mjhjmtu

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?

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.)