On Jul 17, 2013, at 10:48 AM, Nathan Hjelm <[email protected]> wrote:
> I must be missing something here. type_size.c contains MPI_Type_size and
> MPI_Type_size_x and I see all the MPI and PMPI variants in the resulting .so,
> .dylib, and .a.
If you have a nathan.c file with:
-----
void MPI_foo() { ... }
void MPI_bar() { ... }
-----
This will result in defining both symbols in that nathan.o file, which ends up
in libmpi.so.
Then if someone writes a code like this:
-----
int main() {
MPI_Init();
MPI_Foo();
MPI_Bar();
MPI_Finalize();
return 0;
}
-----
And then they interpose their own version of MPI_Bar() with their
libinterposition.so, *it won't work* (meaning their version of MPI_Bar() won't
be called).
This happens because the linker will first see MPI_Foo() in main and resolves
it. When it resolves the MPI_Foo symbol, it pulls *all* symbols out of the .o
from where MPI_Foo came (i.e., nathan.o in libmpi.so) -- i.e., including
MPI_Bar.
So when MPI_Bar goes to get executed, it's *already been resolved* to the one
in nathan.o/libmpi.so, not the one from libinterposition.so.
Even worse, if they reversed the order of foo/bar in main, then the linker
would likely give you a duplicate symbol error because it will first resolve
MPI_Bar from libinterposition.so, and then later resolve MPI_Foo from
libmpi.so, but it will also pull MPI_Bar from libmpi.so -- kaboom.
Linkers are insanely complicated.
--
Jeff Squyres
[email protected]
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