Signal

Segementation Fault Warning!!!!!! (not really lol)

#include <signal.h>

For our C Language learner, signals is not strange, at least segment fault is not. But there are more signals we are using. For example, ctrl + c is the action send termination signal (SIGINT) to the current process. ctrl + z is the action send suspending signal (SIGSTOP) and then we can use shell command fg program to wake it up and use ps to lookup current running prcoesses.

And we can also use kill to send SIGSTOP by argument -STOP + pid to suspend pid process and also with argument -CONT + pid to re-run this process. But Notice, kill program do not terminate a process.

Furthermore, signals can be sent by operation system but also users, but as programmer we can't know when a signal might arrive at process.

Normally, we have 31 signals in linux system

Num	Name	description
1	SIGHUP	Hangup
2	SIGINT	Interrupt
3	SIGQUIT	Quit
4	SIGILL	Illegal instruction
5	SIGTRAP	trace/breakpoint trap
6	SIGABRT	Aborted
7
8
9	SIGKILL	killed
10	SIGUSR1	User defined signal 1
11	SIGSEGV	Segmentation fault
12
13	SIGPIPE	Broken pipe
14
15	SIGTERM	Terminated
16	SIGSTKFLT	Stack fault
17	SIGCHLD	child exited
18	SIGCONT	Continue
19	SIGSTOP	Stopped by signal
20
21
22
23
24
25
26
27	SIGPROF	Profiling timer expired
28
29
30
31

Notice: signal 9 and 19, can never be modified

Signal in C

To do action with signals, we should know the struct of sigaction:

struct sigaction {
    void (*sa_handler)(int);
    void (*sa_sigaction)(int, siginfo_t *, void *);
    sigset_t sa_mask;j
    int sa_flags;
}

• sa_handler is the action when signum signal raise, we can change to a new function to act like we want
• sa_mask point to a signal when the signal raise, the signal will be blocked
• sa_flags can set different flags. if specifies as SA_SIGINFO the sa_sigaction will be action (instead of sa_hanlder) when signum raise
• siginfo_t(a type) is different with SA_SIGINFO (a flag). siginfo_t is a structure with details about signal.

Then we can change the action when signal raise, by following:

• int sigemptyset(sigset_t* set): initialize the signal set to empty, return 0 success, or -1 on error, and errno is set to indicate the error.
• int sigaction(int signum, const struct sigaction *act, struct sigaction *oldact): (except signal 9 and 19) change the signum signal action in act and store the old version into oldact. returns 0 on success; on error, -1 is returned, and errno is set to indicate the error.

Notice, the new action of sa_handler should have the same signature which take the same type and number of parameters.