crt0 (also known as c0) is a set of execution startup routines linked into a C program that performs any initialization work required before calling the program's main function. It generally takes the form of an object file called crt0.o, often written in assembly language, which is automatically included by the linker into every executable file it builds.
crt0 contains the most basic parts of the runtime library. As such, the exact work it performs depends on the program's compiler, operating system and C standard library implementation. Beside the initialization work required by the environment and toolchain, crt0 can perform additional operations defined by the programmer, such as executing C++ global constructors and C functions carrying the GCC's ((constructor)) attribute.
"crt" stands for "C runtime", and the zero stands for "the very beginning". However, when programs are compiled using GCC, it is also used for languages other than C. Alternative versions of crt0 are available for special usage scenarios; for example, the profiler gprof requires its programs to be compiled with gcrt0.