Why doesn't my program bind when changing the order of g ++ arguments?

Question

Possible duplicate:
Why does the -l option in gcc matter?

I'm starting to learn the scope of the Boost Unit Test. I have a minimal test suite:

#define BOOST_TEST_MAIN #define BOOST_TEST_DYN_LINK #include <boost/test/unit_test.hpp> BOOST_AUTO_TEST_CASE( test1 ) { BOOST_CHECK( 2 == 1 ); } 

First I will compile the source:

 g++ -c src/tests.cc -o src/tests.o 

It ends without errors. Then I can link the following:

 g++ -o tests src/tests.o -lboost_unit_test_framework 

It also completes without error. The resulting binary is executed with the expected results. However, if I change the order of src/tests.o and -lboost_unit_test_framework , I get linker errors:

 g++ -o tests -lboost_unit_test_framework src/tests.o 

  src / tests.o: In function `main ':
 tests.cc:(.text+0x29): undefined reference to `boost :: unit_test :: unit_test_main (bool (*) (), int, char **) '
 src / tests.o: In function `test1 :: test_method () ':
 tests.cc:(.text+0x9d): undefined reference to `boost :: unit_test :: unit_test_log_t :: set_checkpoint (boost :: unit_test :: basic_cstring, unsigned int, boost :: unit_test :: basic_cstring) '
 tests.cc:(.text+0x146): undefined reference to `boost :: test_tools :: tt_detail :: check_impl (boost :: test_tools :: predicate_result const &, boost :: unit_test :: lazy_ostream const &, boost :: unit_test :: basic_cstring, unsigned int, boost :: test_tools :: tt_detail :: tool_level, boost :: test_tools :: tt_detail :: check_type, unsigned int, ...) '
 src / tests.o: In function `__static_initialization_and_destruction_0 (int, int) ':
 tests.cc:(.text+0x24d): undefined reference to `boost :: unit_test :: ut_detail :: auto_test_unit_registrar :: auto_test_unit_registrar (boost :: unit_test :: test_case *, unsigned long) '
 src / tests.o: In function `boost :: unit_test :: unit_test_log_t :: unit_test_log_t () ':
 tests.cc:(.text._ZN5boost9unit_test15unit_test_log_tC2Ev[_ZN5boost9unit_test15unit_test_log_tC5Ev†+0x21): undefined reference to `vtable for boost :: unit_test :: unit_test_log_t '
 src / tests.o: In function `boost :: unit_test :: make_test_case (boost :: unit_test :: callback0 const &, boost :: unit_test :: basic_cstring) ':
 tests.cc:(.text._ZN5boost9unit_test14make_test_caseERKNS0_9callback0INS0_9ut_detail6unusedEEENS0_13basic_cstringIKcEE[boost::unit_test::make_test_case_best :: unit_test_all_stest :: basic_best: unit_test: unit_test: unit_test: unit_test: unit_test: unit_test: unit_test: unit_test: Unit_test: unit_test: Unit_test: Unit_test: Unit_test :: basic ut_detail :: normalize_test_case_name (boost :: unit_test :: basic_cstring) '
 tests.cc:(.text._ZN5boost9unit_test14make_test_caseERKNS0_9callback0INS0_9ut_detail6unusedEEENS0_13basic_cstringIKcEE[boost::unit_test::make_test_stest :: unit_test_all_stest :: basic_ Unit_test: unit_test: unit_test: unit_test: unit_test: unit_test: unit_test: unit_test: unit_test: unit_test: unit_test: unit_test: unit_test: unit_test: unit_test: unit_test: unit_test: unit_test: Unit_test: test_case :: test_case (boost :: unit_test :: basic_cstring, boost :: unit_test :: callback0 const &) '
 src / tests.o: In function `boost :: unit_test :: unit_test_log_t :: ~ unit_test_log_t () ':
 tests.cc:(.text._ZN5boost9unit_test15unit_test_log_tD2Ev[_ZN5boost9unit_test15unit_test_log_tD5Ev†+0xb): undefined reference to `vtable for boost :: unit_test :: unit_test_log_t '
 collect2: ld returned 1 exit status

Why does the order of my arguments cause linker errors?

Answer 1

When GCC performs the binding, the libraries are processed specially: from the library, only those characters are missing that are not in the object files that were before the library in the command line list. If you have additional object files after the library, the missing characters from this object are not displayed in the library.

In short, first list your object files and libraries at the end.

Answer 2

The traditional behavior of linkers is to look for external functions from left to right in the libraries specified on the command line. This means that the library containing the function definition must appear after any source files or object files that use it. This includes the libraries specified with the short-cut -l option, as shown in the following command:

http://www.network-theory.co.uk/docs/gccintro/gccintro_18.html