Writing a CLI in C

Table of Contents

The first time I learned C was when I was in the first year of my undergrad studies at IIT Guwahati, India. After learning many other languages like C++ and Java, I wanted to try something out in C. So, I wrote a simple time reminder for my Ubuntu PC that notifies me of the time after every 30 minutes. Currently, this is a simple limit, but this can be changed easily.

Code breakdown

I will explain what all components do.

To notify on linux systems, we can use the notify-send command. We also define the following macros to reuse:

#define SNOTIFY "notify-send"
#define ONE_SECOND_IN_MILLISECONDS 1000000

This function plays the notification sound:

void notification_sound(){
    printf("\a\n");
}

This function updates the time_t object passed to it and sets the value to current time:

void update_current_time(time_t mytime, struct tm ** timeinfo){
    mytime = time(NULL);
    ctime(&mytime);
    *timeinfo = localtime (&mytime);
}

These functions can be used to wait for a specified number of seconds:

void sleep_for_one_second(){
    usleep(ONE_SECOND_IN_MILLISECONDS);
}

void sleep_for_n_seconds(int n){
	int i;
	i = 0;
	while(i < n){
		sleep_for_one_second();
		i++;
	}
}

And here is the main function that contains the core code:

int main(int argc, char const *argv[]){
    int time_in_minutes;
	time_in_minutes = 30;

    int time_in_seconds;
#ifdef DEBUG
    time_in_seconds = 1;
#else
    time_in_seconds = time_in_minutes * 60;
#endif

	time_t mytime;

	// waiting for time to reach :00 or :30
    while(1) {
		struct tm * timeinfo;

	    update_current_time(mytime, &timeinfo);
	    sleep_for_one_second();
#ifdef DEBUG
	    printf("%d:%d:%d\n", timeinfo->tm_hour, timeinfo->tm_min, timeinfo->tm_sec);
	    // stops after 2 mins
	    if (timeinfo->tm_min % 2 == 0) {
	    	break;
	    }
#else
	    if (timeinfo->tm_min % 30 == 0) {
	    	break;
	    }
#endif
	    // free(timeinfo);
    }

    // start alerting user after every 30 minutes from now
    while(1) {
    	notification_sound();

    	char *system_call;
		struct tm * timeinfo;

	    update_current_time(mytime, &timeinfo);
	    sprintf(system_call, "%s \"It's %d:%d\"", SNOTIFY, timeinfo->tm_hour, timeinfo->tm_min);

	    system(system_call);

	    // wait for the specified time
	    sleep_for_n_seconds(time_in_seconds);
    }
	return 0;
}

Build and run

I created a simple MakeFile for the project with the following contents:

time_reminder: time_reminder.c
	gcc -o time_reminder time_reminder.c

After running make inside the folder where our C file is located, an executable time_reminder is generated. This executable can be run using:

./time_reminder