One of the reasons is that GCC can be built and used on (e.g. proprietary Unix systems like MacOSX, Solaris, HPUX or some FreeBSD) systems having their own C standard library.
Even on Linux, you can have a C standard library which is not the GNU Glibc. In particular, you can build GCC (or use it) on Linux systems with musl-libc or with Bionic (Android systems) or with dietlibc, etc. And a Linux system could have the GNU Glibc and use some other C compiler (like Clang or TinyCC).
Also, the C library heavily depends upon the Linux kernel. Some old versions of the kernel might require some particular kind (or version) of
And GCC is buildable as a cross-compiler.
And details like "how to call a
main function" also depends on the compiler, but in fact, those details are supplied by
libc.so on a Linux system.
That is not exactly correct. The
main function is called (in a hosted environment) by the crt0 stuff, some of which is provided by GCC (e.g.
/usr/lib/gcc/x86_64-linux-gnu/6/crtbegin.o on my Debian/Sid/x86-64 is from the
libgcc-6-dev package). Read also about
Actually, there is some half-hidden relation between
libc and GCC, e.g. because many
libc headers are (optionally) using some gcc builtins or function attributes.
(hence the GCC developers and the GNU libc developers do have to interact)
.... if I change the compiler to work with another ABI...
You'll need to ...
/configure the GCC compiler and rebuild it, and you might even need to patch the GCC compiler (to describe your ABI and calling conventions). The x32 ABI is a good example.
At last, some contributors to or maintainers of GCC (including me) have signed a copyright assignment which covers GCC but not the GNU
(regarding GCC license, read carefully GCC runtime library exception)
Notice that some standard headers, like
<stdint.h> are provided by GCC; others, like
<stdlib.h> are "fixed" during GCC build: the compiler build procedure takes them from the Libc implementation and patches them. Still, other standard headers (probably
<stdio.h> and the internal headers it is including) are taken from the
libc. Read more about GCC FIXINCLUDES and Fixed Header Files.
(the fixincludes thing is something I (Basile) still don't understand well)
You could compile with
gcc -v -H to understand more precisely which actual programs are run (since
gcc is a driver, running the
cc1 compiler, the
collect2 linkers, the
as assembler, etc...) and which headers are included, which libraries and object files are linked (even implicitly, including the C standard library and the crt0). Read more about GCC options.
BTW, you can use a C standard library different of the one your GCC expects or was built for (e.g.
musl-libc or some dietlibc), bypassing appropriate extra arguments to