Unix Signals

A signal is a message that can be sent to a running process.

Programs, users, or administrators can initiate signals.

For example, the proper method of telling the Internet Daemon (inetd) to re-read its configuration file is to send it a SIGHUP signal.

For example, if the current process ID (PID) of inetd is 4140, type:

kill -SIGHUP 4140

Another common use of signals is to stop a running process. To stop the inetd process completely, use this command:

kill 4140

By default, the kill command sends the SIGTERM signal. If SIGTERM fails, escalate to using the SIGKILL signal to stop the process:

kill -9 4140

Because SIGKILL cannot be handled, stopping a process with SIGKILL is generally considered a bad idea. Using SIGKILL prevents a process from cleaning up after itself and exiting gracefully.

Handling Signals

Each Unix signal has a default set of effects on a Unix program. Programmers can code their applications to respond in customized ways to most signals. These custom code pieces are called signal handlers.

A signal handler is unable to redefine two signals. SIGKILL always stops a process and SIGSTOP always moves a process from the foreground to the background. A signal handler cannot “catch” these two signals.

FreeBSD Signals

Signal Name Signal Number Signal Description
SIGHUP 1 Terminal line hangup
SIGINT 2 Interrupt program
SIGQUIT 3 Quit program
SIGILL 4 Illegal instruction
SIGTRAP 5 Trace trap
SIGABRT 6 Abort
SIGEMT 7 Emulate instruction executed
SIGFPE 8 Floating-point exception
SIGKILL 9 Kill program
SIGBUS 10 Bus error
SIGSEGV 11 Segmentation violation
SIGSYS 12 Bad argument to system call
SIGPIPE 13 Write on a pipe with no one to read it
SIGALRM 14 Real-time timer expired
SIGTERM 15 Software termination signal
SIGURG 16 Urgent condition on I/O channel
SIGSTOP 17 Stop signal not from terminal
SIGTSTP 18 Stop signal from terminal
SIGCONT 19 A stopped process is being continued
SIGCHLD 20 Notification to parent on child stop or exit
SIGTTIN 21 Read on terminal by background process
SIGTTOU 22 Write to terminal by background process
SIGIO 23 I/O possible on a descriptor
SIGXCPU 24 CPU time limit exceeded
SIGXFSZ 25 File-size limit exceeded
SIGVTALRM 26 Virtual timer expired
SIGPROF 27 Profiling timer expired
SIGWINCH 28 Window size changed
SIGINFO 29 Information request
SIGUSR1 30 User-defined signal 1
SIGUSR2 31 User-defined signal 2
SIGTHR 32 Thread interrupt

Solaris Signals

Signal Name Signal Number Signal Description
SIGHUP 1 Hangs up
SIGINT 2 Interrupts
SIGQUIT 3 Quits
SIGILL 4 Illegal instruction
SIGTRAP 5 Trace trap
SIGABRT 6 Used by abort
SIGEMT 7 EMT instruction
SIGFPE 8 Floating-point exception
SIGKILL 9 Kill (cannot be caught or ignored)
SIGBUS 10 Bus error
SIGSEGV 11 Segmentation violation
SIGSYS 12 Bad argument to system call
SIGPIPE 13 Writes on a pipe with no one to read it
SIGALRM 14 Alarm clock
SIGTERM 15 Software termination
SIGUSR1 16 User-defined signal 1
SIGUSR2 17 User-defined signal 2
SIGCHLD 18 Child status change alias (POSIX)
SIGPWR 19 Power-fail restart
SIGWINCH 20 Window size change
SIGURG 21 Urgent socket condition
SIGPOLL /SIGIO 22 Pollable event occurred or Socket I/O possible
SIGSTOP 23 Stop (cannot be caught or ignored)
SIGTSTP 24 User stop requested from TTY
SIGCONT 25 Stopped process has been continued
SIGTTIN 26 Background TTY read attempted
SIGTTOU 27 Background TTY write attempted
SIGVTALRM 28 Virtual timer expired
SIGPROF 29 Profiling timer expired
SIGXCPU 30 Exceeded CPU limit
SIGXFSZ 31 Exceeded file size limit
SIGWAITING 32 Process’ LWPs are blocked
SIGLWP 33 Special signal used by thread library
SIGFREEZE 34 Special signal used by CPR
SIGTHAW 35 Special signal used by CPR
SIGCANCEL 36 Thread cancellation signal used by libthread
SIGLOST 37 Resource lost
SIGRTMIN 38 Highest priority real-time signal
SIGRTMAX 45 Lowest priority real-time signal

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  • eric

    this was very helpful. finally, information when you ask for it with a simple search