Are standard library required to be standard conformant?

Question

Are standard library required to be standard conformant? I've this feeling that standard library aren't standard conformant. The basis of this feeling is the error messages generated by the compiler(s). For example, sometime GCC gives error messages which starts with prefix __gxx and many others which I don't remember as of now. But seeing them gives me feeling that these are very compiler specific messages, and different compilers wouldn't be able to compile standard library provided by GCC, and vice-versa. Is it true?

The question can be asked in other words as:

• Can standard library provided by one compiler be compiled with other compilers?
• When we say a particular compiler is standard conformant, does it automatically mean that the stdlib which comes with it is also standard-conformant? Or it simply means that this compiler can compile standard-conformant code written by us, programmers?
• Can I use standard library provided by one compiler, in my project which uses a different compiler to compile the project? Is portability same as standard-conformance?

These questions are different angles to look at the same big question. So, please help me understanding what does it exactly mean when we say compiler X is standard-conformant.

Answer 1

The standard library is a detail of implementation. It may not even be 'compiled' in the sense that the standard doesn't require it to consist of 'files' [headers]:

174) A header is not necessarily a source file, nor are the sequences delimited by < and > in header names necessarily valid source file names (16.2).

The standard carefully eases the requirements on the implementation, so that the library may be 'built-in' to the compiler (a.k.a intrinsics). For example, extending the std namespace, or #defining a name used in the standard library gives you undefined behavior.