Issues with Segmentation Faults when deploying custom externs in simple_switch_grpc

[aneeshnotfound]

Hi,

I followed the instructions on implementing custom externs as given by #1140

However, I am running into segmentation faults when trying to modify P4 variables inside a C++ function within simple_switch_grpc. While passing variables as const or by value works for reading, arithmetic, and logging, passing them as non-const references leads to garbage memory allocation and crashes. I need to ensure that changes made to these variables in C++ properly reflect back in the P4 code. Does simple_switch_grpc support mutable P4 variable references in external C++ functions, and if so, what is the correct way to pass them?

I ran the following command to compile my p4 code:

p4c-bm2-ss —emit-externs -o program.json program.p4 —p4runtime-files p4info.txtpb

I ran the same executable command for simple_switch_grpc as mentioned on the README but with the addition of —load-modules=/extern_lib/definition.so

[jafingerhut]

At least for the primitive types like bit<W>, int<W>, bool, and several others, P4 only supports "directions" of in, out, and inout as parameters to externs. These are "copy in" for direction in and inout, and "copy out" for direction out and inout.

You can see examples of dealing with in and out parameters to extern method calls in the PR that you linked.

Method increment_by has one in parameter, which in the corresponding C++ code has a const reference parameter.

Method read has one out parameter, which in the corresponding C++ code has a reference parameter, which is assigned to in the C++ body of that code that modifies the value and should be seen by the P4 code after the extern method call returns.

I don't see any example of how to handle an inout parameter, but I assume it would be the same reference parameter in C++ as shown in the example for a direction out parameter, and the body of the C++ method should be able to examine the value, or assign to it.

[aneeshnotfound]

You are correct. However if I try to access (read from/write to) the reference parameter, I experience a segmentation fault. I am still able to read from the const reference parameter however (of course I can’t write to it since it is a constant).

Let me try providing an example from the PR: if I try logging or accessing “count” from the “read” function (a non constant reference) in “CustomCounter”, the switches crash. However, if I try logging or accessing “amount” from the “increment_by” function (a constant reference), it works.

[jafingerhut]

If you can create a public fork of this repository that contains the changes you are trying to make, and ideally also a P4 program you are compiling and running that causes the segmentation fault, that would help someone else reproduce the issue, and/or look at your code and perhaps see by inspection what is going wrong.