| lh | 9ed821d | 2023-04-07 01:36:19 -0700 | [diff] [blame] | 1 | Mini FAQ about the misc libc/gcc crt files. |
| 2 | |
| 3 | |
| 4 | Some definitions: |
| 5 | PIC - position independent code (-fPIC) |
| 6 | PIE - position independent executable (-fPIE -pie) |
| 7 | crt - C runtime |
| 8 | |
| 9 | |
| 10 | |
| 11 | crt0.o crt1.o etc... |
| 12 | Some systems use crt0.o, while some use crt1.o (and a few even use crt2.o |
| 13 | or higher). Most likely due to a transitionary phase that some targets |
| 14 | went through. The specific number is otherwise entirely arbitrary -- look |
| 15 | at the internal gcc port code to figure out what your target expects. All |
| 16 | that matters is that whatever gcc has encoded, your C library better use |
| 17 | the same name. |
| 18 | |
| 19 | This object is expected to contain the _start symbol which takes care of |
| 20 | bootstrapping the initial execution of the program. What exactly that |
| 21 | entails is highly libc dependent and as such, the object is provided by |
| 22 | the C library and cannot be mixed with other ones. |
| 23 | |
| 24 | On uClibc/glibc systems, this object initializes very early ABI requirements |
| 25 | (like the stack or frame pointer), setting up the argc/argv/env values, and |
| 26 | then passing pointers to the init/fini/main funcs to the internal libc main |
| 27 | which in turn does more general bootstrapping before finally calling the real |
| 28 | main function. |
| 29 | |
| 30 | glibc ports call this file 'start.S' while uClibc ports call this crt0.S or |
| 31 | crt1.S (depending on what their gcc expects). |
| 32 | |
| 33 | crti.o |
| 34 | Defines the function prologs for the .init and .fini sections (with the _init |
| 35 | and _fini symbols respectively). This way they can be called directly. These |
| 36 | symbols also trigger the linker to generate DT_INIT/DT_FINI dynamic ELF tags. |
| 37 | |
| 38 | These are to support the old style constructor/destructor system where all |
| 39 | .init/.fini sections get concatenated at link time. Not to be confused with |
| 40 | newer prioritized constructor/destructor .init_array/.fini_array sections and |
| 41 | DT_INIT_ARRAY/DT_FINI_ARRAY ELF tags. |
| 42 | |
| 43 | glibc ports used to call this 'initfini.c', but now use 'crti.S'. uClibc |
| 44 | also uses 'crti.S'. |
| 45 | |
| 46 | crtn.o |
| 47 | Defines the function epilogs for the .init/.fini sections. See crti.o. |
| 48 | |
| 49 | glibc ports used to call this 'initfini.c', but now use 'crtn.S'. uClibc |
| 50 | also uses 'crtn.S'. |
| 51 | |
| 52 | Scrt1.o |
| 53 | Used in place of crt1.o when generating PIEs. |
| 54 | gcrt1.o |
| 55 | Used in place of crt1.o when generating code with profiling information. |
| 56 | Compile with -pg. Produces output suitable for the gprof util. |
| 57 | Mcrt1.o |
| 58 | Like gcrt1.o, but is used with the prof utility. glibc installs this as |
| 59 | a dummy file as it's useless on linux systems. |
| 60 | |
| 61 | crtbegin.o |
| 62 | GCC uses this to find the start of the constructors. |
| 63 | crtbeginS.o |
| 64 | Used in place of crtbegin.o when generating shared objects/PIEs. |
| 65 | crtbeginT.o |
| 66 | Used in place of crtbegin.o when generating static executables. |
| 67 | crtend.o |
| 68 | GCC uses this to find the start of the destructors. |
| 69 | crtendS.o |
| 70 | Used in place of crtend.o when generating shared objects/PIEs. |
| 71 | |
| 72 | |
| 73 | |
| 74 | General linking order: |
| 75 | crt1.o crti.o crtbegin.o [-L paths] [user objects] [gcc libs] [C libs] [gcc libs] crtend.o crtn.o |
| 76 | |
| 77 | |
| 78 | |
| 79 | More references: |
| 80 | http://gcc.gnu.org/onlinedocs/gccint/Initialization.html |