arithmetic on a pointer to an incomplete type 'ebpf::USDT'

[juchem]

Summary:

When including bcc/BPF.h there's a compilation error regarding arithmetic on a pointer to an incomplete type 'ebpf::USDT'.

Upon inspection of the bcc/BPF.h source file, I verified that it indeed forward declares class USDT then creates a std::vector of it somewhere inside class BPF. The class definition for USDT only comes later in the file. The implementation of std::vector itself is the one responsible for the pointer arithmetic.

The fix for this issue seems straightforward: swap the order of the class declarations in the bcc/BPF.h file, after which things compile successfully.

bcc version

This has been observed in bcc v0.25.0.

Environment:

Clang 14.0.6 with libstdc++-12-dev version 12.1.0-8 under Debian Sid. Linux kernel version: Linux bst 5.18.0-2-amd64 #1 SMP PREEMPT_DYNAMIC Debian 5.18.5-1 (2022-06-16) x86_64 GNU/Linux.

Error message:

In file included from /usr/lib/gcc/x86_64-linux-gnu/12/../../../../include/c++/12/functional:62:
In file included from /usr/lib/gcc/x86_64-linux-gnu/12/../../../../include/c++/12/vector:64:
/usr/lib/gcc/x86_64-linux-gnu/12/../../../../include/c++/12/bits/stl_vector.h:367:35: error: arithmetic on a pointer to an incomplete type 'ebpf::USDT'
        _M_impl._M_end_of_storage - _M_impl._M_start);
        ~~~~~~~~~~~~~~~~~~~~~~~~~ ^
/usr/lib/gcc/x86_64-linux-gnu/12/../../../../include/c++/12/bits/stl_vector.h:526:7: note: in instantiation of member function 'std::_Vector_base<ebpf::USDT, std::allocator<ebpf::USDT>>::~_Vector_base' requested here
      vector() = default;
      ^
/usr/local/include/bcc/BPF.h:59:50: note: in defaulted default constructor for 'std::vector<ebpf::USDT>' first required here
                   const std::vector<USDT>& usdt = {});
                                                 ^
/usr/local/include/bcc/BPF.h:43:7: note: forward declaration of 'ebpf::USDT'
class USDT;
      ^
fatal error: too many errors emitted, stopping now [-ferror-limit=]

Compiler flags

/usr/bin/c++  -I/src -I/src/out -D__STRICT_ANSI__ -std=c++2b -static -Wall -Werror -Wno-bitwise-instead-of-logical -Wno-comment -Wno-gnu-string-literal-operator-template -Wno-overlength-strings -Wno-pointer-bool-conversion -Wshadow-field-in-constructor -Wshadow-field-in-constructor-modified -Wshadow-ivar -Wshadow-uncaptured-local -Wundef -x c++ -save-temps=obj -ggdb3 -O1 -Wno-unused-const-variable -Wno-unused-local-typedef -Wno-unused-private-field -ferror-limit=1 -o kprobe.cpp.o -c kprobe.cpp

Compiler version:

$ /usr/bin/c++ --version
clang version 14.0.6 (https://github.com/llvm/llvm-project.git f28c006a5895fc0e329fe15fead81e37457cb1d1)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /usr/bin