How to discard unused function after optimization in GCC

Question

I got problems with GCC optimization,

when I use gcc test.c -Os or gcc test.c -O2 , the main() function will put at start address.

but some unused function like <_start> <exit> <memset> , why are they still there?

I have try to use -ffunction-sections -fdata-sections or -gc-sections , it doesn't work.

my simple code:

#include <stdio.h>
int main(){
char buff[20];
sprintf(buff, "%s%d", "hello", 6);
return 0; }

Answer 1

_start is generally the real entrypoint of your executable (the one that initializes the C runtime before calling main ); exit is implicitly invoked when main terminates. memset is implicitly invoked by the compiler in a variety of situations (for example when zero-initializing buffers on the stack), or may be used by the printf implementation.