Why do I see symbol linking error messages when the symbols are present? - dll

Question

I have two object files, one with a couple of symbols defined and one in which those symbols are undefined:
nm ./src/freertos/freertos.o |grep pvPortMalloc
000000000000d045 T pvPortMalloc
...and...
nm ./src/clib/new_delete.o
U _GLOBAL_OFFSET_TABLE_
U _Z12pvPortMalloci
U _Z9vPortFreePv
0000000000000019 T _ZdlPv
0000000000000000 T _Znwm
When I attempt to link them together, I see this:
ld ./src/clib/new_delete.o ./src/freertos/freertos.o
ld: warning: cannot find entry symbol _start; defaulting to 0000000000401000
ld: ./src/clib/new_delete.o: in function `operator new(unsigned long)':
new_delete.cc:(.text+0x13): undefined reference to `pvPortMalloc(int)'
ld: ./src/clib/new_delete.o: in function `operator delete(void*)':
new_delete.cc:(.text+0x2d): undefined reference to `vPortFree(void*)'
Anyone have any ideas?

Answer 1

Your nm listings do not show there is any undefined symbol reported in ./src/clib/new_delete.o
that is defined in ./src/freertos/freertos.o.
There is a similarity between the
symbol pvPortMalloc defined in ./src/freertos/freertos.o and the undefined
symbol _Z12pvPortMalloci in ./src/clib/new_delete.o which I guess leads you to suppose that they are
the same, but the linker does not equate symbols on the basis of any similarity short of identity.
The symbol _Z12pvPortMalloci is the C++ mangled name of:
$ c++filt _Z12pvPortMalloci
pvPortMalloc(int)
showing that ./src/clib/new_delete.o was compiled with a C++ compiler. ./src/freertos/freertos.o,
on the other hand, was compiled with a C compiler, which does not do name-mangling.
If you want to tell your C++ compiler that the external name pvPortMalloc in
your source code is to have C linkage language, and therefore not to mangle it,
you must declare it extern "C".
Wrong
$ cat wrong.cpp
extern void * pvPortMalloc(int);
int main()
{
return pvPortMalloc(42) != nullptr;
}
$ g++ -Wall -c wrong.cpp
$ nm wrong.o
U _GLOBAL_OFFSET_TABLE_
0000000000000000 T main
U _Z12pvPortMalloci
Right
$ cat right.cpp
extern "C" {
void * pvPortMalloc(int);
// And any more...
}
int main()
{
return pvPortMalloc(42) != nullptr;
}
$ g++ -Wall -c right.cpp
$ nm right.o
U _GLOBAL_OFFSET_TABLE_
0000000000000000 T main
U pvPortMalloc