Runtime

Overview

The C runtime is a collection of object files provided by the compiler toolchain. It contains functionality that wraps around the actual invocation of main. Namely, the following files are included:

• crt0.o or crt1.o (the number indicates the ABI version)
• crti.o (used for partially constructing the .init and .fini sections)
• crtn.o (used for partially constructing the .init and .fini sections)

The _init and _fini functions are assembled by the linker in three parts:

1. The function prologues are built from crti.o
2. The body is built from linked objects
3. The function epilogues are built from crtn.o

The .init_array and .fini_array sections supersede the .init and .fini sections respectively.

Startup

The main function is typically described as the special function serving as the entry point to C programs. Though this is not technically true, from a practical perspective it is true enough. It can have several different prototypes, but the following two are always possible:

int main(void);
int main(int argc, char* argv[argc+1]);

The only two return values guaranteed to work on all platform is EXIT_SUCCESS and EXIT_FAILURE. Reaching the end of main is equivalent to a return with value EXIT_SUCCESS.

In hosted environments, a third argument char *envp[] is included. This points to a null-terminated array of pointers to char, each of which points to a string encoding an environment variable as NAME=value.

Shutdown

Returning from main is semantically equivalent to invoking the exit function. On exit, any functions registered with atexit are called in reverse order of registration. A number of cleanup operations (e.g. closing open file descriptors, flushing buffers, etc.) are also performed.

_Noreturn void exit(int status);

Other termination functions exist:

• quick_exit
  • Functions registered with at_quick_exit are called in reverse order.
  • Other cleanup operations are implementation-defined.
• _Exit
  • No functions registered with atexit or at_quick_exit are run.
  • Other cleanup operations are implementation-defined.

Unlike the above three, abort can be used to cause abnormal program termination.

_Noreturn void abort(void);