c - working with pointers struct automatically updated -

i have product sales management program have structure storing product data, store customer data , store sales data. when insert new sale has associated existing product serial number , id of existing customer. how guarantee when data products , customers struct changed sales struct updated?

here's have:

typedef struct{  char serialnumber[10] char description[100] float price }stproducts;  typedef struct{ int id; char name[50] char adress[100] int phonenumber }stcustomers;  typedef struct{ int idcustomersale; char serialnumberproductsale[10] float endprice; }stsales;   int main() {    stproducts pr[1000];    int countproducts =0;    stcustomers cust[500];    int countcustomers=0;     stsales sal[1000];    int countsales=0;  } 

part of function insert sale:

void insertsale(stsales sal[], int *countsales, stproduct pr[], int countproduct,stcustomers cust[], int countcustomers) { char psale[10]; int number;   consultproducts(pr, countproducts);  consultcustomers(cust,countcustomers);  printf("insert product serial number of sale:");  fgets(psale, sizeof psale, stdin);  strcpy(sal[*countsales].serialnumberproductsale,psale);  printf("insert customer id of sale:");  scanf ("%d", &number);  sal[*countsales].idcustomersale=number;  //....................   } 

for example let's imagine id field changed on customer struct, automatically customer id associated sale (idcustomersale) must updated.

the answer quite simple , fundamental principle independent of language use: avoid repeating/copying information around.

make sale refer other objects (i.e. product , customer). you'd in relational db, using foreign keys.

assuming each sale made of single product:

typedef struct {     stcustomers* customer;     stproducts* product; } stsales; 

so, let's sales kept in static array

of sales instances, have done

unsigned int n=0; // count of sales reached @ point in time stsales sal[maxsales]; stsales *p; ... p = &sal[n++]; 

or of pointers , allocated on demand

stsales *sal[maxsales]; p = malloc(sizeof(stsales)); // no error checking, demo! sal[n++] = p; 

in both cases can access fields had copied on (and other field) with

p->customer->id; p->product->serialnumber; 

i recommend manage data structures (for sales , other objects) via dynamic memory allocation.

so, assuming sal , n global variables (i.e. declared outside main, not design practice, now), insertsale function become

stsales *insertsale(stcustomers* customer, stproducts* product) {     p = malloc(sizeof(stsales)); // no error checking, demo!     if (p != null) {         sal[n++] = p;         p->customer = customer;         p->product = product;     }     return(p); } 

of course, sales instances should freed if/when deleted. leave exercise find out how you. :)