crt0o

crt0.o is a startup object file used by many C toolchains, including GCC, to provide the program’s initial runtime entry. It is part of the C runtime startup sequence and is normally linked in automatically by the compiler driver as part of building a C program. The standard file is named crt0.o (with a dot); some contexts or build configurations may refer to a variant as crt0o, but the conventional runtime file uses the period.

The primary role of crt0.o is to supply the program entry point, typically the symbol _start, which the linker uses as the entry address. The code in crt0.o performs minimal, low-level initialization and then transfers control to the C library startup routine. On many Linux systems using glibc, this involves calling __libc_start_main, passing arguments such as argc, argv, envp, and a pointer to the program’s main function. The library startup routine then calls main and ensures proper cleanup when main returns or when exit is invoked.

crt0.o is part of a family of startup and runtime objects that may also include crt1.o, crti.o, and crtn.o. These files together establish the environment required for a C program to run, including relocation when dynamic linking is used and basic initialization before main executes. Variants exist across platforms (for example, Windows toolchains such as MinGW have their own startup objects and entry points like mainCRTStartup) and for different linking modes (static vs. dynamic, PIE vs. non-PIE).

In practice, developers rarely interact with crt0.o directly; it is a built artifact of the toolchain. Removing startup files or using -nostdlib/-nostartfiles can alter the behavior, in which case a custom entry point would be required.