Signal

Signals have a unique number or name. Either can be passed to a process by means of the kill(1) command. In OpenBSD the signals are defined in /usr/include/sys/signal.h

#define SIGHUP  1       /* hangup */
#define SIGINT  2       /* interrupt */
#define SIGQUIT 3       /* quit */
#define SIGILL  4       /* illegal instruction (not reset when caught) */
#define SIGTRAP 5       /* trace trap (not reset when caught) */
#define SIGABRT 6       /* abort() */
#ifndef _POSIX_SOURCE
#define SIGIOT  SIGABRT /* compatibility */
#define SIGEMT  7       /* EMT instruction */
#endif
#define SIGFPE  8       /* floating point exception */
#define SIGKILL 9       /* kill (cannot be caught or ignored) */
#define SIGBUS  10      /* bus error */
#define SIGSEGV 11      /* segmentation violation */
#define SIGSYS  12      /* bad argument to system call */
#define SIGPIPE 13      /* write on a pipe with no one to read it */
#define SIGALRM 14      /* alarm clock */
#define SIGTERM 15      /* software termination signal from kill */
#define SIGURG  16      /* urgent condition on IO channel */
#define SIGSTOP 17      /* sendable stop signal not from tty */
#define SIGTSTP 18      /* stop signal from tty */
#define SIGCONT 19      /* continue a stopped process */
#define SIGCHLD 20      /* to parent on child stop or exit */
#define SIGTTIN 21      /* to readers pgrp upon background tty read */
#define SIGTTOU 22      /* like TTIN for output if (tp->t_local&LTOSTOP) */
#ifndef _POSIX_SOURCE
#define SIGIO   23      /* input/output possible signal */
#endif
#define SIGXCPU 24      /* exceeded CPU time limit */
#define SIGXFSZ 25      /* exceeded file size limit */
#define SIGVTALRM 26    /* virtual time alarm */
#define SIGPROF 27      /* profiling time alarm */
#ifndef _POSIX_SOURCE
#define SIGWINCH 28     /* window size changes */
#define SIGINFO 29      /* information request */
#endif
#define SIGUSR1 30      /* user defined signal 1 */
#define SIGUSR2 31      /* user defined signal 2 */

Programs can set up "signal handlers" (traps) to execute a set of instructions after receiving the signal or to ignore signals. Of all signals SIGSTOP (17) and SIGKILL (9) cannot be handled or ignored.