crt*.o stuff
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@ -26,3 +26,5 @@
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* ["Everything you always wanted to know about Hello World" (video)](https://archive.fosdem.org/2017/schedule/event/hello_world/)
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* [link-time GNU hash stuff (src)](https://sourceware.org/git/?p=binutils.git;a=blob_plain;f=bfd/elf.c)
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* [runtime GNU hash stuff (src)](https://sourceware.org/git/?p=glibc.git;a=blob_plain;f=elf/dl-lookup.c)
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* ["Linkers and loaders" (book, pdf), very extensive](http://becbapatla.ac.in/cse/naveenv/docs/LL1.pdf)
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@ -28,3 +28,84 @@ isn't exactly a hassle anymore.
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For exiting the program when not using `__libc_start_main`, you can just use a
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bare syscall, or `int3`.
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### What are all those crtfoo.o-files for?
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(Pilfered from [dev.gentoo.org](https://dev.gentoo.org/~vapier/crt.txt) -pcy)
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> Some definitions:
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> PIC - position independent code (-fPIC)
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> PIE - position independent executable (-fPIE -pie)
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> crt - C runtime
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>
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>
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>
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> crt0.o crt1.o etc...
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> Some systems use crt0.o, while some use crt1.o (and a few even use crt2.o
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> or higher). Most likely due to a transitionary phase that some targets
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> went through. The specific number is otherwise entirely arbitrary -- look
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> at the internal gcc port code to figure out what your target expects. All
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> that matters is that whatever gcc has encoded, your C library better use
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> the same name.
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>
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> This object is expected to contain the _start symbol which takes care of
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> bootstrapping the initial execution of the program. What exactly that
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> entails is highly libc dependent and as such, the object is provided by
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> the C library and cannot be mixed with other ones.
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>
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> On uClibc/glibc systems, this object initializes very early ABI requirements
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> (like the stack or frame pointer), setting up the argc/argv/env values, and
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> then passing pointers to the init/fini/main funcs to the internal libc main
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> which in turn does more general bootstrapping before finally calling the real
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> main function.
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>
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> glibc ports call this file 'start.S' while uClibc ports call this crt0.S or
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> crt1.S (depending on what their gcc expects).
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>
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> crti.o
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> Defines the function prologs for the .init and .fini sections (with the _init
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> and _fini symbols respectively). This way they can be called directly. These
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> symbols also trigger the linker to generate DT_INIT/DT_FINI dynamic ELF tags.
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>
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> These are to support the old style constructor/destructor system where all
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> .init/.fini sections get concatenated at link time. Not to be confused with
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> newer prioritized constructor/destructor .init_array/.fini_array sections and
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> DT_INIT_ARRAY/DT_FINI_ARRAY ELF tags.
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>
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> glibc ports used to call this 'initfini.c', but now use 'crti.S'. uClibc
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> also uses 'crti.S'.
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>
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> crtn.o
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> Defines the function epilogs for the .init/.fini sections. See crti.o.
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>
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> glibc ports used to call this 'initfini.c', but now use 'crtn.S'. uClibc
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> also uses 'crtn.S'.
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>
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> Scrt1.o
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> Used in place of crt1.o when generating PIEs.
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> gcrt1.o
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> Used in place of crt1.o when generating code with profiling information.
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> Compile with -pg. Produces output suitable for the gprof util.
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> Mcrt1.o
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> Like gcrt1.o, but is used with the prof utility. glibc installs this as
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> a dummy file as it's useless on linux systems.
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>
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> crtbegin.o
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> GCC uses this to find the start of the constructors.
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> crtbeginS.o
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> Used in place of crtbegin.o when generating shared objects/PIEs.
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> crtbeginT.o
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> Used in place of crtbegin.o when generating static executables.
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> crtend.o
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> GCC uses this to find the start of the destructors.
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> crtendS.o
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> Used in place of crtend.o when generating shared objects/PIEs.
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>
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>
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>
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> General linking order:
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> crt1.o crti.o crtbegin.o [-L paths] [user objects] [gcc libs] [C libs] [gcc libs] crtend.o crtn.o
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>
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>
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>
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> More references:
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> http://gcc.gnu.org/onlinedocs/gccint/Initialization.html
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