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diff --git a/docs/crt.txt b/docs/crt.txt new file mode 100644 index 000000000..41a44dcb4 --- /dev/null +++ b/docs/crt.txt @@ -0,0 +1,80 @@ +Mini FAQ about the misc libc/gcc crt files. + + +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 |