Using channels and ports in C/C++

From XCore Exchange

Currently ports and channel interaction in C or C++ has to be achieved by the user writing their own XC functions that can handle the various operations they require. These XC functions can then be called from C or C++ when needed.

Channel and Port Types in C/C++

You can have a channel as an argument to a C function by using the 'xccompat.h' include. For example:

#include <xccompat.h>
#include "channel_funcs.h"

void c_func( chanend c )
{
    unsigned v;
    while (1)
    {
        xc_channel_out(c, v);
        v = xc_channel_in( c );
    }
}

Where the channel interaction functions would be defined in 'channel_funcs.xc' such as:

void xc_channel_out(chanend c, unsigned v)
{
    c <: v;
}

Name mangling

To avoid name mangling with C++, you need to give the C++ functions called from XC, C linkage. For example, the header 'channel_funcs.h', for the C++ file channel_funcs.ccp would be:

#ifdef __cplusplus
extern "C" {
#endif

void cpp_func(chanend c);

#ifdef __cplusplus
}
#endif

For more information on mixing C and C++ see [1].

Passing Streaming Channels to C Functions

Using the <xccompat.h> header, you can easily pass channels into C functions as it has a typedef for the chanend type. However, if you have a streaming channel in XC and want to pass that to a C function, your code will not compile. The 'streaming' modifier is unknown and you'll get various errors such as:

error: conversion from transmissive type requested
function xyz changes streaming qualifier of type

The solution is very simple. You need to add:

1. define streaming

directly after your C source file include of the xccompat.h file:

#include <xccompat.h>
#define streaming

Your C code needs to know implicitly that the channel is a streaming type and treat it as such - no handshake token, etc. Similar to the examples above with a C program calling an XC function with a channel, you can call an XC function with a streaming channel.

In your XC, you can use:

// channel function called from C
// lets a C function that deals with pointers, floats and other things XC can't
// stuff a channel from C

void xc_channel_out(streaming chanend c, unsigned v)
{
    c <: v;
}

In your C code, you need to define the XC function prototype:

extern void xc_channel_out(streaming chanend c, unsigned v);

in order to call back into XC with a streaming channel.