--warn-backrefs gives a warning when an undefined symbol reference is
resolved by a definition in an archive to the left of it on the command line.
A linker such as GNU ld makes a single pass over the input files from left to
right maintaining the set of undefined symbol references from the files loaded
so far. When encountering an archive or an object file surrounded by
--end-lib that archive will be searched for resolving
symbol definitions; this may result in input files being loaded, updating the
set of undefined symbol references. When all resolving definitions have been
loaded from the archive, the linker moves on the next file and will not return
to it. This means that if an input file to the right of a archive cannot have
an undefined symbol resolved by a archive to the left of it. For example:
ld def.a ref.o
will result in an
undefined reference error. If there are no cyclic
references, the archives can be ordered in such a way that there are no
backward references. If there are cyclic references then the
--end-group options can be used, or the same archive can be placed on
the command line twice.
LLD remembers the symbol table of archives that it has previously seen, so if
there is a reference from an input file to the right of an archive, LLD will
still search that archive for resolving any undefined references. This means
that an archive only needs to be included once on the command line and the
--end-group options are redundant.
A consequence of the differing archive searching semantics is that the same linker command line can result in different outcomes. A link may succeed with LLD that will fail with GNU ld, or even worse both links succeed but they have selected different objects from different archives that both define the same symbols.
warn-backrefs option provides information that helps identify cases
where LLD and GNU ld archive selection may differ.
% ld.lld –warn-backrefs … -lB -lAld.lld: warning: backward reference detected: system in A.a(a.o) refers to B.a(b.o)% ld.lld –warn-backrefs … –start-lib B/b.o –end-lib –start-lib A/a.o –end-libld.lld: warning: backward reference detected: system in A/a.o refers to B/b.o
# To suppress the warning, you can specify –warn-backrefs-exclude=<glob> to match B/b.o or B.a(b.o)
--warn-backrefs option can also provide a check to enforce a
topological order of archives, which can be useful to detect layering
violations (albeit unable to catch all cases). There are two cases where GNU ld
will result in an
undefined reference error:
If adding the dependency does not form a cycle: conceptually
Ais higher level library while
Bis at a lower level. When you are developing an application
Pwhich depends on
A, but does not directly depend on
B, your link may fail surprisingly with
undefined symbol: symbol_defined_in_Bif the used/linked part of
Ahappens to need some components of
B. It is inappropriate for
Pto add a dependency on
Pdoes not use
If adding the dependency forms a cycle, e.g.
B->C->A ~> B.
Ais supposed to be at the lowest level while
Bis supposed to be at the highest level. When you are developing
C, your link may fail surprisingly with
undefined symbolif there is somehow a dependency on some components of
B. You could fix the issue by adding the missing dependency (
B), however, then every test (
C_test) will link against every library. This breaks the motivation of splitting
Ainto separate libraries and makes binaries unnecessarily large. Moreover, the layering violation makes lower-level libraries (e.g.
A) vulnerable to changes to higher-level libraries (e.g.
Add a dependency from
The reference may be unintended and can be removed.
The dependency may be intentionally omitted because there are multiple libraries like
B. Consider linking
Bwith object semantics by surrounding it with
In the case of circular dependency, sometimes merging the libraries are the best.
There are two cases like a library sandwich where GNU ld will select a different object.
A.a B A2.so:
A.amay be used as an interceptor (e.g. it provides some optimized libc functions and
Bdoes not need to know about
A.amay be pulled into the link by other part of the program. For linker portability, consider
A.a B A2.a: similar to the above case but
--warn-backrefsdoes not flag the problem, because
A2.amay be a replicate of
A.a, which is redundant but benign. In some cases
Bshould be surrounded by a pair of
--end-group. This is especially common among system libraries (e.g.
-lc __isnanl references -lm,
-lc _IO_funlockfile references -lpthread,
-lc __gcc_personality_v0 references -lgcc_eh, and
-lpthread _Unwind_GetCFA references -lunwind).
In C++, this is likely an ODR violation. We probably need a dedicated option for ODR detection.