Permute signal handlers

Hello everyone,

I am trying to write a program that includes signals handling.
My program should not stop when he receives a SIGINT signal, he should printf a message instead.
This message should change at every single time my program receives a SIGQUIT signal.
So my program prints "message number 1" at the first reception of a SIGINT signal. then, if he receives a SIGQUIT signal, then the next time he receives a SIGINT signal, that's "message number 2" who should be displayed...

I am trying to do this using *sig_before, it seems quite logical.
However I am not sure that I should use three handlers like shown in the code.

Can someone get me some help here ?

thanks in advance :redface:

Code:

---

#include <signal.h>
#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
#include <string.h>
#include <unistd.h>

#define True 1

void (*sig_before)(int); /* to use for permutation */

void work() {
  while(True){
    sleep(100000);
  }
}

void work() __attribute__((noreturn));

/* signal handlers */
void sighdl1(int n) {
  printf("message number 1 \n");
}
void sighdl2(int n) {
  printf("message number 2 \n");
}
void quithdlr(int n) {
  printf("signal %i received, messages permutation", n);
}

/* the main of my program*/
int main() {
/* here I would like to use the call system signal and sig_before so that my program can swap from one signal handler to another when he receives a QUIT signal */
  signal(SIGINT, sighdl1);
  work();

}

---

You need to be careful what you do in a signal handler. It's best that they do as little as possible. In particular, functions like printf are generally not supposed to be used inside them. signal-safety(7) - Linux manual page

Instead of defining your own True, just use <stdbool.h> which defines true, false, and bool.

You'll need to kill the program from another terminal:

Code:

---

$ ps -C a.out
  PID TTY          TIME CMD
10374 pts/0    00:00:00 a.out
$ kill -s USR1 10374

---

Keys:
SIGINT: ctrl+C
SIGQUIT: ctrl+\ (or maybe ctrl+Y)

Code:

---


#include <signal.h>
#include <stdbool.h>
#include <stdio.h>
#include <string.h>
 
#include <sys/types.h>
#include <unistd.h>
 
#define NumMessages 2
 
sig_atomic_t g_msg_num = -1;
sig_atomic_t g_cur_msg_num = 0;
 
void work() {
    static const char *msgs[] = {
        "message 1",
        "message 2"
    };
    while (true) {
        sleep(1); // one second is enough
        if (g_msg_num != -1) {
            printf("%s\n", msgs[g_msg_num]);
            g_msg_num = -1;
        }
    }
}
 
void inthdlr(int n) {
    g_msg_num = g_cur_msg_num;
    signal(SIGINT, inthdlr);
}
 
void quithdlr(int n) {
    g_cur_msg_num = (g_cur_msg_num + 1) % NumMessages;
    signal(SIGQUIT, quithdlr);
}
 
int main() {
    signal(SIGINT, inthdlr);
    signal(SIGQUIT, quithdlr);
    work();
}

---

And... avoid to use signal()... it is outdated.
Use sigaction().

Quote:

---

Originally Posted by flp1969

And... avoid to use signal()... it is outdated.
Use sigaction().

---

that is noted, thank u for the advice :redface:

Quote:

---

Originally Posted by john.c

You need to be careful what you do in a signal handler. It's best that they do as little as possible. In particular, functions like printf are generally not supposed to be used inside them. signal-safety(7) - Linux manual page

Instead of defining your own True, just use <stdbool.h> which defines true, false, and bool.

You'll need to kill the program from another terminal:

Code:

---

$ ps -C a.out
  PID TTY          TIME CMD
10374 pts/0    00:00:00 a.out
$ kill -s USR1 10374

---

Keys:
SIGINT: ctrl+C
SIGQUIT: ctrl+\ (or maybe ctrl+Y)

Code:

---


#include <signal.h>
#include <stdbool.h>
#include <stdio.h>
#include <string.h>
 
#include <sys/types.h>
#include <unistd.h>
 
#define NumMessages 2
 
sig_atomic_t g_msg_num = -1;
sig_atomic_t g_cur_msg_num = 0;
 
void work() {
    static const char *msgs[] = {
        "message 1",
        "message 2"
    };
    while (true) {
        sleep(1); // one second is enough
        if (g_msg_num != -1) {
            printf("%s\n", msgs[g_msg_num]);
            g_msg_num = -1;
        }
    }
}
 
void inthdlr(int n) {
    g_msg_num = g_cur_msg_num;
    signal(SIGINT, inthdlr);
}
 
void quithdlr(int n) {
    g_cur_msg_num = (g_cur_msg_num + 1) % NumMessages;
    signal(SIGQUIT, quithdlr);
}
 
int main() {
    signal(SIGINT, inthdlr);
    signal(SIGQUIT, quithdlr);
    work();
}

---

---

Thank u John for ur reply, it was very helpful !! However, in this program, I would like to use the fact that signal() call system returns a pointer to the last signal handler used to handle the same type of signal (which is SIGINT in our case). That is why I used void (*sig_before)(int); in the beginning.

You can do the same with sigaction.

Code:

---

      int sigaction(int signum, const struct sigaction *act,
                    struct sigaction *oldact);

---

@Ramsis94, get the GNU C Library manual and take a look at printf() funcion at 12.12.7 (page 277):

Attachment 16043

Notice the "AS-Unsafe" at "Preliminary". This means, printf() isn't safe to use in signal handlers (Async-Signal safety).
The definition of these terms are in 1.2.2.1 - "POSIX Safety Concepts".

As @john.c told you, printf() isn't signal safe.

You may ask: "How can I print a simple string in stdout, then?". Using write() function (which is AS-Safe):

Code:

---

static const char str[] = "hello";
...
write( STDOUT_FILENO, str, sizeof str - 1 );

---

I see, thanks a lot

Quote:

---

Originally Posted by Salem

You can do the same with sigaction.

Code:

---

      int sigaction(int signum, const struct sigaction *act,
                    struct sigaction *oldact);

---

---

Apparently yes, but I am not familiar with sigaction() and I don't know how to use it :frown:

> Apparently yes, but I am not familiar with sigaction() and I don't know how to use it
And?

Presumably there were previous times when you didn't know what signal() was, or what printf() was, and that didn't stop you.

Quote:

---

Originally Posted by Salem

> Apparently yes, but I am not familiar with sigaction() and I don't know how to use it
And?

Presumably there were previous times when you didn't know what signal() was, or what printf() was, and that didn't stop you.

---

You are right ! I am learning too much new things these days :wink: