C Standard Library -<signal.h>

Introduction

<signal.h>It is a header file in the C standard library used for handling signals.

signal.hThe header file defines a variable typesig_atomic_ttwo function calls, and some macros to handle different signals reported during program execution.

A signal is an asynchronous notification mechanism that allows a process to execute a predefined handler function when a specific event occurs.

Below is a simple example program demonstrating how to use the signal function to catch the SIGINT signal (usually generated by Ctrl+C).

Example

#include <stdio.h>
#include <signal.h>
#include <unistd.h>

// Global variable indicating whether the program should exit
volatile sig_atomic_t stop = 0;

void handle_sigint(int sig) {
printf("Caught signal %d\n", sig);
stop = 1; // Set the exit flag
}

int main() {
// Set the handler for the SIGINT signal to the handle_sigint function
signal(SIGINT, handle_sigint);

while (!stop) { // Check whether to exit
printf("Running...\n");
sleep(1);
}

printf("Exiting...\n");

return 0;
}

In the above code, when the program is running, if the user pressesCtrl+C, it will catch the SIGINT signal and call the handle_sigint function, printing the signal number.

Compile and execute the above code, the output is as follows:

Running...
Running...
Running...
^CCaught signal 2
Exiting...

Code Analysis:

volatile sig_atomic_t stop = 0;: define a global variablestop, used to indicate whether the program should exit. Usingvolatilethe keyword ensures that the compiler will not optimize away accesses to this variable, because it may be modified in a signal handler.sig_atomic_tThe type guarantees that accesses to this variable are atomic.
Inhandle_sigintIn the signal handler function, setstopto 1, indicating the program should exit.
In the main loop, checkstopthe value of the variable; if it is set to 1, break out of the loop and end the program.

Library variables

The following are variable types defined in the header file signal.h:

Serial Number	Variables & Description
1	sig_atomic_t This isintType, used as a variable in signal handlers. It is an integer type of an object that can be accessed as an atomic entity even in the presence of asynchronous signals.
2	sigset_t A data type used to represent a signal set.

Library macros

In the C standard library,SIG_Prefix macros andsignalUsed together with functions to define and handle signal behavior. The following are commonSIG_Macros and their uses:

Serial Number	Macros & Description
1	SIG_DFL Represents the default handler for a signal. Use this macro to restore a signal to its default behavior.
2	SIG_ERR Represents the return value when an error occurs in setting a signal handler function.
3	SIG_IGN Ignore signal.

Signal types:

Constants	Description
`SIGABRT`	by`abort`The signal generated by the function, indicating abnormal termination
`SIGALRM`	by`alarm`The timer expiration signal set by the function
`SIGBUS`	Illegal memory access (for example, accessing an unaligned memory address)
`SIGCHLD`	Child process status changed (exited or stopped)
`SIGCONT`	Continue executing a suspended process
`SIGFPE`	Arithmetic errors (for example, division by zero, floating-point exceptions)
`SIGHUP`	Hangup signal (usually used to detect terminal disconnect)
`SIGILL`	Illegal instruction
`SIGINT`	Interrupt signal (usually generated by Ctrl+C)
`SIGKILL`	Immediate termination signal (cannot be caught or ignored)
`SIGPIPE`	Write data to a pipe with no read process
`SIGQUIT`	Terminal quit signal (usually generated by Ctrl+\), produces a core dump
`SIGSEGV`	Segmentation fault (illegal memory access)
`SIGSTOP`	Stops process execution (cannot be caught or ignored)
`SIGTERM`	Termination signal
`SIGTSTP`	Suspends the process (usually generated by Ctrl+Z)
`SIGTTIN`	Signal generated when a background process reads data from the terminal
`SIGTTOU`	Signal generated when a background process writes data to the terminal
`SIGUSR1`	User-defined signal 1
`SIGUSR2`	User-defined signal 2
`SIGPOLL`	I/O event (replaces SIGIO)
`SIGPROF`	Timer expiration signal (by`setitimer`the set profiling timer)
`SIGSYS`	Illegal system call
`SIGTRAP`	Breakpoint or trap instruction
`SIGURG`	Socket urgent condition signal
`SIGVTALRM`	Virtual clock timer expiration signal
`SIGXCPU`	Exceeded CPU time limit
`SIGXFSZ`	Exceeded file size limit

Library functions

The following are functions defined in the header file signal.h:

Function	Description
`void (signal(int sig, void (func)(int)))(int);`	Set the signal handler.
`int raise(int sig);`	Send a signal to the current process.
`int kill(pid_t pid, int sig);`	Send a signal to the specified process.
`int sigprocmask(int how, const sigset_t set, sigset_t oldset);`	Check or change the blocked signal set.
`int sigaction(int sig, const struct sigaction act, struct sigaction oldact);`	Check or change the signal handler.
`int sigsuspend(const sigset_t *mask);`	Temporarily replace the current signal mask and suspend the process until a signal is caught.
`int sigpending(sigset_t *set);`	Check the pending signal set.
`int sigemptyset(sigset_t *set);`	Initialize the signal set to an empty set.
`int sigfillset(sigset_t *set);`	Initialize the signal set to the full signal set.
`int sigaddset(sigset_t *set, int signum);`	Add the specified signal to the signal set.
`int sigdelset(sigset_t *set, int signum);`	Remove the specified signal from the signal set.
`int sigismember(const sigset_t *set, int signum);`	Check whether the specified signal is in the signal set.
`void abort(void);`	raise`SIGABRT`The signal causes abnormal termination of the process.
`unsigned int alarm(unsigned int seconds);`	After the specified number of seconds, send`SIGALRM`signal to the calling process.
`int pause(void);`	Suspend the process until a signal is caught.
`void psignal(int sig, const char *s);`	Print signal description information.
`char *strsignal(int sig);`	Return the signal description string.
`int sigwait(const sigset_t set, int sig);`	Block waiting for a signal and handle it.

other extensions