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Re: [Chicken-users] call Chicken Scheme from C and pass a bytevector

From: Claude Marinier
Subject: Re: [Chicken-users] call Chicken Scheme from C and pass a bytevector
Date: Thu, 27 Jun 2013 17:24:39 -0400 (EDT)
User-agent: Alpine 2.02 (DEB 1266 2009-07-14)

On Thu, 27 Jun 2013, Dan Leslie wrote:
There's a section on accessing external objects that covers this sort of thing:

This addresses a different issue. The PCAP event handler in C calls the packet handler in Scheme. I am looking for a way to create saddr_o and daddr_o and give them to scheme like this.

  vector = C_alloc( C_SIZEOF_VECTOR(7) );
  vector_o = C_vector( &vector, 7,
    saddr_o, daddr_o,
    C_fix(type), C_fix(code),
    C_fix(tot_len) );

  ret = process_packet( vector_o );  // call Scheme code

It's possible to allocate C structures under the control of the Chicken GC:

Hum ... this has potential.

  struct ipv4_addr_struct {
    C_header tag;
    uint8_t *octets;
  typedef struct ipv4_addr_struct ipv4_addr;

  static const C_header BTREE_TAG =
    ((sizeof(ipv4_addr) - sizeof(C_header)) /
    sizeof(C_word)) | C_BYTEVECTOR_TYPE;

This would be handled by the GC, right?

Would Scheme be able to convert or use this as a u8vector or would it be a blob which I could convert with blob->u8vector/shared ?

However, you can also leave control up to the user rather than the GC and they can use free:

I prefer to let the GC handle it.

Personally, I like to allocate blobs and srfi-4 vectors in scheme and pass them as parameters to C functions to be mutated.

Wrong direction: I call scheme from C.

On 6/27/2013 10:07 AM, Claude Marinier wrote:

A function in pcap-interface.c calls Chicken Scheme. It builds a vector containing a bunch of things, e.g. C_fix(ethtype), and the source and destination addresses as vectors. The scheme code converts the address vectors to u8vector (u16vector for IPv6). This is a lot of work: just under 20% of CPU time is spent in vector->u8vector.

If I could create a blob in C and pass it to scheme, the scheme code could use blob->u8vector/shared and blob->u16vector/shared which would achieve the same result with much less work.

What is the recommended way to pass a blob to scheme from C?

Is there a way to create a u8vector or u16vector directly in C?

Thank you.

Claude Marinier

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