Print "Hello world" before main() function in C

Question

The following program copied from Quora, that print "Hello world" before main() function.

#include <stdio.h>
#include <unistd.h>

int main(void)
{
  return 0;
}

void _start(void)
{
  printf ("hello, world\n");
  int ret = main();
  _exit (ret);
}

Then, I compiled above program on Ubuntu-14.04 GCC compiler using following command

gcc -nostartfiles hello.c

And ran a.out executable file, But I got Segmentation fault (core dumped)? So, Why Segmentation fault?

Answer 1

_start is the real entrypoint of the executable, that is normally taken by the C runtime to initialize its stuff - including stdio -, call functions marked with the constructor attribute and then call your main entrypoint. If you take it and try to use stuff from the standard library (such as printf) you are living dangerously, because you are using stuff that hasn't been initialized yet.

What you can do, however, is to bypass the C runtime completely, and print using a straight syscall, such as write.