C: incomplete definition of type 'struct date' error

Question

i started learning C and just ran into an issue.

I created a date ADT and would like to test it out :)

basically, i would like to read in a string from standard input, convert it to the date and print it out on standard output.

after compiling these files i got the following errors:

datetest.c:15:45: error: incomplete definition of type 'struct date'
  printf("Year: %d Month: %d Day: %d", d->year, d->month, d->day);
                                       ~^
./date.h:4:16: note: forward declaration of 'struct date'
typedef struct date Date;

What am i doing wrong?

date.c:

#include "date.h"
#include <stdlib.h>
#include <stdio.h>
#include <string.h>

struct date {
  int day;
  int month;
  int year;
};

/*
 * date_create creates a Date structure from `datestr`
 * `datestr' is expected to be of the form "dd/mm/yyyy"
 * returns pointer to Date structure if successful,
 *         NULL if not (syntax error)
 */
Date *date_create(char *datestr) {
  Date *d = (Date *)malloc(sizeof(Date));
  const char delimiter[2] = "/";
  char *token;

  if (d != NULL) {  
    token = strtok(datestr, delimiter);
    d->day = atoi(token);
    token = strtok(NULL, delimiter);
    d->month = atoi(token);
    token = strtok(NULL, delimiter);
    d->year = atoi(token);
    //printf("Day: %d Month: %d Year: %d\n", d->day, d->month, d->year);    
    //printf("Day: %p Month: %p Year: %p\n", *d->day, *d->month, *d->year);
  }
  return d;
};

/*
 * date_duplicate creates a duplicate of `d'
 * returns pointer to new Date structure if successful,
 *         NULL if not (memory allocation failure)
 */
Date *date_duplicate(Date *d) {
  Date *dd = (Date *)malloc(sizeof(Date));
  if (dd != NULL) {
    dd->day = d->day;
    dd->month = d->month;
    dd->year = d->year;
  }
  return dd;
};

/*
 * date_compare compares two dates, returning <0, 0, >0 if
 * date1<date2, date1==date2, date1>date2, respectively
 */
int date_compare(Date *date1, Date *date2) {
  if (date1->year < date2->year)
    return -1;
  else if (date1->year > date2->year)
    return 1;
  else {
    if (date1->month < date2->month)
      return -1;
    else if (date1->month > date2->month)
      return 1;
    else {
      if (date1->day < date2->day)
    return -1;
      else if (date1->day > date2->day)
    return 1;
      else
    return 0;
    }
  }
};

/*
 * date_destroy returns any storage associated with `d' to the system
 */
void date_destroy(Date *d) {
  if (d != NULL)
    free(d);
};

datetest.c:

#include "date.h"
#include <stdlib.h>
#include <stdio.h>
#include <string.h>

int main() {
  Date *d;
  char buf[1024], *s;

  while (fgets(buf, sizeof(buf), stdin) != NULL) {
    if (!(d = date_create(buf))) {
    fprintf(stderr, "Unable to create a date.\n");
    return -1;
    }
      printf("Year: %d Month: %d Day: %d", d->year, d->month, d->day);
  }
}

date.h:

#ifndef _DATE_H_INCLUDED_
#define _DATE_H_INCLUDED_

typedef struct date Date;

/*
 * date_create creates a Date structure from `datestr`
 * `datestr' is expected to be of the form "dd/mm/yyyy"
 * returns pointer to Date structure if successful,
 *         NULL if not (syntax error)
 */
Date *date_create(char *datestr);

/*
 * date_duplicate creates a duplicate of `d'
 * returns pointer to new Date structure if successful,
 *         NULL if not (memory allocation failure)
 */
Date *date_duplicate(Date *d);

/*
 * date_compare compares two dates, returning <0, 0, >0 if
 * date1<date2, date1==date2, date1>date2, respectively
 */
int date_compare(Date *date1, Date *date2);

/*
 * date_destroy returns any storage associated with `d' to the system
 */
void date_destroy(Date *d);

#endif /* _DATE_H_INCLUDED_ */

Answer 1

You're defining struct date in date.c, datetest.c has no idea what it is. Declare it in date.h instead. Currently it's an opaque type - anything that includes date.h can make a pointer to it, but can't access the members.

Answer 2

datetest.c:15:45: error: incomplete definition of type 'struct date'
  printf("Year: %d Month: %d Day: %d", d->year, d->month, d->day);
                                       ~^
./date.h:4:16: note: forward declaration of 'struct date'
typedef struct date Date;

When the compiler parses date.h, it detects that date.h doesnt have struct date but it uses date. So it throws that note note: forward declaration of 'struct date'

In datetest.c you have included date.h but not the actual definition which is in date.c and the compiler is not able to detect the type. Thats why it throws the error

error: incomplete definition of type 'struct date'

To fix this,

struct date {
  int day;
  int month;
  int year;
};

Move this to date.h file.