dereferencing pointer to incomplete type when time.h header is already included

Question

I'm writing a header, timedate.h, which begins as follows:

#ifndef _TIMEDATE_H_
#define _TIMEDATE_H_

int timetounixtime(int year, int month, int day, int hour, int minute, int second)
{
  struct tm *time;
  time->tm_year = year;
  time->tm_mon = month;
  time->tm_mday = day;
  time->tm_hour = hour;
  time->tm_min = minute;
  time->tm_sec = second;
  return mktime(time);
}

/*...*/

#endif

And is then included in one of my main .c files as follows:

#include <stdio.h>
#include <string.h>
#include <sys/time.h>
#include "timedate.h"

int main(int argv, char **argc)
{
/*...*/
}

It seems to me that this should work since time.h is included in the main code before timedate.h is called. However, when I make, I get the following errors:

XXXXXXXXXX$ make
gcc file2nav.c -o file2nav
In file included from file2nav.c:4:0:
timedate.h: In function ‘timetounixtime’:
timedate.h:10:7: error: dereferencing pointer to incomplete type
timedate.h:11:7: error: dereferencing pointer to incomplete type
timedate.h:12:7: error: dereferencing pointer to incomplete type
timedate.h:13:7: error: dereferencing pointer to incomplete type
timedate.h:14:7: error: dereferencing pointer to incomplete type
timedate.h:15:7: error: dereferencing pointer to incomplete type

Can you help me understand what's going on? I note that if I #include <time.h> in timedate.h, the error goes away...But why? It's already included in file2nav.c.

Answer 1

In your file timedate.h you use

struct tm *time;

but struct tm is not defined yet. You need to include the header #include <time.h>.

A second problem in your code is that you're using an uninitialized pointer time. You can use a local variable:

struct tm time;
time.tm_year = year;

or malloc a pointer (remember to free):

struct tm* time = malloc(sizeof(struct tm));

A better practice, as Ryan points out, is to declare functions in .h and define them in .c:

/* timedate.h */
#ifndef _TIMEDATE_H_
#define _TIMEDATE_H_

int timetounixtime(int year, int month, int day, int hour, int minute, int second);

#endif

and

/* timedate.c */
#include "timedate.h"
#include <time.h>

int timetounixtime(int year, int month, int day, int hour, int minute, int second)
{
  struct tm time;
  time.tm_year = year;
  time.tm_mon = month;
  time.tm_mday = day;
  time.tm_hour = hour;
  time.tm_min = minute;
  time.tm_sec = second;
  return mktime(time);
}

You need to include all header files to make your program compile. C++ Header order suggests one possible order:

• corresponded header file
• necessary project headers
• 3rd party libraries headers
• standard libraries headers
• system headers

In this order you will not miss any of your header files that forgot to include libraries by their own. (Thank Josh for this point).

Answer 2

You include the wrong header, it should be <time.h>, not <sys/time.h>.

<sys/time.h> probably simply doesn't define the struct you are trying to use.