Processing both I/O and timed events

I have some aspirations of one day programming a MUD engine, and I will need to be able to handle both user I/O and timed events. I have written a little program that uses SIGALRM to update the time. I am wondering if I have a sensible "architecture" for the program, given that global variables should generally be avoided. The program is:

Code:

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

volatile unsigned int alarmTrig;

static void alrmHandler(int x)  //indicates alarm triggered and resets same
{
        alarmTrig = 1;
        alarm(1);
        return;
}

int main(int argc, char *argv[])
{
        struct sigaction theHandler;    //sigaction struct
        char inpChar;
        int i, bytesRead = 0;
        sigset_t emptyMask;

        sigemptyset(&emptyMask);
        theHandler.sa_handler = alrmHandler;
        theHandler.sa_mask = emptyMask; //nothing special
        theHandler.sa_flags = 0;        //nothing special here
        theHandler.sa_restorer = NULL;  //option obsolete

        sigaction(SIGALRM, &theHandler, NULL);
        alarmTrig=0;

        alarm(1);
        for(i=0; i<20; i++)
        {
                while(alarmTrig == 0)
                {
                        bytesRead = fread(&inpChar, 1, 1, stdin);
                        if(bytesRead == 1)
                        {
                                printf("You entered %c.\n",inpChar);
                        }
                }
                if (alarmTrig == 1)
                {
                        alarmTrig=0;
                        printf("%d seconds have gone by.\n",i);
                        alarm(1);
                }
        }

        return 0;
}

Ignoring for now that I do not continually flush the input buffer, is this a reasonable approach, with the alarm handler setting a global variable? Thank you.

>> ... given that global variables should generally be avoided.
There are still times when they're needed.

>> ... is this a reasonable approach, with the alarm handler setting a global variable?
Yes - It's an ideal signal handler in general. However, global should be of type "volatile sig_atomic_t".

gg

The select() system call with a timeout allows you to monitor i/o for activity, and allows you to observe the passage of time.

Originally Posted by Codeplug

>>
>> ... is this a reasonable approach, with the alarm handler setting a global variable?
Yes - It's an ideal signal handler in general. However, global should be of type "volatile sig_atomic_t".

gg

Thanks. It's nice to know that my microcontroller work is applicable to a computer system.

I was looking up sig_atomic_t, and it appears to be typedefed int, with its typedef being performed between

Code:

__BEGIN_NAMESPACE_STD

and 

__END_NAMESPACE_STD

in <signal.h>. This ensures that changes are atomic. I have tried to find these in gcc's info file, but have not found them. What section would these be under? Thank you again.

Originally Posted by Salem

The select() system call with a timeout allows you to monitor i/o for activity, and allows you to observe the passage of time.

Thanks. I knew Python had something similar for serial ports, but I had forgotten about select's timeout.

Originally Posted by drunken_scot

Thanks. It's nice to know that my microcontroller work is applicable to a computer system.

I was looking up sig_atomic_t, and it appears to be typedefed int, with its typedef being performed between

Code:

__BEGIN_NAMESPACE_STD

and 

__END_NAMESPACE_STD

in <signal.h>. This ensures that changes are atomic. I have tried to find these in gcc's info file, but have not found them. What section would these be under? Thank you again.

If you are asking what __BEGIN/__END_NAMESPAC_STD does, then that's for when you use <csignal> in C++, which included <signal.h>.

The point is that sig_atomic_t is GUARANTEED to be of a type that can be accessed as a single operation, which is not guaranteed [but still possible] about other types such as int or long. [It happens that the C runtime for this environment KNOWS that int is OK for this purpose, but it's not necessarily the case for another environment].

The select() solution is definitely one worth looking at.

--
Mats

Originally Posted by matsp

If you are asking what __BEGIN/__END_NAMESPAC_STD does, then that's for when you use <csignal> in C++, which included <signal.h>.

That makes sense. Thanks.