Standard C Library (libc, -lc)
#include <signal.h>
void (*
  
  bsd_signal(int
    sig, void
    (*func)(int)))(int);
The bsd_signal() function provides a partially
  compatible interface for programs written to historical system interfaces.
The previous action is returned on a successful call. Otherwise,
  SIG_ERR is returned and the global variable
  errno is set to indicate the error.
The bsd_signal() function conforms to
  IEEE Std 1003.1-2004 (“POSIX.1”). It was
  moved from X/Open to zbase in Issue 4 version 2. It first appeared in
  NetBSD 8.