Re: Const symbols suppressed

[Thomas Maeder]

PlayDough <pladow@gmail.com> writes:

> test.cpp:
> const char test_str[] = "abcd";
>
> And I compile with g++ (mb-g++ in my case).  So I do a "mb-g++ -c
> test.cpp".  I'd expect test_str to be visible in the output.  So, I do
> nm (mb-nm in my case), and get....nothing.

Good. The const gives test_str internal linkage. g++ has to make sure
that it can't be referenced from different translation units.


> The test_str is not exported and is not visible.  In fact, dumping
> the output (objdump -s) shows nothing.  And dumping the headers
> (objdump - h) shows zero lengths for .text, .data, and .bss.  No
> output at all.
>
> But change things up.  Change the filename to test.c and recompile
> using gcc (mb-gcc).  Now test_str is exported, and is placed in
> the .rodata section.  Exactly as expected.
>
> The only fix I have found for this is to do one of two things.
>
> test.cpp:
> extern const char test_str[];
> const char test_str[] = "abcd";

The extern declaration causes test_str to have external linkage
despite the const.


> test.cpp:
> extern const char test_str[] = "abcd";

Ditto.


> I dislike the latter case, since we only use extern with declarations,
> and leave off the extern for allocations.  That is, we'll put the
> extern statement in a header, and leave off the extern in the source
> file.
>
> So, only with the extern keyword does the symbol get exported.
>
> Is this a C++-ism, gcc (4.1.1)-ism, or a Xilinx-ism?

This behavior is specified in the ISO C++ Standard.