The C runtime

Even though C is a relatively low-level language, some assembly code is needed to get the process in an acceptable state to the language standard.

First of all, main isn’t the standard entry point, _start is. This function is provided by your compiler, typically from a file called crt*.o. This function:

  1. aligns the stack
  2. reads argc, argv, environ and the auxiliary vector from the stack (see the links to LWN in Process creation for stack content details)
  3. sets up TLS, locales, and other crap
  4. calls __libc_start_user, which first calls the constructors of all global (C++) variables, then calls main, and exit automatically as well.

Of course, it’s huge, but we can provide our own: step 3 can be skipped, 1 and 2 are really small to implement in assembly, and 4 is replaced by calling main. Of course, the second step can be skipped as well when you don’t need args/environ.

However, when external libs need to read from the environment (eg. X11 needs $DISPLAY), you still have to call __libc_start_user; it’s a portable interface that asks for argc, argv, envp and main. With smol, this 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 -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: