Re: [ruby-ffi] How to manage an Array of Structs

Hi P,

Just FYI, the first parameter to MemoryPointer.new is the type_size, and the second parameter is count - so you probably wanted to do something like this (for an array consisting of a single Sembuf):

ops = FFI::MemoryPointer.new(Shared::Sembuf) # count defaults to 1

Now then, there's probably a better way to do this, but this should work:

sembuf_count = ... # however many Sembufs you want to write

ops = FFI::MemoryPointer.new(Shared::Sembuf, sembuf_count)

# get the bytes

struct_bytes = op.get_bytes(0, Sembuf.size) # (offset, length)

# Pointer#[n] returns a new pointer with the address incremented by (n * Pointer#type_size) - just like a C array

ops[0].put_bytes(0, struct_bytes) # (offset, byte_string)

# now you repeat with ops[1], ops[2], ops[n-1]

ops[1].put_bytes ..

...

Again, there might be a better way to do this. I figured that Pointer#[]= would do the trick, but I guess not.

HTH,

Charles

On Wed, Apr 27, 2011 at 11:58 AM, P4010 <paolo.bosetti@gmail.com> wrote:

Hi all,
I am writing an FFI interface to IP shared memory and semaphores. I
wonder if someone here can give me advice on how to manage an Array of
Structs.

I am mapping the semop() function in FFI. It has the following
signature:

#include <sys/sem.h>
int
semop(int semid, struct sembuf *sops, size_t nsops);

I wrote something like:

module Shared
extend FFI::Library
ffi_lib FFI::Library::LIBC

class Sembuf < FFI::Struct
layout :sem_num, :ushort,
:sem_op, :short,
:sem_flg, :short
end

attach_function :semop, [:int, :pointer, :size_t], :int

end

Now the Shared#semop wants a pointer to an array of Shared::Sembuf
structs. How can I build such a pointer? I guess I should start with

op = Shared::Sembuf.new
op[:sem_num] = 0
op[:sem_op] = -1
op[:sem_flg] = 0

ops = FFI::MemoryPointer.new(Shared::Sembuf, Shared::Sembuf.size * 1)

But then I could not find how to add the op instance to the array
pointed to by ops...

I'll appreciate any suggestion...

Cheers,
-P.