SIGCONT Handler for terminal application

Question

I'm writing a C++ application for POSIX environment with terminal-based User Interface. I'd like to make my application's UI be consistent between Ctrl-Z and fg(i.e: SIGSTOP and SIGCONT).

Here is a minimal example (without using longjmp. this code uses C and POSIX headers only, but entire application will be C++):

#include <cstdio>
#include <cstdlib>
#include <csignal>
#include <unistd.h>
void PrintMessage() {
 printf("Press q to quit");
}
sig_atomic_t signalCheck;
void SigCONTHandler(int sig) {
 if(sig != SIGCONT) {
 return;
 } 
 printf("From Signal Handler\n");
 fflush(stdout);
 if(signalCheck != 0) {
 PrintMessage();
 } 
}
int main() {
 setbuf(stdout, NULL);
 signal(SIGCONT, SigCONTHandler);
 int ch = 0;
 while(true) {
 PrintMessage();
 signalCheck = 1;
 ch = getchar();
 signalCheck = 0;
 if(ch == 'q') {
 break;
 } 
 if(ch == EOF) {
 printf("I asked you to process q.\n");
 return EXIT_FAILURE;
 } 
 } 
 return EXIT_SUCCESS;
}

Usage(Ubuntu 16.04, g++ 5.4, TERM=linux):

$ ./app
Press q to quit
Press q to quit
Press q to quit
Press q to quit
Press q to quit^Z
[2]+ Stopped ./app
$ fg
./app
From Signal Handler
Press q to quit
Press q to quitq

This code works as intended, but I feel this code is Bad. signalCheck variable keep own state to check whether the application need to print PrintMessage() or not. However, if a application become complex, I don't think I can print whole thing to stdout in SigContHandler. Moreover, it won't work without setbuf(stdout, NULL), so I'm trying to make it with longjmp to restore state.

#include <cstdio>
#include <cstdlib>
#include <csignal>
#include <csetjmp>
#include <unistd.h>
sigjmp_buf jumpBuffer;
void PrintMessage() {
 printf("Press q to quit");
}
void SigCONTHandler(int sig) {
 if(sig != SIGCONT) {
 return;
 } 
 printf("From Signal Handler\n");
 siglongjmp(jumpBuffer, 1); 
}
int main() {
 // setbuf(stdout, NULL);
 signal(SIGCONT, SigCONTHandler);
 int ch = 0;
 while(true) {
 if(sigsetjmp(jumpBuffer, 1) == 0) {
 PrintMessage();
 ch = getchar();
 if(ch == 'q') {
 break;
 } 
 if(ch == EOF) {
 printf("I asked you to process q.\n");
 return EXIT_FAILURE;
 } 
 } 
 } 
 return EXIT_SUCCESS;
}

This code works, and I don't need to turn off stdout buffer, but this code also looks bad.

If I need to choose one of them, what I need to choose it? Is it portable from latest Linux to old Unix box?

• vnp: I am not sure I understand what you are trying to achieve. In any case, calling printf from a signal handler is an undefined behavior.
• Byoungchan Lee: @vnp I didn't know that. It is okay to change it to write(STDOUT_FILENO, ...)? Can I use printf after longjmp?
• vnp: Since write translates directly to a system call, it it kind of safe. In doubt, consult man 7 signal-safety.