bpf_trace_printk does not exist but it does?

[a8nguyen]

Hello,
I am trying to write some bpf probes for an application written in C++. When I write bpf probes in bpf syntax, it works fine. But when I used the bpf C binding, my compiler kept saying that

socket.cc:198:5: error: ‘bpf_trace_printk’ was not declared in this scope; did you mean ‘BPF_FUNC_trace_printk’?
  198 |     bpf_trace_printk("%d\n", sizeof(tcps), tcps->snd_cwnd);
      |     ^~~~~~~~~~~~~~~~
      |     BPF_FUNC_trace_printk

But it does exist!

$grep bpf_trace_printk /usr/include/linux/bpf.h
  long bpf_trace_printk(const char *fmt, u32 fmt_size, ...)
               Also, note that **bpf_trace_printk**\ () is slow, and should
               than **bpf_trace_printk**\ () for sharing data with user
               bpf program is consistent with what **bpf_trace_printk**\ () does for now.
               bpf_trace_printk() is to be avoided as it places too small
               **bpf_trace_printk**\ () does for now.
               Behaves like **bpf_trace_printk**\ () helper, but takes an array of u64

[chenhengqi]

What are bpf syntax and bpf C binding ? Could you share the code here ?

[a8nguyen]

of course!

The snippet of code that is in bpf that works fine is from here

Here is my code using the C wrapper that throws me an error

void TCPSocket::trace_tcp_cwnd(void){
    Address::raw address;
    struct tcp_sock * tcps = &address;//address-> as_sockaddr;
    bpf_trace_printk("%d\n", sizeof(tcps), tcps->snd_cwnd);
    //BPF_FUNC_trace_printk("%d\n", sizeof(tcps), tcps->snd_cwnd);
}

[chenhengqi]

Please refer to https://github.com/iovisor/bcc/tree/master/examples/cpp

[a8nguyen]

So I tried to run the example script from there. but it couldnt compile either. The following is the error

