GSoC Weekly Report #6

Weekly Report #6
Submitted on 2010-06-28
Covers 2010-06-21 to 2010-06-28
Corresponding Draft Schedule Item:
Create the RPC framework that can call functions on the GPP side from the DSP and return values back to the DSP.  Implement and unit-test the POSIX function wrappers according to the planned order.

Status and Accomplishments
DSP->GPP RPC code is now integrated into the C6Run sources and works fine alongside with the regular C6RunApp C I/O calls. The RPC layer is compiled/linked together with the user-provided sources for now as putting them into the C6Run library caused some strange problems (see issues section).
the GPP side RPC stubs are now compiled into a dynamically linked library (.so) and the GPP side server uses dlfcn functions (dlopen, dlsym..) to locate the function with the given name and invoke it. the stubs still pull the parameters from the buffer on their own - assembly code for automating this is still in the works

Plans and Tasks
finalize the GPP-side assembly function caller so that GPP side stubs don't need to be provided at all
experiment with different combinations of parameters, including doubles/long integers and shared-mem allocated buffers to make sure everything is working correctly with the marshaller, DSP/Link transfers, unmarshaller and GPP-side server
finalize the DSP-side stubs for the POSIX functions which don't take/return any buffers as parameters

Risks, issues, blockers
One strange thing that occured: if I compile the RPC marshalling function together with the user sources (thus having the RPC layer in the user code, since readmsg/writemsg is accessible from there) given to c6runapp-cc everything works fine, but when I try to include the functions in the DSP-side C6Run library, something goes wrong. The function is a variadic function whose definition looks like this:

void rpc_marshal(char *function_name, char *function_signature, ...)

When I compile the DSP side libs with this function inside, the parameters after (and including) the second parameter are not passed correctly. I've observed that even the address of the formal parameter goes awry. To illustrate, let's say I put a printf call inside this function (thanks to C6Run CIO :)):

printf("Stack addresses: %x %x values: %s %s", &function_name, &function_signature, function_name, function_signature);

If I compile and use this function alongside with the user-provided sources to c6runapp-cc, everything works correctly, so I get output that looks like this (I've made up the stack addresses but they were similar to these):

printf("Stack addresses: 0x80000800 0x8000804 values: rpc_function iii@");

but if used from inside the library:

printf("Stack addresses: 0x80000800 0x80000854 values: rpc_function ");

Not sure why this is happening - linker settings? a problem with passing variadic parameters? (since if I remove the ... at the end the two parameters get passed correctly, but of course then the others don't). Since it works fine if provided alongside user sources I decided to stick with that for now.