Error with 'extern' usage -- unresolved external symbol

I got the following error message while compiling, but I'm sure I have the variables declared in an external Project2.c file. Could someone give me a hint what I did wrong? Thank you

1>main.obj : error LNK2001: unresolved external symbol _num_days
1>main.obj : error LNK2019: unresolved external symbol _countDays referenced in function _testIt
1>main.obj : error LNK2001: unresolved external symbol _what_birthday
1>main.obj : error LNK2001: unresolved external symbol _birth_day
1>main.obj : error LNK2001: unresolved external symbol _birth_month
1>main.obj : error LNK2001: unresolved external symbol _birth_year

Source code:

#include <stdio.h>

/* the following five variables are defined inside project2.c */
extern int birth_year; 
ext开发者_开发知识库ern int birth_month;
extern int birth_day;
extern int what_birthday;
extern int num_days;

void countDays(void); // a declaration of your function

int num_tests_passed = 0;

void testIt(int test_num, 
        int year, int month, int day, int bday, int expected) {
    birth_year = year;
    birth_month = month;
    birth_day = day;
    what_birthday = bday;
    countDays();
    printf("your answer to test %d was %d, should have been %d\n", 
        test_num, num_days, expected);

    /* check the correctness and quality of their solution */
    if (num_days == expected) {
        if ((birth_year == year) && (birth_month == month) 
                && (birth_day == day)) {
            num_tests_passed += 1;
        } else {
            printf("you got the answer correct,"
                "but you don't follow instructions very well\n");
        }
    } else {
        printf("oops.\n");
    }
}


int main(void) {
    /* test #1: Born Feb 4, 1964 and about to turn 41 */
    testIt(1, 1964, 2, 4, 41, 41 * 365 + 11); // feb 29ths in '64, 68, 72,
                            // 76, 80, 84, 88, 92, 96, 00, and 04 (11 days)

    /* test #2: Born Jan 22, 1998 and about to turn 2 */
    testIt(2, 1998, 1, 22, 2, 2 * 365); // no leap days

    /* test #3: Born March 22, 1998 and about to turn 2 */
    testIt(3, 1998, 3, 22, 2, 2 * 365 + 1); // feb 29, 2000

    /* test #4: Born March 22, 1898 and about to turn 2 */
    testIt(4, 1898, 3, 22, 2, 2 * 365); // no leap days

    /* test #5: Born Jan 22, 1996 and about to turn 4 */
    testIt(5, 1996, 1, 22, 4, 4 * 365 + 1); // feb 29, 1996

    /* test #6: Born March 22, 1996 and about to turn 4 */
    testIt(6, 1996, 3, 22, 4, 4 * 365 + 1); // feb 29, 2000

    /* test #7: Born March 22, 1796 and about to turn 4 */
    testIt(7, 1796, 3, 22, 4, 365 * 4); // no leap days

    /* test #8: Born Jan 1, 1800 and about to turn 101 */
    testIt(8, 1800, 1, 1, 101, 101 * 365 + 24); // There are 26 years divisible by 4
                    // between 1800 and 1901, all of these except 1800 and 1900 are leap years
                    // so there are 24 leap days
    /* test #9: born Jan 1, 1900 and about to turn 100 */
    testIt(9, 1900, 1, 1, 100, 100 * 365 + 24); // 26 years divisible by 4
                    // 1900 was not a leap year, and the 100th brithday comes
                    // before Feb 29, 2000, so only 24 leap days.


    if (num_tests_passed == 9) {
        printf("all tests passed successfully.  Well done!\n");
    } else {
        printf("you only passed %d of the 9 tests.  Looks like you've got some work to do\n",
            num_tests_passed);
    }
}

In your project2.c, make sure you have those variables defined, and that the file is indeed part of your solution. If the source file is omitted than understandably, the linker will complain as it could not find the definitions.

Normally, the best practice is to put those variables into a header file and defined.

For a simplistic example, call this header file project2.h:

/* project2.h */
extern int birth_year; 
extern int birth_month;
extern int birth_day;
extern int what_birthday;
extern int num_days;

Here, to follow on the example here's your code with the extern variables omitted and in place an #include instead. Let's also give this file an name called myproject1.c

#include <stdio.h>
#include "project2.h"

void countDays(void); // a declaration of your function

int num_tests_passed = 0;

void testIt(int test_num, 
        int year, int month, int day, int bday, int expected) {
    birth_year = year;
    birth_month = month;
    birth_day = day;
    what_birthday = bday;
    countDays();
    printf("your answer to test %d was %d, should have been %d\n", 
        test_num, num_days, expected);

    /* check the correctness and quality of their solution */
    if (num_days == expected) {
        if ((birth_year == year) && (birth_month == month) 
                        && (birth_day == day)) {
                num_tests_passed += 1;
        } else {
                printf("you got the answer correct,"
                        "but you don't follow instructions very well\n");
        }
    } else {
        printf("oops.\n");
    }
}


int main(void) {
    /* test #1: Born Feb 4, 1964 and about to turn 41 */
    testIt(1, 1964, 2, 4, 41, 41 * 365 + 11); // feb 29ths in '64, 68, 72,
                                                // 76, 80, 84, 88, 92, 96, 00, and 04 (11 days)

    /* test #2: Born Jan 22, 1998 and about to turn 2 */
    testIt(2, 1998, 1, 22, 2, 2 * 365); // no leap days

    /* test #3: Born March 22, 1998 and about to turn 2 */
    testIt(3, 1998, 3, 22, 2, 2 * 365 + 1); // feb 29, 2000

    /* test #4: Born March 22, 1898 and about to turn 2 */
    testIt(4, 1898, 3, 22, 2, 2 * 365); // no leap days

    /* test #5: Born Jan 22, 1996 and about to turn 4 */
    testIt(5, 1996, 1, 22, 4, 4 * 365 + 1); // feb 29, 1996

    /* test #6: Born March 22, 1996 and about to turn 4 */
    testIt(6, 1996, 3, 22, 4, 4 * 365 + 1); // feb 29, 2000

    /* test #7: Born March 22, 1796 and about to turn 4 */
    testIt(7, 1796, 3, 22, 4, 365 * 4); // no leap days

    /* test #8: Born Jan 1, 1800 and about to turn 101 */
    testIt(8, 1800, 1, 1, 101, 101 * 365 + 24); // There are 26 years divisible by 4
                                // between 1800 and 1901, all of these except 1800 and 1900 are leap years
                                // so there are 24 leap days
    /* test #9: born Jan 1, 1900 and about to turn 100 */
    testIt(9, 1900, 1, 1, 100, 100 * 365 + 24); // 26 years divisible by 4
                                // 1900 was not a leap year, and the 100th brithday comes
                                // before Feb 29, 2000, so only 24 leap days.


    if (num_tests_passed == 9) {
        printf("all tests passed successfully.  Well done!\n");
    } else {
        printf("you only passed %d of the 9 tests.  Looks like you've got some work to do\n",
                num_tests_passed);
    }
}

Then in project2.c you would do this

/* ... */
int birth_year; 
int birth_month;
int birth_day;
int what_birthday;
int num_days;

/* functions... */

And that your solution composes of

• project2.h
• myproject1.c
• project2.c

Then the project should compile without any linker errors.