linux - From where in libc source code, is open() getting linked? -

i need customize few linux system call interfaces (say sys_open) purpose. aware of gnu linker ld --wrap=symbol option , use logic alter open() libc wrapper. although serves purpose, want know in libc source codes, actual implementation comes play.

the following 2 places major suspects (note fcntrl.h has declarations)

• glibc_dir/io/open.c
• glibc_dir/ports/sysdeps/unix/sysv/linux/generic/open.c

sample driver:

#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; } 

concerned 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, had prepared libc sources executable statically. careful enough make gcc rightly pick custom libc.a. tried adding puts statement mentioned 2 source codes not getting invoked @ all. taking @ assembly of executable [shown above], sys_open call (0x2 in __open_nocancel) has been somehow placed in executable.

so question following:

• from in libc, open()-related code logic magically come?
• how linker able hook open() function when there no function explicitly named open in libc source tree?

from in libc, open()-related code logic magically come?

in comes sysdeps/unix/syscall-template.s

how linker able hook open() function when there no function explicitly named open in libc source tree?

if preprocess above source correct -dsyscall_symbol=..., you'll discover there is mention of open in source.