1--warn-backrefs 2=============== 3 4``--warn-backrefs`` gives a warning when an undefined symbol reference is 5resolved by a definition in an archive to the left of it on the command line. 6 7A linker such as GNU ld makes a single pass over the input files from left to 8right maintaining the set of undefined symbol references from the files loaded 9so far. When encountering an archive or an object file surrounded by 10``--start-lib`` and ``--end-lib`` that archive will be searched for resolving 11symbol definitions; this may result in input files being loaded, updating the 12set of undefined symbol references. When all resolving definitions have been 13loaded from the archive, the linker moves on the next file and will not return 14to it. This means that if an input file to the right of a archive cannot have 15an undefined symbol resolved by a archive to the left of it. For example: 16 17 ld def.a ref.o 18 19will result in an ``undefined reference`` error. If there are no cyclic 20references, the archives can be ordered in such a way that there are no 21backward references. If there are cyclic references then the ``--start-group`` 22and ``--end-group`` options can be used, or the same archive can be placed on 23the command line twice. 24 25LLD remembers the symbol table of archives that it has previously seen, so if 26there is a reference from an input file to the right of an archive, LLD will 27still search that archive for resolving any undefined references. This means 28that an archive only needs to be included once on the command line and the 29``--start-group`` and ``--end-group`` options are redundant. 30 31A consequence of the differing archive searching semantics is that the same 32linker command line can result in different outcomes. A link may succeed with 33LLD that will fail with GNU ld, or even worse both links succeed but they have 34selected different objects from different archives that both define the same 35symbols. 36 37The ``warn-backrefs`` option provides information that helps identify cases 38where LLD and GNU ld archive selection may differ. 39 40 | % ld.lld --warn-backrefs ... -lB -lA 41 | ld.lld: warning: backward reference detected: system in A.a(a.o) refers to B.a(b.o) 42 43 | % ld.lld --warn-backrefs ... --start-lib B/b.o --end-lib --start-lib A/a.o --end-lib 44 | ld.lld: warning: backward reference detected: system in A/a.o refers to B/b.o 45 46 # To suppress the warning, you can specify --warn-backrefs-exclude=<glob> to match B/b.o or B.a(b.o) 47 48The ``--warn-backrefs`` option can also provide a check to enforce a 49topological order of archives, which can be useful to detect layering 50violations (albeit unable to catch all cases). There are two cases where GNU ld 51will result in an ``undefined reference`` error: 52 53* If adding the dependency does not form a cycle: conceptually ``A`` is higher 54 level library while ``B`` is at a lower level. When you are developing an 55 application ``P`` which depends on ``A``, but does not directly depend on 56 ``B``, your link may fail surprisingly with ``undefined symbol: 57 symbol_defined_in_B`` if the used/linked part of ``A`` happens to need some 58 components of ``B``. It is inappropriate for ``P`` to add a dependency on 59 ``B`` since ``P`` does not use ``B`` directly. 60* If adding the dependency forms a cycle, e.g. ``B->C->A ~> B``. ``A`` 61 is supposed to be at the lowest level while ``B`` is supposed to be at the 62 highest level. When you are developing ``C_test`` testing ``C``, your link may 63 fail surprisingly with ``undefined symbol`` if there is somehow a dependency on 64 some components of ``B``. You could fix the issue by adding the missing 65 dependency (``B``), however, then every test (``A_test``, ``B_test``, 66 ``C_test``) will link against every library. This breaks the motivation 67 of splitting ``B``, ``C`` and ``A`` into separate libraries and makes binaries 68 unnecessarily large. Moreover, the layering violation makes lower-level 69 libraries (e.g. ``A``) vulnerable to changes to higher-level libraries (e.g. 70 ``B``, ``C``). 71 72Resolution: 73 74* Add a dependency from ``A`` to ``B``. 75* The reference may be unintended and can be removed. 76* The dependency may be intentionally omitted because there are multiple 77 libraries like ``B``. Consider linking ``B`` with object semantics by 78 surrounding it with ``--whole-archive`` and ``--no-whole-archive``. 79* In the case of circular dependency, sometimes merging the libraries are the best. 80 81There are two cases like a library sandwich where GNU ld will select a 82different object. 83 84* ``A.a B A2.so``: ``A.a`` may be used as an interceptor (e.g. it provides some 85 optimized libc functions and ``A2`` is libc). ``B`` does not need to know 86 about ``A.a``, and ``A.a`` may be pulled into the link by other part of the 87 program. For linker portability, consider ``--whole-archive`` and 88 ``--no-whole-archive``. 89 90* ``A.a B A2.a``: similar to the above case but ``--warn-backrefs`` does not 91 flag the problem, because ``A2.a`` may be a replicate of ``A.a``, which is 92 redundant but benign. In some cases ``A.a`` and ``B`` should be surrounded by 93 a pair of ``--start-group`` and ``--end-group``. This is especially common 94 among system libraries (e.g. ``-lc __isnanl references -lm``, ``-lc 95 _IO_funlockfile references -lpthread``, ``-lc __gcc_personality_v0 references 96 -lgcc_eh``, and ``-lpthread _Unwind_GetCFA references -lunwind``). 97 98 In C++, this is likely an ODR violation. We probably need a dedicated option 99 for ODR detection. 100