crt*.o stuff

explain-dot-md.md

@@ -25,4 +25,6 @@
 * ["How we get to `main()`" (video)](https://www.youtube.com/watch?v=dOfucXtyEsU)
 * ["Everything you always wanted to know about Hello World" (video)](https://archive.fosdem.org/2017/schedule/event/hello_world/)
 * [link-time GNU hash stuff (src)](https://sourceware.org/git/?p=binutils.git;a=blob_plain;f=bfd/elf.c)
-* [runtime GNU hash stuff (src)](https://sourceware.org/git/?p=glibc.git;a=blob_plain;f=elf/dl-lookup.c)
+* [runtime GNU hash stuff (src)](https://sourceware.org/git/?p=glibc.git;a=blob_plain;f=elf/dl-lookup.c)
+* ["Linkers and loaders" (book, pdf), very extensive](http://becbapatla.ac.in/cse/naveenv/docs/LL1.pdf)
+

explain/crt.md

@@ -28,3 +28,84 @@ isn't exactly a hassle anymore.
 For exiting the program when not using `__libc_start_main`, you can just use a
 bare syscall, or `int3`.
 
+### What are all those crtfoo.o-files for?
+
+(Pilfered from [dev.gentoo.org](https://dev.gentoo.org/~vapier/crt.txt) -pcy)
+
+> Some definitions:
+> PIC - position independent code (-fPIC)
+> PIE - position independent executable (-fPIE -pie)
+> crt - C runtime
+>
+>
+>
+> crt0.o crt1.o etc...
+>   Some systems use crt0.o, while some use crt1.o (and a few even use crt2.o
+>   or higher).  Most likely due to a transitionary phase that some targets
+>   went through.  The specific number is otherwise entirely arbitrary -- look
+>   at the internal gcc port code to figure out what your target expects.  All
+>   that matters is that whatever gcc has encoded, your C library better use
+>   the same name.
+>
+>   This object is expected to contain the _start symbol which takes care of
+>   bootstrapping the initial execution of the program.  What exactly that
+>   entails is highly libc dependent and as such, the object is provided by
+>   the C library and cannot be mixed with other ones.
+>
+>   On uClibc/glibc systems, this object initializes very early ABI requirements
+>   (like the stack or frame pointer), setting up the argc/argv/env values, and
+>   then passing pointers to the init/fini/main funcs to the internal libc main
+>   which in turn does more general bootstrapping before finally calling the real
+>   main function.
+>
+>   glibc ports call this file 'start.S' while uClibc ports call this crt0.S or
+>   crt1.S (depending on what their gcc expects).
+>
+> crti.o
+>   Defines the function prologs for the .init and .fini sections (with the _init
+>   and _fini symbols respectively).  This way they can be called directly.  These
+>   symbols also trigger the linker to generate DT_INIT/DT_FINI dynamic ELF tags.
+>
+>   These are to support the old style constructor/destructor system where all
+>   .init/.fini sections get concatenated at link time.  Not to be confused with
+>   newer prioritized constructor/destructor .init_array/.fini_array sections and
+>   DT_INIT_ARRAY/DT_FINI_ARRAY ELF tags.
+>
+>   glibc ports used to call this 'initfini.c', but now use 'crti.S'.  uClibc
+>   also uses 'crti.S'.
+>
+> crtn.o
+>   Defines the function epilogs for the .init/.fini sections.  See crti.o.
+>
+>   glibc ports used to call this 'initfini.c', but now use 'crtn.S'.  uClibc
+>   also uses 'crtn.S'.
+>
+> Scrt1.o
+>   Used in place of crt1.o when generating PIEs.
+> gcrt1.o
+>   Used in place of crt1.o when generating code with profiling information.
+>   Compile with -pg.  Produces output suitable for the gprof util.
+> Mcrt1.o
+>   Like gcrt1.o, but is used with the prof utility.  glibc installs this as
+>   a dummy file as it's useless on linux systems.
+>
+> crtbegin.o
+>   GCC uses this to find the start of the constructors.
+> crtbeginS.o
+>   Used in place of crtbegin.o when generating shared objects/PIEs.
+> crtbeginT.o
+>   Used in place of crtbegin.o when generating static executables.
+> crtend.o
+>   GCC uses this to find the start of the destructors.
+> crtendS.o
+>   Used in place of crtend.o when generating shared objects/PIEs.
+>
+>
+>
+> General linking order:
+> crt1.o crti.o crtbegin.o [-L paths] [user objects] [gcc libs] [C libs] [gcc libs] crtend.o crtn.o
+>
+>
+>
+> More references:
+>     http://gcc.gnu.org/onlinedocs/gccint/Initialization.html