I'm pretty sure mmap is more fundamental. shm() will also work, but I prefer mmap, it's more flexible. I also like the idea of being able to map in massive things from disk without wasting untold amounts of memory; whereas the memory the system will allow you to shm() may be fairly limited, and eats swap. Not wrong, no. It illustrates an important feature of shm, getting at anonymous memory from the outside. Just knowing the name can get you the memory, where mmap() would need to map an actual file to get at already-in-use segments a process didn't inherit. Remember though, a UNIX system is supposed to back all memory with disk one way or another. If you don't let it be on disk, it may just end up in swap. Well, how large are your structures? Think in terms of available memory, not in clients. Many programs have that kind of configurable maximum; use this much memory but no more. mmap makes this simple because you can map in the max amount of memory without actually using it. The OS will just give you more when you use it, up to the max you requested.

Many programs have a configurable limit for that sort of upper max. I recall a chess program that took a configurable amount of space for its memory buffer. It couldn't run as a limited user until fiddled with settings in /proc though because shm only gives limited users 16 megs or so by default in Linux. With file-backed mmap, hundreds of megs is the practical limit for one segment in 32-bit; and on 64-bit, the sky is the limit. You only want it to grow to a point. You don't want it to eat beyond all available memory and grind your machine into swapdeath. Pick a fraction of total RAM maybe.

Code:

// one way
struct mystruct
{
        int a;
        char b;
        float c;
};

// No arbitrary sizes
struct mystruct *myarray=malloc(sizeof(mystruct)*5000);
// or
struct mystruct *myarray=mmap(...);
// or shmat, etc etc.

struct otherstruct
{
        int a[5000];
        char b[5000];
        float c[5000];
};

// Size stuck at 5000
struct otherstruct otherarray;

It could be that I misunderstood the intent of the structure you posted though. I have no idea how you plan to organize this memory yet.