Basic Signals

• kill - send a process a signal

  •        #include <sys/types.h>
           #include <signal.h>
    
           int kill(pid_t pid, int sig);
           

  • pid is the process id you want the signal delivered to.
    • 0 means every process in the process group
    • if pid is positive it means that process.
    • if pid is -1, all signals that this process has permission to send to
    • If pid is less than -1, all process in -pid process group
    • But only if you have permission to send signals to the processes
  • sig is the signal you wish to send.
    • You can use numbers (not portable)
    • Or names (portable)
    • 0 indicates that you want to check to see if the process exists.
  • PID 1 can only be sent signals for which it has a signal handler installed.
• signal: Handling signals the bad way.

  •        #include <signal.h>
           typedef void (*sighandler_t)(int);
           sighandler_t signal(int signum, sighandler_t handler);
    	

  • For compatibility, and quick examples.
  • signum is the signal number, or better the constant name.
  • handler is
    • Either a pointer to a signal function
    • SIG_IGN : cause the signal to be ignored.
    • SIG_DFL : cause the default action to be used for the signal
  • The return value is
    • The previous value
    • SIG_ERR on error.
• This shows up all over the place, especially older code
• BUT DON'T USE THIS.
  • The functionality is limited
  • There are problems with uniformity across implementations
• When a process receives a signal
  • The process is interpreted and the signal handler is called
  • 
  • But possibly not that simple, play with usr.
  • More on this to come in chapter 21.
  • In general, the handler should do as little as possible.
• Oh, and many functions are not safe to call in signals, but we will do so for now.
• What signal did I get?

  •        #include <string.h>
           char *strsignal(int sig);
           extern const char * const sys_siglist[];