CRT: C Run Time Before Starting main()

There are a lot of functions called before & after the main execution. As an application developer you need not worry about this stuff, but yes! if you are a core developer who works in Linux kernel, Binutils, compiler or embedded system-related development, then you must know these things. Here in “CRT: C run time before starting main”, we will see some pointers related to it.

What Is crt?

• crt stands for C runtime.
• crt is a set of execution startup routines compiled into a program which performs any initialization work required before calling the program’s main function. It is a basic runtime library/system.
• The work performed by crt depends on the ABI, machine, compiler, operating system and C standard library implementation.

CRT: C Run Time Before Starting main()

When an executable is loaded in the memory, the control does not immediately jump into main()(here is what happen). Before going to main it goes to an intermediate start-up-routine/symbol(first symbol) called _start which does some setup/initialization work like:

• Initialize the stacks.
• Preparing the stdin / stdout / stderr streams;
• Pushing argc and argv (or whatever arguments are provided by the shell) onto the stack so that main() can find them.
• If required, copy the contents of the .data (initialized data) section from non-volatile memory.
• If required, copy the contents of the .fast section from non-volatile memory to SRAM.
• Initialize the .bss section to zero.
• Initialize the heap.
• Any other kind of preparation the OS or hardware might require.
• Call the main entry point.

The shutdown/cleanup code (that makes sure that exit() is called when main() returns) is defined in a specific object file commonly called crt0.o . Other possible names are crt1.o(/usr/lib/crt1.o – used if ctors/dtors are there) or similar.

Typical Runtime files

• There could be crt1.o, crt2.o & so on, depending upon implementation, crt0.c is runtime 0 & runs first.
• glibc calls this file start.S while uClibc calls this crt0.S or crt1.S
• General linking order is crt1.o crti.o crtbegin.o [-L paths] [user objects] [gcc libs] [C libs] [gcc libs] crtend.o crtn.o.

What Is the Need for C Startup Routine?

Calling main() is a C thing while calling _start() is a kernel thing, indicated by the entry point in the binary format header. (for clarity: the kernel doesn’t want or need to know that we call it _start).

Why Doesn’t a Compiler Give the Address of Main() as a Starting Point?

That’s because typical libc implementations want to do some initializations before really starting the program.

Changing Entry Point

$ cat centrypoint.c
int disp() { printf("Display !\n"); exit(0); }
int main() { printf("not called\n"); }
$ gcc centrypoint.c -e disp
$ ./a.out
Display !

Some compiler uses --entry or -Wl,-edisp. The -Wl,... thing passes arguments to the linker, and the linker takes a -e argument to set the entry function

Flow of x86 C Program With Runtime

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