On Fri, 2010-03-26 at 06:38 +0100, Lubomir Rintel wrote: > Hi, > > I'm wondering if anyone could enlighten me about why does --as-needed > make a difference here? (let alone the order in which -lGL appears). Because order matters. Linker arguments are positional. Object files are walked left to right along the command line, and are added to the link in order. Archive members are added to the link output if and only if a symbol in that archive member satisfies a reference to a symbol mentioned earlier in the link. --as-needed extends this behaviour to DSOs, such that libraries will only be added to the link if they satisfy a symbol reference from an earlier object. So in your first link line: > [lkundrak@localhost VirtualBox-3.1.6_OSE]$ g++ -Wl,--as-needed \ > > -o out/linux.x86/release/obj/VBoxTestOGL/VBoxTestOGL \ > > out/linux.x86/release/obj/VBoxTestOGL/generic/OpenGLTestApp.o \ > > -L/usr/X11R6/lib -L/usr/lib \ > > -lGL \ > > obj/lib/VBoxOGLhostspuload.a obj/bin/VBoxOGLhostcrutil.so obj/lib/VBoxOGL2D.a \ > > obj/bin/VBoxRT.so obj/bin/VBoxRT.so obj/lib/VBoxREM.so obj/bin/VBoxVMM.so \ > > -lXcursor -lXext -lX11 \ > > obj/lib/VBoxCOM.a obj/bin/VBoxXPCOM.so \ > > /usr/lib/libQtCore.so /usr/lib/libQtGui.so /usr/lib/libQtOpenGL.so OpenGLTestApp.o is added. libGL is looked at, but not added, since the first .o (presumably) doesn't mention any symbol defined in libGL. Later you add the VBoxOGL2D.a archive; VBoxGLSupportInfo.o provides some symbol that one of the earlier objects needs, but adds references to symbols that happen to be defined in libGL. At no point later do you mention libGL again. [1] Therefore: > /usr/bin/ld: obj/lib/VBoxOGL2D.a(VBoxGLSupportInfo.o): undefined reference to symbol 'glGetString' Whereas: > [lkundrak@localhost VirtualBox-3.1.6_OSE]$ g++ -Wl,--as-needed \ > > -o out/linux.x86/release/obj/VBoxTestOGL/VBoxTestOGL \ > > out/linux.x86/release/obj/VBoxTestOGL/generic/OpenGLTestApp.o \ > > -L/usr/X11R6/lib -L/usr/lib \ > > obj/lib/VBoxOGLhostspuload.a obj/bin/VBoxOGLhostcrutil.so obj/lib/VBoxOGL2D.a \ > > -lGL \ > > obj/bin/VBoxRT.so obj/bin/VBoxRT.so obj/lib/VBoxREM.so obj/bin/VBoxVMM.so \ > > -lXcursor -lXext -lX11 \ > > obj/lib/VBoxCOM.a obj/bin/VBoxXPCOM.so \ > > /usr/lib/libQtCore.so /usr/lib/libQtGui.so /usr/lib/libQtOpenGL.so Here you link libGL _after_ an archive that requires it, so it's included. And: > [lkundrak@localhost VirtualBox-3.1.6_OSE]$ g++ \ > > -o out/linux.x86/release/obj/VBoxTestOGL/VBoxTestOGL \ > > out/linux.x86/release/obj/VBoxTestOGL/generic/OpenGLTestApp.o \ > > -L/usr/X11R6/lib -L/usr/lib \ > > -lGL \ > > obj/lib/VBoxOGLhostspuload.a obj/bin/VBoxOGLhostcrutil.so obj/lib/VBoxOGL2D.a \ > > obj/bin/VBoxRT.so obj/bin/VBoxRT.so obj/lib/VBoxREM.so obj/bin/VBoxVMM.so \ > > -lXcursor -lXext -lX11 \ > > obj/lib/VBoxCOM.a obj/bin/VBoxXPCOM.so \ > > /usr/lib/libQtCore.so /usr/lib/libQtGui.so /usr/lib/libQtOpenGL.so Here you link without --as-needed, so a DT_NEEDED note for libGL is emitted unconditionally; therefore, by the time you get to VBoxOGL2D.a, libGL is included in the link already. [1] - The new linker behaviour in F13 does change things. Recall that the default used to be "--add-needed", which has since been renamed --copy-dt-needed-entries. What this means is, when producing a dynamic object as output, any DT_NEEDED entries in dynamic libraries mentioned on the link line will be copied into the output. The last library in that link line, libQtOpenGL.so, almost certainly has a DT_NEEDED entry for libGL.so already. So, when doing --copy-dt-needed-entries, that link line would have worked: the first mention of -lGL would have been skipped over, but then it would have been included since libQtOpenGL had a DT_NEEDED for it. Now, in F13, since --no-copy-dt-needed-entries is the default, this doesn't happen; DT_NEEDED entries are only emitted for things you really declare that you need. This is desirable; it allows libraries to change the libraries they themselves depend on, without necessarily requiring applications to also be recompiled. - ajax
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