crt0 is a set of execution startup routines compiled into a program which perform any initialization work required before calling the program's main function. The work performed by crt0 depends on the program's language, compiler, operating system and C standard library

C standard library

The C Standard Library is the standard library for the programming language C, as specified in the ANSI C standard.. It was developed at the same time as the C POSIX library, which is basically a superset of it...

 implementation.

"crt" stands for "C

C (programming language)

C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....

 runtime" (the zero stands for "the very beginning"). However, when compiling using GCC

GNU Compiler Collection

The GNU Compiler Collection is a compiler system produced by the GNU Project supporting various programming languages. GCC is a key component of the GNU toolchain...

, it is also included for other languages than C.

Alternative versions of crt0 are available for special situations. For example, the profiler gprof requires its programs to be compiled with "gcrt0".