {
int whatever;
struct rdev* rdev;
}

struct nuda_conf {
char* name;
int whotever;
struct dev_info disks*
}

...

struct nuda_conf* conf;
conf = kzalloc(sizeof(struct nuda_conf));
conf->disks = kzalloc(sizeof(struct dev_info) * NUMER_OF_DISKS);

But variable-size structures allows to do only one allocation :

struct nuda_conf {
char* name;
int whotever;
struct dev_info disks[0];
}

struct nuda_conf* conf;

conf = kzalloc(sizeof(struct nuda_conf) + sizeof(struct dev_info) * NUMBER_OF_DISKS);

This will allocate the memory for the char* and the int of nuda_conf, and the memory for a number of “dev_info”. To access them, one can do conf->disks[N] where N is the disk index.

But, this is limited to one variable-size array. If you do :

struct nuda_conf {
char* name;
int whotever;
struct dev_info disks[0];
struct indirection_line indirection_table[0];
}

The “indirection_table” and the “disks” pointer will point to the same location ! Even if you allocate size for both ! The solution is either to use a classical pointer as in the first example, or handle the memory access yourself :

struct nuda_conf {
char* name;
int whotever;
}
...
conf = kzalloc (sizeof(struct nuda_conf) + sizeof(struct dev_info) * NUMBER_OF_DISKS + sizeof(struct indirection_line)*NUMBER_OF_CHUNKS);
struct dev_info* disks = (struct dev_info*)(conf + 1);
struct indirection_line* indirection_table = (struct dev_info*)(disks + NUMBER_OF_DISKS);

But there is no interest in doing so, and both disks and indirection_table pointers have to be manually computed each time you need them… The use for that is really really really specific (dynamic metadata in packets for example).

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