IPC – Inter-process Communication – is the way running programs can communicate with each other in a Linux system when there are conflicts or user has need to interrupt the running program. The following are a list of the various signals that the Linux Kernel handles. Most of us are familiar with several of them already. For example when you run kill -9 process ID runs the number 9 SIGKILL.
| Number | Name | Default action | Corresponding event |
|---|---|---|---|
| 1 | SIGHUP | Terminate | Terminal line hangup |
| 2 | SIGINT | Terminate | Interrupt from keyboard |
| 3 | SIGQUIT | Terminate | Quit from keyboard |
| 4 | SIGILL | Terminate | Illegal instruction |
| 5 | SIGTRAP | Terminate and dump core (1) | Trace trap |
| 6 | SIGABRT | Terminate and dump core (1) | Abort signal from abort function |
| 7 | SIGBUS | Terminate | Bus error |
| 8 | SIGFPE | Terminate and dump core (1) | Floating point exception |
| 9 | SIGKILL | Terminate (2) | Kill program |
| 10 | SIGUSR1 | Terminate | User-defined signal 1 |
| 11 | SIGSEGV | Terminate and dump core (1) | Invalid memory reference (seg fault) |
| 12 | SIGUSR2 | Terminate | User-defined signal 2 |
| 13 | SIGPIPE | Terminate | Wrote to a pipe with no reader |
| 14 | SIGALRM | Terminate | Timer signal from alarm function |
| 15 | SIGTERM | Terminate | Software termination signal |
| 16 | SIGSTKFLT | Terminate | Stack fault on coprocessor |
| 17 | SIGCHLD | Ignore | A child process has stopped or terminated |
| 18 | SIGCONT | Ignore | Continue process if stopped |
| 19 | SIGSTOP | Stop until next SIGCONT (2) | Stop signal not from terminal |
| 20 | SIGTSTP | Stop until next SIGCONT | Stop signal from terminal |
| 21 | SIGTTIN | Stop until next SIGCONT | Background process read from terminal |
| 22 | SIGTTOU | Stop until next SIGCONT | Background process wrote to terminal |
| 23 | SIGURG | Ignore | Urgent condition on socket |
| 24 | SIGXCPU | Terminate | CPU time limit exceeded |
| 25 | SIGXFSZ | Terminate | File size limit exceeded |
| 26 | SIGVTALRM | Terminate | Virtual timer expired |
| 27 | SIGPROF | Terminate | Profiling timer expired |
| 28 | SIGWINCH | Ignore | Window size changed |
| 29 | SIGIO | Terminate | I/O now possible on a descriptor |
| 30 | SIGPWR | Terminate | Power failure |
All but two of these signals can have handlers that overwrite the default. The two signals that can’t be overwritten are the SIGSTOP and SIGKILL signals.
The following is an example program that overwrites the SIGING (cntl-c signal) and SIGTSTP (cntl-z signal). You can play around and overwrite the handler for any other linux signals.
[c]
#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
void handler1(int sig)
{
printf("Not so simple — I can’t be stopped with a ctr-c!n");
}
void handler2(int sig)
{
printf("Nope — I refuse to be put in the background!n");
}
int main()
{
/* install the SIGINT handlers */
/* only code changes go right here! */
if (signal(SIGINT, handler1) == SIG_ERR)
/* install the SIGINT handlers */
/* only code changes go right here! */
if (signal(SIGTSTP, handler2) == SIG_ERR)
/* Don’t change rest of code */
printf("Just try to stop me!n");
while (1)
sleep(1);
}
[/c]