Unix signals are initiated either by cron, running programs, computer administrators, or operators. One example of a signal is the SIGHUP signal, which is sent to the internet to instruct the Internet Daemon, also known as inetd, to repeat reading the Daemon’s configuration file.
It is also possible to use signals to terminate a currently running process. To interrupt the Applicaton’s currently running process (ID, or PID), key in the command kill -SIGHUP
Handling Unix Signals.
Every Unix signal results in a default range of effects when used with a Unix program. Computer programmers can opt to code their own applications for a customized response upon the receipt of most signals. The term used to describe these individually designed strings of code is “signal handlers”.
Signal handlers are unable to redefine two unique signals. In every situation, SIGKILL is used to end a process. Similarly, SIGSTOP always sends a running application to the background when it is in the foreground.
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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