Undefined reference to boost::system::generic_category() although libs are given to g++

Question

What are the linker flags I'm missing here?

I try to compile this on an Ubuntu 18.04 LTS and it fails, it works inside a Debian 9 docker image:

#include <boost/dll.hpp>

// Trying to compile it with:
// g++ -o program -lboost_filesystem -ldl -lboost_system program.cpp

int main() {
  boost::dll::program_location();
  return 0;
}

The error I get is:

/tmp/ccKlWUUd.o: In function `__static_initialization_and_destruction_0(int, int)':
program.cpp:(.text+0x68): undefined reference to `boost::system::generic_category()'
program.cpp:(.text+0x74): undefined reference to `boost::system::generic_category()'
program.cpp:(.text+0x80): undefined reference to `boost::system::system_category()'
/tmp/ccKlWUUd.o: In function `boost::system::error_code::error_code()':
program.cpp:(.text._ZN5boost6system10error_codeC2Ev[_ZN5boost6system10error_codeC5Ev]+0x17): undefined reference to `boost::system::system_category()'
/tmp/ccKlWUUd.o: In function `boost::system::error_category::std_category::equivalent(int, std::error_condition const&) const':
program.cpp:(.text._ZNK5boost6system14error_category12std_category10equivalentEiRKSt15error_condition[_ZNK5boost6system14error_category12std_category10equivalentEiRKSt15error_condition]+0xb8): undefined reference to `boost::system::generic_category()'
program.cpp:(.text._ZNK5boost6system14error_category12std_category10equivalentEiRKSt15error_condition[_ZNK5boost6system14error_category12std_category10equivalentEiRKSt15error_condition]+0xf3): undefined reference to `boost::system::generic_category()'
/tmp/ccKlWUUd.o: In function `boost::system::error_category::std_category::equivalent(std::error_code const&, int) const':
program.cpp:(.text._ZNK5boost6system14error_category12std_category10equivalentERKSt10error_codei[_ZNK5boost6system14error_category12std_category10equivalentERKSt10error_codei]+0xb8): undefined reference to `boost::system::generic_category()'
program.cpp:(.text._ZNK5boost6system14error_category12std_category10equivalentERKSt10error_codei[_ZNK5boost6system14error_category12std_category10equivalentERKSt10error_codei]+0xf3): undefined reference to `boost::system::generic_category()'
program.cpp:(.text._ZNK5boost6system14error_category12std_category10equivalentERKSt10error_codei[_ZNK5boost6system14error_category12std_category10equivalentERKSt10error_codei]+0x1d2): undefined reference to `boost::system::generic_category()'
/tmp/ccKlWUUd.o: In function `boost::dll::detail::report_error(boost::system::error_code const&, char const*)':
program.cpp:(.text._ZN5boost3dll6detail12report_errorERKNS_6system10error_codeEPKc[_ZN5boost3dll6detail12report_errorERKNS_6system10error_codeEPKc]+0x2a): undefined reference to `dlerror'
/tmp/ccKlWUUd.o: In function `boost::filesystem::read_symlink(boost::filesystem::path const&, boost::system::error_code&)':
program.cpp:(.text._ZN5boost10filesystem12read_symlinkERKNS0_4pathERNS_6system10error_codeE[_ZN5boost10filesystem12read_symlinkERKNS0_4pathERNS_6system10error_codeE]+0x36): undefined reference to `boost::filesystem::detail::read_symlink(boost::filesystem::path const&, boost::system::error_code*)'
collect2: error: ld returned 1 exit status

The system here is:

gcc: 7.0.3
boost: 1.65.1
libc6: 2.73

Answer 1

Your build command is in the wrong order, and order matters.

GCC reads left-to-right, taking symbols from libraries when it already knows it needs them. As you put program.cpp last, you don't make that known until all listed libraries have already been identified and discarded.

Put program.cpp first, then the libraries it needs.

g++ -o program program.cpp -lboost_filesystem -ldl -lboost_system

Yes, it's kind of weird. (Even weirder that it worked on Debian! Though apparently only some "recent" Linuxy distributions default --as-needed on, which is what causes the behaviour you see, showing that the behaviour isn't necessarily guaranteed. Perhaps Debian 9 just outright does not do that.)

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More info:

• Why does the order in which libraries are linked sometimes cause errors in GCC?