C Windows - Memory Mapped File - dynamic array within a shared struct

Question

I'm trying to share a struct similar to the following example:

typedef struct { 
    int *a; 
    int b; 
    int c;
} example;

I'm trying to share this struct between processes, the problem that I find is that when I initialize 'a' with malloc, I won't be able to access the array from within the second process. Is it possible to add this dynamic array to the memory mapped file?

Answer 1

You can have it as

typedef struct { 
    int b; 
    int c;
    int asize; // size of "a" in bytes - not a number of elements
    int a[0];
} example;

/* allocation of variable */
#define ASIZE   (10*sizeof(int))
example * val = (example*)malloc(sizeof(example) + ASIZE);
val->asize = ASIZE;

/* accessing "a" elements */
val->a[9] = 125;

the trick is zero sized a array at the end of the structure and malloc larger then size of structure by actual size of a.

You can copy this structure to mmapped file. You should copy sizeof(example)+val->asize bytes. On the other side, just read asize and you know how many data you should read - so read sizeof(example) bytes, realloc and read additional asize bytes.