Using Integer Variable as Index in scanf

Question

I'm trying to do the below in my function but keep getting a segmentation fault error. It's failing when I'm trying to use [iModify - 1] as my index.

Can you not use an int variable calculation as an index in C?

int modify(pb *PhoneBook)
{
   int x;
   int iModify = 0;
   char name_num[] = {'\0'};
   print(PhoneBook);
   printf("\nWhich entry would you like to modify? ");
   scanf("%d", &iModify);
   printf("\niModify - 1: %d\n", iModify - 1);

   printf("\nModify name or number? ");
   scanf("%s", name_num);
   convert_u(name_num);
   if (strcmp(name_num, "NAME") == 0) {
      printf("\nEnter new name: ");
      scanf("%s %s", PhoneBook[iModify - 1].cFirstName, PhoneBook[iModify - 1].cLastName); //fails here
   }
   else if (strcmp(name_num, "NUMBER") == 0) {
      printf("\nEnter new number: ");
      scanf("%s", PhoneBook[iModify - 1].cNumber); //also fails here
   }
}

Answer 1

The problem here is

 char name_num[] = {'\0'};

here, name_num is having a length of 1 char, which will not be sufficient for holding a string at a later point. So, when you do

scanf("%s", name_num);

you're essentially writing out of bound which invokes undefined behavior.

Reference: C11, chapter §6.7.9

If an array of unknown size is initialized, its size is determined by the largest indexed element with an explicit initializer. [...]

To compare with your code, name_num is an array of unknown size which is being initialized by only a single element in a brace enclosed initializer, so the size of the array will be 1.

Solution: You have to mention the size explicitly at the time of definition. You'll be needing something like

char name_num[32] = {'\0'};     //32 is just for example purpose
....
scanf("%31s", name_num);  // make sure longer inputs don't blow up the buffer

or similar.

---

Having said that, please notice your int modify() function does not return any value. If the returned value is used in the caller, it will again invoke undefined behavior.

Answer 2

You have used char name_num[] = {'\0'}; in your code. name_num has the length of 1 char, which is too short to hold a string later.

Therefore, you are writing out-of-bounds when you read name_num.