Print "Hello world" before main() function in C
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, worldn");
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 ?
_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 .
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