How to use alternate glibc with existing libstdc++?

Question

I need to use a self-compiled version of glibc (2.18), newer than the default one on the system (2.15). I can compile&link a C++ program, but when I try to run it, I get errors about libstdc++.so.6. (C programs seems to work just fine.) Do I need to recompile gcc against the newer glibc for this to work? Why? (Update: I figured this part out, but I have a few other questions at the bottom.)

Here is a sample C++ program:

#include <iostream>
int main()
{
    std::cout << "ok\n";
    return 0;
}

Following this answer, I compiled it with:

g++ -Wl,--rpath=/path/to/glibc-2.18/lib -Wl,--dynamic-linker=/path/to/glibc-2.18/lib/ld-2.18.so a.cpp

It compiles with no errors, then ldd says:

$ ldd a.out 
    linux-vdso.so.1 =>  (0x00007fff421fe000)
    libstdc++.so.6 => /usr/lib/x86_64-linux-gnu/libstdc++.so.6 (0x00007f3b96e7f000)
    libc.so.6 => /path/to/glibc-2.18/lib/libc.so.6 (0x00007f3b96ad1000)
    libm.so.6 => /path/to/glibc-2.18/lib/libm.so.6 (0x00007f3b967cf000)
    /path/to/glibc-2.18/lib/ld-2.18.so => /lib64/ld-linux-x86-64.so.2 (0x00007f3b9719d000)
    libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007f3b965b9000)

But when I try to run it:

$ ./a.out 
./a.out: error while loading shared libraries: libstdc++.so.6: cannot open shared object file: No such file or directory

This is confusing because it looks like ldd finds libstdc++.so.6 just fine (the specific version is libstdc++.so.6.0.16).

Update: The problem seems to have been (not sure) that the new 2.18 dynamic linker is using its own library path which includes only subfolders of /path/to/glibc-2.18/lib. I got the program to run by adding this new path, followed by the standard paths (/lib', '/usr/lib, etc) to /path/to/glibc-2.18/etc/ld.so.conf and running /path/to/glibc-2.18/sbin/ldconfig. More questions:

1. Do I absolutely need the new 2.18 dynamic linker to run a program with glibc-2.18? Can't the standard linker do it? (This would avoid me having to set up and continuously update the paths of the 2.18 dynamic linker.)

2. If I compile with the 2.18 dynamic linker but without --rpath, the program doesn't work. Why?

3. Should I be using -L/path/to/glibc-2.18/lib in the compilation command (in addition to --rpath and --dynamic-linker)?

Answer 1

Do I absolutely need the new 2.18 dynamic linker to run a program with glibc-2.18?

Yes (well, almost. See footnote).

This would avoid me having to set up and continuously update the paths of the 2.18 dynamic linker.

A common technique is to create a g++ shell wrapper, e.g. g++glibc2.18, and encapsulate adding the necessary link arguments there. Then a simple make CXX=g++glibc2.18 would do the right thing.

Can't the standard linker do it?

No. See this answer for explanation.

If I compile with the 2.18 dynamic linker but without --rpath, the program doesn't work. Why?

See the same answer.

Should I be using -L/path/to/glibc-2.18/lib in the compilation command (in addition to --rpath and --dynamic-linker)?

Yes, if you want to use symbols that are present in glibc-2.18 but not present in your system library. Otherwise, no.

Footnote:

As an alternative, you could build your program without the special flags, then use "explicit loader invocation" to actually run it: /path/to/glibc-2.18/lib/ld-2.18.so /path/to/a.out.

Beware: this doesn't always work: if the program likes to re-exec itself (and under other rare conditions). You may also have trouble debugging it when it is invoked that way.