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## Introduction `` is a header file in the C standard library used for handling signals. The **signal.h** header file defines a variable type **sig_atomic_t**, two function calls, and some macros to handle the different signals reported during program execution. Signals are a form of asynchronous notification mechanism, allowing a process to execute predefined handler functions when specific events occur. Here is a simple example program demonstrating how to use the signal function to catch the SIGINT signal (typically generated by Ctrl+C). ## Example #include #include #include // Global variable indicating whether the program should exit volatile sig_atomic_t stop =0; void handle_sigint(int sig){ printf("Caught signal %dn", sig); stop =1;// Set exit flag } int main(){ // Set the handler for SIGINT signal to the handle_sigint function signal(SIGINT, handle_sigint); while(!stop){// Check if should exit printf("Running...n"); sleep(1); } printf("Exiting...n"); return 0; } In the above code, when the program is running, if the user presses Ctrl+C, the SIGINT signal is caught and the handle_sigint function is called, printing the signal number. Compiling and executing the above code yields the following output: Running...Running...Running...^CCaught signal 2Exiting... Code Analysis: * `volatile sig_atomic_t stop = 0;`: Defines a global variable `stop` used to indicate whether the program should exit. The `volatile` keyword ensures the compiler does not optimize away accesses to this variable, as it may be modified within the signal handler. The `sig_atomic_t` type guarantees that access to this variable is atomic. * Inside the `handle_sigint` signal handler function, `stop` is set to 1, indicating the program should exit. * In the main loop, the value of the `stop` variable is checked. If it is set to 1, the loop is broken out of, ending the program. ## Library Variables Below is the variable type defined in the signal.h header file: | S.No. | Variable & Description | | --- | --- | | 1 | **sig_atomic_t** This is an **int** type, used as a variable within signal handlers. It is an integer type of an object that can be accessed as an atomic entity, even when asynchronous signals are present. | | 2 | **sigset_t** A data type used to represent a signal set. | ## Library Macros < In the C standard library, macros with the `SIG_` prefix are used together with the `signal` function to define and handle signal behavior. Below are common `SIG_` macros and their purposes: | S.No. | Macro & Description | | --- | --- | | 1 | **SIG_DFL** Represents the default handler for a signal. Using this macro restores a signal to its default behavior. | | 2 | **SIG_ERR** Represents the return value for signal handling function errors. | | 3 | **SIG_IGN** Ignore the signal. | Signal Types: | Constant | Description | | --- | --- | | `SIGABRT` | Signal generated by the `abort` function, indicating abnormal termination | | `SIGALRM` | Signal generated when a timer set by the `alarm` function expires | | `SIGBUS` | Illegal memory access (e.g., accessing misaligned memory addresses) | | `SIGCHLD` | Child process status changed (exited or stopped) | | `SIGCONT` | Continue a paused process | | `SIGFPE` | Arithmetic error (e.g., division by zero, floating-point exception) | | `SIGHUP` | Hangup signal (often used to detect terminal disconnection) | | `SIGILL` | Illegal instruction | | `SIGINT` | Interrupt signal (typically generated by Ctrl+C) | | `SIGKILL` | Immediate termination signal (cannot be caught or ignored) | | `SIGPIPE` | Writing to a pipe with no reading process | | `SIGQUIT` | Terminal quit signal (typically generated by Ctrl+), generates a core dump | | `SIGSEGV` | Segmentation fault (illegal memory access) | | `SIGSTOP` | Stop process execution (cannot be caught or ignored) | | `SIGTERM` | Termination signal | | `SIGTSTP` | Suspend process (typically generated by Ctrl+Z) | | `SIGTTIN` | Signal generated when a background process reads from the terminal | | `SIGTTOU` | Signal generated when a background process writes to the terminal | | `SIGUSR1` | User-defined signal 1 | | `SIGUSR2` | User-defined signal 2 | | `SIGPOLL` | I/O event (replaces SIGIO) | | `SIGPROF` | Timer expiration signal (profiling timer set by `setitimer`) | | `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 Below are the functions defined in the signal.h header file: | Function | Description | | --- | --- | | `void (*signal(int sig, void (*func)(int)))(int);` | Sets a signal handler. | | `int raise(int sig);` | Sends a signal to the current process. | | `int kill(pid_t pid, int sig);` | Sends a signal to a specified process. | | `int sigprocmask(int how, const sigset_t *set, sigset_t *oldset);` | Examines or changes the blocked signal set. | | `int sigaction(int sig, const struct sigaction *act, struct sigaction *oldact);` | Examines or changes the signal handler. | | `int sigsuspend(const sigset_t *mask);` | Temporarily replaces the current signal mask and suspends the process until a signal is caught. | | `int sigpending(sigset_t *set);` | Examines the pending signal set. | | `int sigemptyset(sigset_t *set);` | Initializes the signal set to empty. | | `int sigfillset(sigset_t *set);` | Initializes the signal set to contain all signals. | | `int sigaddset(sigset_t *set, int signum);` | Adds the specified signal to the signal set. | | `int sigdelset(sigset_t *set, int signum);` | Removes the specified signal from the signal set. | | `int sigismember(const sigset_t *set, int signum);` | Checks if the specified signal is in the signal set. | | `void abort(void);` | Generates the `SIGABRT` signal, causing abnormal termination of the process. | | `unsigned int alarm(unsigned int seconds);` | Sends the `SIGALRM` signal to the calling process after the specified number of seconds. | | `int pause(void);` | Suspends the process until a signal is caught. | | `void psignal(int sig, const char *s);` | Prints signal description information. | | `char *strsignal(int sig);` | Returns a string describing the signal. | | `int sigwait(const sigset_t *set, int *sig);` | Blocks and waits for a signal to handle. |