What does exactly the warning mean about hidden symbol being referenced by DSO?

Question

I have a problem linking some shared library with g++. It gives me a warning like:

hidden symbol XXX in YYY is referenced by DSO /usr/lib/... 

I've read some related questions about particular problems, but I want to understand it in a whole - what does this warning mean and what is a cause:

1. What is DSO?
2. What is a hidden symbol?
3. How can it be referenced, if it's hidden?

Answer 1

What is a DSO?

A DSO is a Dynamic Shared Object, or less formally a shared library.

What is a hidden symbol?

A hidden symbol is a symbol (i.e. name of function or data object) that has been compiled with hidden linkage, e.g. as per the (GCC specific) declaration:

int x __attribute__ ((visibility ("hidden"))); 

If x is defined in one DSO then dynamic linkage cannot reference it from a different DSO. The linker can see x (it is not static), but it is not available for dynamic linkage. Documentation here

How can it be referenced, if it's hidden?

It can't be, which is what you are being warned about. E.g. the linktime warning:

hidden symbol `stat' in /usr/lib/libc_nonshared.a(stat.oS) is referenced by DSO

is telling you that a DSO in the linkage references the symbol stat, and the linker can locate a definition of stat in /usr/lib/libc_nonshared.a, but (obviously) that definition is not in the DSO that references it and cannot be referenced from that DSO, because it is hidden.

You get this problem if the problem DSO has not been built correctly for use as a DSO. See this example and follow the follow-ups for the solution.

Continued for OP's followups

If hidden symbol is already referenced by some DSO, then why the problem is with DSO?

The linker is saying:

The DSO X contains a reference to symbol S. I can find a definition of symbol S is another linked module Y, but that definition will not be available to satisfy the reference in X dynamically (i.e. at runtime) because S has hidden linkage in Y.

I can confirm that the problem comes with a non-shared object [...][but] I don't explicitly hide those symbols in my non-shared object.

You may not have explicitly marked any symbols hidden in the non-shared object. Depending on how it was built, symbols may be hidden by default unless explicitly marked otherwise.

Say the non-shared object is libnonshared.a and the allegedly hidden symbol is foo. Run:

objdump -t libnonshared.a 

to get info about the symbols in libnonshared.a. In the output, look for the entry for foo. Does it contain the tag .hidden? - e.g.

0000000000000000 g     F .text  000000000000000b .hidden foo 

This entry says that foo is a global symbol (marked g - that's why the linker is able to see it) but it's hidden for dynamic linkage.

If this turns out to be the case then you need to go and fix the build of libnonshared.a so that it doesn't hide foo. If not, then I'm stumped.