crt*.o stuff

master
PoroCYon 3 years ago
parent deb23819c8
commit 7d2e9cda74
  1. 4
      explain-dot-md.md
  2. 81
      explain/crt.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)

@ -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

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