Date program for xv6- improvement and testing

Question

This is my first post on this forum. I have read the guidelines and will try to adhere to them, but I apologize if I miss something. The code I am posting is intended to duplicate the output of "date -u" in the XV6 OS. It does this, with the exception of dates before the 1700s (This wasn't relevant to the task, and the math is a little different, from what I read). My goal with this post is to seek advice on improving the code in terms of safety and performance, with an emphasis on best practices that I can generalize to future C code (I realize this may not be as doable for something intended to run in XV6). Advice on both my code and better techniques for posting here is appreciated!

#include "types.h"
#include "stat.h"
#include "user.h"
#include "date.h"

int check_leap(int year);
void print_day(int year, int month, int day);
void print_month(int month);
int month_key(int month);

int
main(void)
{
  struct rtcdate current_date;
  if(date(&current_date) != 0)
    exit();
  print_day(current_date.year, current_date.month, current_date.day);
  print_month(current_date.month);
  printf(1, " %d",current_date.day);
  printf(1," %d:%d:%d", current_date.hour, current_date.minute, current_date.second);
  printf(1," %d", current_date.year);
  printf(1," UTC\n");
  exit();
}

int 
check_leap(int year)
{
  if(year % 4 == 0)
  {
    if(year % 100 == 0)
    {
      if(year % 400 == 0)
        return 1; //Leap year
    }
  }
  return 0;
}

int
month_key(int month)
{
  int key;
  switch (month)
  {
    case 1: key = 1;
            break;
    case 2: key = 4;
            break;
    case 3: key = 4;
            break;
    case 4: key = 0;
            break;
    case 5: key = 2;
            break;
    case 6: key = 5;
            break;
    case 7: key = 0;
            break;
    case 8: key = 3;
            break;
    case 9: key = 6;
            break;
    case 10: key = 1;
             break;
    case 11: key = 4;
             break;
    case 12: key = 6;
             break;
    default: key = -1;
             break;
  }
  return key;
}

void 
print_day(int year, int month, int day)
{
  int week_day;
  int decade = year % 100;
  int century = year / 100;
  int leap = check_leap(year);
  int key = month_key(month);
  if(key < 0)
  {
    printf(1, "%s", "Invalid value for month!\n");
    exit();
  }

  week_day = decade / 4;
  week_day += day;
  week_day += key;
  if((leap) && ((month == 1) || (month == 2)))
  {
    week_day -= 1;
  }
  switch (century)
  {
    case 17: week_day += 4;
             break;
    case 18: week_day += 2;
             break;
    case 19: break;
    case 20: week_day += 6;
             break;
    default: printf(1, "%s", "Centuries before 1700 or after 2000 not yet supported. \n");
             break;
  }
  week_day += decade;
  week_day = (week_day % 7) - 1;

  if(week_day < 0 || week_day > 6)
  {
    printf(1, "Invalid Day error!"); //TS code- to be removed
  }
  switch(week_day)
  {
    case 0: printf(1, "Sun ");
            break;
    case 1: printf(1, "Mon ");
            break;
    case 2: printf(1, "Tue ");
            break;
    case 3: printf(1, "Wed ");
            break;
    case 4: printf(1, "Thu ");
            break;
    case 5: printf(1, "Fri ");
            break;
    case 6: printf(1, "Sat ");
            break;
    default: printf(1, "Unknown day!");
            break;  
 }
 return;
}

void
print_month(int month)
{
  switch (month)
  {
    case 1: printf(1, "Jan");
            break;
    case 2: printf(1, "Feb");
            break;
    case 3: printf(1, "Mar");
            break;
    case 4: printf(1, "Apr");
            break;
    case 5: printf(1, "May");
            break;
    case 6: printf(1, "Jun");
            break;
    case 7: printf(1, "Jul");
            break;
    case 8: printf(1, "Aug");
            break;
    case 9: printf(1, "Sep");
            break;
    case 10: printf(1, "Oct");
            break;
    case 11: printf(1, "Nov");
            break;
    case 12: printf(1, "Dec");
            break;
    default: printf(1, "Month Error");
            break;
 }
 return;
}

Answer 1

XV6 bizarreness

XV6 has made at least one ill-advised decision in forcing a non-standard signature of printf that requires a file descriptor be passed in. Maybe they should have omitted printf entirely and made you use fprintf instead, but anyway: at the least, you should make a stdout constant equal to 1 and use this instead of the numeric literal.

You could actually make #define shims in a header file that centralize the XV6-to-C99 translation, including a definition for fprintf that you can call as you would the standard version, but in fact calls the XV6 printf internally. If you're in a course that forces non-standard C but you still want to learn standard C this would be a sane option.

Also, does returning from main not work? If it does, return instead of exit().

month_key

Re-express this long, continuous switch as a static const lookup array that you index into, after having performed a bounds check. The same is possible for your switch (week_day) and switch (month).