/usr/bin/ld: /tmp/ccqSCRYo.o: in function `main':
tcpsendstack.cpp:(.text+0x5c): undefined reference to `ebpf::bpf_module_rw_engine_enabled()'
/usr/bin/ld: tcpsendstack.cpp:(.text+0x11b): undefined reference to `ebpf::BPF::init(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::vector<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::allocator<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > > const&, std::vector<ebpf::USDT, std::allocator<ebpf::USDT> > const&)'
/usr/bin/ld: tcpsendstack.cpp:(.text+0x1c3): undefined reference to `ebpf::BPF::attach_kprobe(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, unsigned long, bpf_probe_attach_type, int)'
/usr/bin/ld: tcpsendstack.cpp:(.text+0x2bb): undefined reference to `ebpf::BPF::detach_kprobe(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, bpf_probe_attach_type)'
/usr/bin/ld: tcpsendstack.cpp:(.text+0x3c3): undefined reference to `ebpf::BPF::get_stack_table(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, bool, bool)'
/usr/bin/ld: tcpsendstack.cpp:(.text+0x539): undefined reference to `ebpf::BPFStackTable::get_stack_symbol[abi:cxx11](int, int)'
/usr/bin/ld: tcpsendstack.cpp:(.text+0x6a8): undefined reference to `ebpf::BPFStackTable::get_stack_symbol[abi:cxx11](int, int)'
/usr/bin/ld: tcpsendstack.cpp:(.text+0x85b): undefined reference to `ebpf::BPFStackTable::~BPFStackTable()'
/usr/bin/ld: tcpsendstack.cpp:(.text+0x8a6): undefined reference to `ebpf::BPF::~BPF()'
/usr/bin/ld: tcpsendstack.cpp:(.text+0xa29): undefined reference to `ebpf::BPFStackTable::~BPFStackTable()'
/usr/bin/ld: tcpsendstack.cpp:(.text+0xa7e): undefined reference to `ebpf::BPF::~BPF()'
/usr/bin/ld: /tmp/ccqSCRYo.o: in function `ebpf::Path::Path(std::initializer_list<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >)':
tcpsendstack.cpp:(.text._ZN4ebpf4PathC2ESt16initializer_listINSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEEE[_ZN4ebpf4PathC5ESt16initializer_listINSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEEE]+0x4f): undefined reference to `ebpf::Path::DELIM[abi:cxx11]'
/usr/bin/ld: tcpsendstack.cpp:(.text._ZN4ebpf4PathC2ESt16initializer_listINSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEEE[_ZN4ebpf4PathC5ESt16initializer_listINSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEEE]+0x108): undefined reference to `ebpf::Path::DELIM[abi:cxx11]'
/usr/bin/ld: /tmp/ccqSCRYo.o: in function `ebpf::BPF::BPF(unsigned int, ebpf::TableStorage*, bool, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, bool)':
tcpsendstack.cpp:(.text._ZN4ebpf3BPFC2EjPNS_12TableStorageEbRKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEEb[_ZN4ebpf3BPFC5EjPNS_12TableStorageEbRKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEEb]+0x88): undefined reference to `ebpf::BPFModule::BPFModule(unsigned int, ebpf::TableStorage*, bool, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, bool)'
/usr/bin/ld: /tmp/ccqSCRYo.o: in function `ebpf::BPFHashTable<stack_key_t, unsigned long> ebpf::BPF::get_hash_table<stack_key_t, unsigned long>(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)':
tcpsendstack.cpp:(.text._ZN4ebpf3BPF14get_hash_tableI11stack_key_tmEENS_12BPFHashTableIT_T0_EERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE[_ZN4ebpf3BPF14get_hash_tableI11stack_key_tmEENS_12BPFHashTableIT_T0_EERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE]+0x34): undefined reference to `ebpf::TableStorage::iterator::iterator()'
/usr/bin/ld: tcpsendstack.cpp:(.text._ZN4ebpf3BPF14get_hash_tableI11stack_key_tmEENS_12BPFHashTableIT_T0_EERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE[_ZN4ebpf3BPF14get_hash_tableI11stack_key_tmEENS_12BPFHashTableIT_T0_EERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE]+0xed): undefined reference to `ebpf::TableStorage::Find(ebpf::Path const&, ebpf::TableStorage::iterator&) const'
/usr/bin/ld: tcpsendstack.cpp:(.text._ZN4ebpf3BPF14get_hash_tableI11stack_key_tmEENS_12BPFHashTableIT_T0_EERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE[_ZN4ebpf3BPF14get_hash_tableI11stack_key_tmEENS_12BPFHashTableIT_T0_EERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE]+0x138): undefined reference to `ebpf::TableStorage::iterator::operator->[abi:cxx11]() const'
/usr/bin/ld: tcpsendstack.cpp:(.text._ZN4ebpf3BPF14get_hash_tableI11stack_key_tmEENS_12BPFHashTableIT_T0_EERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE[_ZN4ebpf3BPF14get_hash_tableI11stack_key_tmEENS_12BPFHashTableIT_T0_EERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE]+0x196): undefined reference to `ebpf::TableStorage::iterator::~iterator()'
/usr/bin/ld: tcpsendstack.cpp:(.text._ZN4ebpf3BPF14get_hash_tableI11stack_key_tmEENS_12BPFHashTableIT_T0_EERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE[_ZN4ebpf3BPF14get_hash_tableI11stack_key_tmEENS_12BPFHashTableIT_T0_EERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE]+0x230): undefined reference to `ebpf::TableStorage::iterator::~iterator()'
/usr/bin/ld: /tmp/ccqSCRYo.o: in function `ebpf::BPFTableBase<stack_key_t, unsigned long>::first(void*)':
tcpsendstack.cpp:(.text._ZN4ebpf12BPFTableBaseI11stack_key_tmE5firstEPv[_ZN4ebpf12BPFTableBaseI11stack_key_tmE5firstEPv]+0x3e): undefined reference to `bpf_get_first_key'
/usr/bin/ld: /tmp/ccqSCRYo.o: in function `ebpf::BPFTableBase<stack_key_t, unsigned long>::next(void*, void*)':
tcpsendstack.cpp:(.text._ZN4ebpf12BPFTableBaseI11stack_key_tmE4nextEPvS3_[_ZN4ebpf12BPFTableBaseI11stack_key_tmE4nextEPvS3_]+0x37): undefined reference to `bpf_get_next_key'
/usr/bin/ld: /tmp/ccqSCRYo.o: in function `ebpf::BPFTableBase<stack_key_t, unsigned long>::lookup(void*, void*)':
tcpsendstack.cpp:(.text._ZN4ebpf12BPFTableBaseI11stack_key_tmE6lookupEPvS3_[_ZN4ebpf12BPFTableBaseI11stack_key_tmE6lookupEPvS3_]+0x37): undefined reference to `bpf_lookup_elem'
/usr/bin/ld: /tmp/ccqSCRYo.o: in function `ebpf::BPFTableBase<stack_key_t, unsigned long>::update(void*, void*)':
tcpsendstack.cpp:(.text._ZN4ebpf12BPFTableBaseI11stack_key_tmE6updateEPvS3_[_ZN4ebpf12BPFTableBaseI11stack_key_tmE6updateEPvS3_]+0x3a): undefined reference to `bpf_update_elem'
/usr/bin/ld: /tmp/ccqSCRYo.o: in function `ebpf::BPFTableBase<stack_key_t, unsigned long>::remove(void*)':
tcpsendstack.cpp:(.text._ZN4ebpf12BPFTableBaseI11stack_key_tmE6removeEPv[_ZN4ebpf12BPFTableBaseI11stack_key_tmE6removeEPv]+0x2f): undefined reference to `bpf_delete_elem'
collect2: error: ld returned 1 exit status

[chenhengqi]

https://github.com/iovisor/bcc/blob/master/INSTALL.md#source