If you are using TCC/TinyCC and you are trying to get it to place code or data
in a specific section with __attribute__((section("..."))) and it is not
creating the section, or if you are trying to get the start/end of the section
with __start_SECTION and __stop_SECTION and you are getting linker errors
about undefined symbols, don’t worry. It’s not you. And it’s not exactly TCC,
either.
$ gcc const.c && ./a.out
f: 0x5c97a966d020, g: 0x5c97a966d030
start: 0x5c97a966d048, end 0x5c97a966d050
$ clang const.c && ./a.out
f: 0x574924409030, g: 0x574924409040
start: 0x574924409068, end 0x574924409070
$ tcc const.c
tcc: error: undefined symbol '__stop_const_heap'
tcc: error: undefined symbol '__start_const_heap'
$
The fix is to add this snippet to the top of your code (after including the glibc headers):
#ifdef __TINYC__
#undef __attribute__
#endif
and then it will magically start working.
It turns out that this is because glibc headers define __attribute__ to do
nothing if the compiler either does not pretend to be GCC with __GNUC__ or
if it is an old version of GCC. This means it gets stripped out by the
preprocessor, so the compiler never sees it. To undo that, we #undef
__attribute__, which passes through __attribute__ annotations to the
compiler.
I found this out from Allan Wind, who found it out from the tcc-devel mailing list.
I found no mention of this in my searches, so hopefully this post will help anyone who runs into this in the future. Perhaps they can avoid scrolling through TCC source code, downloading TCC, instrumenting it, running GDB on it… and of course, calling in Tom Hebb.