Where is the connection open () in the libc source?

I basically need to configure multiple linux system call interfaces (like sys_open) for my purpose. I know very well the GNU Linker ld -wrap = symbol option and use this logic to modify the open () libc wrapper. While this serves a purpose, I really want to know where in the libc sources the actual implementation comes into play.

The next two places are my main suspects (note that fcntrl.h only has declarations)

• GLIBC_DIR / I.O. / open.c
• GLIBC_DIR / ports / sysdeps / Unix / SysV / Linux / general / open.c

Driver example:

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>

int main(int argc, char *argv[])
{
    int fd;

    if ((fd = open("sample.c", O_RDONLY)) == -1) {
        fprintf(stderr, "file not found\n");
        exit(1);
    }

    return 0;
}

      

        
        
        
      

    

Worried snippet:

main:
  401dd1:       bf 44 90 48 00          mov    $0x489044,%edi
  401dd6:       b8 00 00 00 00          mov    $0x0,%eax
  401ddb:       e8 10 03 03 00          callq  4320f0 <__libc_open>

......
......

 __libc_open:
  4320f0:       83 3d 69 8e 28 00 00    cmpl   $0x0,0x288e69(%rip)        
  4320f7:       75 14                   jne    43210d <__open_nocancel+0x14>

__open_nocancel:
  4320f9:       b8 02 00 00 00          mov    $0x2,%eax
  4320fe:       0f 05                   syscall 

      

        
        
        
      

    

For simplicity, I've prepared all libc binaries statically . Was also careful enough for GCC to pick up the custom libc.a correctly. I tried adding a puts statement, but the two source codes mentioned DO NOT run at all. Taking a look at the assembly of the executable [shown above], the sys_open call (0x2 in __open_nocancel) was somehow placed in the executable.

So my question is the following:

• Where exactly in libc the open () logic logic is related to magic?
• How can the linker successfully hook up the open () function when there is no function explicitly named open in the libc source tree?