Why Does My C Program That Uses A Function To Total Months in a Year Fail? -

i've written program that's gradually grown simple structure holds information month, array of 12 structures of same information non-leap year. now, i'm trying include function that, "when given month number, returns total days in year including month. assume structure template of question 3 , appropriate array of such structures declared externally."

when press run 2 errors, neither of understand. here are: "undefined symbols architecture x86_64: "_months", referenced from: _days in main.o ld: symbol(s) not found architecture x86_64 clang: error: linker command failed exit code 1 (use -v see invocation)"

any appreciated, thanks.

#include <stdio.h> int days(int monthnum); struct month { char name[10]; char abbreviaton[4]; int days; int monthnum; }; int main(void) {  struct month months[12] = {     {"january", "jan", 31, 1},     {"february", "feb", 28, 2},     {"march", "mar", 31, 3},     {"april", "apr", 30, 4},     {"may", "may", 31, 5},     {"june", "jun", 30, 6},     {"july", "jul", 31, 7},     {"august", "aug", 31, 8},     {"september", "sep", 30, 9},     {"october", "oct", 31, 10},     {"november", "nov", 30, 11},     {"december", "dec", 31, 12}, };  struct month *sign;  sign = &months[12];  days(months[12].monthnum);  return 0; } extern struct month months[12]; int days(int monthnum) { int index, total;  if (monthnum < 1 || monthnum > 12)     return(-1); else {     (index = 0, total = 0; index < monthnum; index++)         total += months[index].days; return(total); } } 

here code have now. program works:

#include <stdio.h> int days(int monthnum); struct month { char name[10]; char abbreviaton[4]; int days; int monthnum; }; struct month months[12] = {     {"january", "jan", 31, 1},     {"february", "feb", 28, 2},     {"march", "mar", 31, 3},     {"april", "apr", 30, 4},     {"may", "may", 31, 5},     {"june", "jun", 30, 6},     {"july", "jul", 31, 7},     {"august", "aug", 31, 8},     {"september", "sep", 30, 9},     {"october", "oct", 31, 10},     {"november", "nov", 30, 11},     {"december", "dec", 31, 12}, }; int main(void) { int value; int count = 0; struct month *sign;  sign = &months;  printf("enter month number: ");  scanf("%d", &months[count].monthnum);  value = days(sign->monthnum);  printf("%d", value);  return 0; } extern struct month months[]; int days(int monthnum) { int index, total;  if (monthnum < 1 || monthnum > 12)     return(-1); else {     (index = 0, total = 0; index < monthnum; index++)         total += months[index].days; return(total); } } 

the immediate problem due line;

extern struct month months[12]; 

there no array definition reference here; months declared inside of main, else. rid of line.

next have logical issues:

days(months[12].monthnum); 

you have overrun bounds of array. arrays 0 indexed, i.e., array of twelve elements contains valid indices 0-11. 12 1 far.

as aside, not need specify dimension of array when explicitly initializing each element. use:

struct month months[] = {     {"january", "jan", 31, 1},     {"february", "feb", 28, 2},     {"march", "mar", 31, 3},     {"april", "apr", 30, 4},     {"may", "may", 31, 5},     {"june", "jun", 30, 6},     {"july", "jul", 31, 7},     {"august", "aug", 31, 8},     {"september", "sep", 30, 9},     {"october", "oct", 31, 10},     {"november", "nov", 30, 11},     {"december", "dec", 31, 12}, }; 

the compiler knows there twelve elements because told so. don't have change dimension when/if add or remove element.

next problem; variable scope. have inside of days function:

total += months[index].days; 

well, days has no access months because months local main. need study , understand variable scope.