Difference between the linker flags

Question

I'm adding c++ runtime and exception support to the Linux kernel. For that, I need to provide my own lib/gcc and lib/libstdc++instead of the standard libraries provided by the compiler.

So, I am confused with the flags that are to be passed to the linker. In a normal kernel's top-level Makefile, LD = $(CROSS_COMPILE)ld which enables the kernel to use the default standard libraries and startup files. For my kernel I'm using LD = $(CROSS_COMPILE)ld -nostdlib -nodefaultlibs -nostartfiles as said in a documentation. What I understood from gcc documentation is that passing -nostdlib to the linker is that of passing both -nodefaultlibs -nostartfiles. What's actually the difference between these flags?

Answer 1

These flags are defined in GCC’s spec files, so the best way to determine the differences between them is to look there:

gcc -dumpspecs

The relevant part is the link_command definition. This shows that -nostdlib, -nodefaultlibs and -nostartfiles have the following impact:

• %{!nostdlib:%{!nodefaultlibs:%:pass-through-libs(%(link_gcc_c_sequence))}} — this adds libgcc, libpthread, libc, libieee as necessary, using a macro and the lib and libgcc spec strings;
• %{!nostdlib:%{!nostartfiles:%S}} — this adds the startfile spec string, , which specifies the object files to add to handle startup (crti.o etc.)
• %{!nostdlib:%{fvtable-verify=std: -lvtv -u_vtable_map_vars_start -u_vtable_map_vars_end} %{fvtable-verify=preinit: -lvtv -u_vtable_map_vars_start -u_vtable_map_vars_end}} — this adds virtual table verification using libvtv
• %{!nostdlib:%{!nodefaultlibs:%{mmpx:%{fcheck-pointer-bounds: %{static:--whole-archive -lmpx --no-whole-archive %:include(libmpx.spec)%(link_libmpx)} %{!static:%{static-libmpx:-Bstatic --whole-archive} %{!static-libmpx:--push-state --no-as-needed} -lmpx %{!static-libmpx:--pop-state} %{static-libmpx:--no-whole-archive -Bdynamic %:include(libmpx.spec)%(link_libmpx)}}}}%{mmpx:%{fcheck-pointer-bounds:%{!fno-chkp-use-wrappers: %{static:-lmpxwrappers} %{!static:%{static-libmpxwrappers:-Bstatic} -lmpxwrappers %{static-libmpxwrappers: -Bdynamic}}}}}}} — this handles libmpx
• %{!nostdlib:%{!nodefaultlibs:%{%:sanitize(address): %{static-libasan:%:include(libsanitizer.spec)%(link_libasan)} %{static:%ecannot specify -static with -fsanitize=address}} %{%:sanitize(thread): %{static-libtsan:%:include(libsanitizer.spec)%(link_libtsan)} %{static:%ecannot specify -static with -fsanitize=thread}} %{%:sanitize(undefined):%{static-libubsan:-Bstatic} -lubsan %{static-libubsan:-Bdynamic} %{static-libubsan:%:include(libsanitizer.spec)%(link_libubsan)}} %{%:sanitize(leak): %{static-liblsan:%:include(libsanitizer.spec)%(link_liblsan)}}}} — this handles the various sanitisation options
• %{!nostdlib:%{!nodefaultlibs:%(link_ssp) %(link_gcc_c_sequence)}} — this adds the stack protection options and repeats the C link sequence (whose libraries were already specified at the start)
• %{!nostdlib:%{!nostartfiles:%E}} — this adds the endfile spec string, which specifies the object files to add to handle left-overs (crtfastmath.o, crtend.o etc.)

As you understood from the documentation, -nostdlib is a superset of -nodefaultlibs and -nostartfiles. It also disables virtual table verification.

So -nostdlib is sufficient to disable all the related features; -nodefaultlibs and -nostartfiles don’t add anything to it. (But it doesn’t hurt to mention them too.)