Segmentation fault when creating custom tcl channel driver - c++

Question

I am trying to create custom tcl channel and use it to get the output of tcl Interpreter. I added the implementation of few function of Tcl_ChannelType but I am getting segfault.
#include <tcl.h>
#include <iostream>
int driverBlockModeProc(ClientData instanceData, int mode) {
std::cout << "driverBlockModeProc\n";
return 0;
}
int driverCloseProc(ClientData instanceData, Tcl_Interp *interp) {
std::cout << "driverCloseProc\n";
return 0;
}
int driverInputProc(ClientData instanceData, char* buf, int bufSize, int *errorCodePtr) {
std::cout << "driverInputProc\n";
return 0;
}
int driverOutputProc(ClientData instanceData, const char* buf, int toWrite, int *errorCodePtr) {
std::cout << "driverOutputProc\n";
return 0;
}
int main() {
Tcl_ChannelType *typePtr = new Tcl_ChannelType;
typePtr->blockModeProc = driverBlockModeProc;
typePtr->outputProc = driverOutputProc;
typePtr->closeProc = driverCloseProc;
typePtr->inputProc = driverInputProc;
typePtr->seekProc = NULL;
typePtr->setOptionProc = NULL;
typePtr->getOptionProc = NULL;
typePtr->watchProc = NULL;
typePtr->getHandleProc = NULL;
typePtr->close2Proc = NULL;
typePtr->blockModeProc = NULL;
typePtr->flushProc = NULL;
typePtr->handlerProc = NULL;
typePtr->wideSeekProc = NULL;
typePtr->threadActionProc = NULL;
ClientData data = new char[200];
Tcl_CreateChannel(typePtr, "im_chanel", data, TCL_WRITABLE | TCL_READABLE);
}
I cant debug the segfault because its source are not available. I think the segfault is because a function is called which is NULL. I only need to use the channel to get the output of interpreter. Which functions I needn't implement here and is it right direction to solve the problem.

Answer 1

You're advised to download the source to Tcl when working at this level. I'm not sure what version you're using, but all official distributions of the source going back a very long way are on the SourceForge file distribution system; pick the exact match for the version you've got.
Creating a custom channel driver is not easy. There's a significant amount of complexity involved, and it isn't especially well-documented what “methods” within the channel driver type are mandatory and what are optional. (They're not C++ methods in a class — Tcl is pure C code for reasons too long to go into here — but they function in a conceptually similar way.)
If we look at the documentation for Tcl_CreateChannel, we see (quite a long way down that page) a definition of the channel type structure. The channel type structure should be statically allocated (Tcl's implementation assumes very strongly that it never changes location) and the following fields must be set to something meaningful:
typeName — This is the name of the channel type, useful for debugging!
version — This is the version of the channel type; you should set it to the latest version supported by your target source level. (You're recommended to use at least TCL_CHANNEL_VERSION_2 or things get rather more complex.)
closeProc or close2Proc — Channels must be closeable, but you have two choices for ways to do it. Bidirectional channels ought to use the close2Proc, but aren't strictly required to.
inputProc — Only needed if you're reading; take care to handle this correctly.
outputProc — Only needed if you're writing; take care to handle this correctly.
watchProc — Called to tell the channel driver to install itself into the event system so as to receive suitable events (as instructed by the supplied bitmask). Channels that don't have backing OS handles use timer events, or simply never actually generate events (in which case they'll never become readable or writable from the perspective of fileevent).
Looking at your code, I see that you're missing a watchProc. I know it's hard to see (not many people write channel drivers, to be honest, so the documentation isn't very hard “tested”) but it's really necessary.