Writing a CLI in C

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