Why is my Struct member changing when it isn't supposed to? - c

Question

I've written this code in C where I want to increment the balance of a account if it is found with a matching string ID using the increaseBalance function, and although it does increment the balance value, it also changes the value of the structs ID as well. Here is my code
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#define MAX_ACCOUNT_NUM 1000
#define NAME_LEN 15
#define ACCOUNT_NUM_LEN 6
#define FILE_LEN 20
struct bank_account
{
char owner_first_name[NAME_LEN], owner_last_name[NAME_LEN], account_num[ACCOUNT_NUM_LEN];
int balance;
};
typedef struct bank_account Account;
void reg_new_acc(Account account_reg[], int *nrOfAccounts);
Account create_user(char owner_first_name[], char owner_last_name[], char account_num[]);
void accountManagement(Account account_reg[],int *nrOfAccounts);
void print_account(Account account_reg[], int *pNrOfAccounts);
int checkAccountNum(Account account_reg[],char account_num[], int *nrOfAccounts);
void increaseBalance(Account account_reg[], char accountNum[], int amount, int *nrOfAccounts);
int main(void)
{
Account account_reg[MAX_ACCOUNT_NUM];
int nrOfAccounts = 0;
char accountFile[FILE_LEN];
//readFromFile(account_reg, &nrOfAccounts, accountFile);
accountManagement(account_reg, &nrOfAccounts);
return 0;
}
void accountManagement(Account account_reg[],int *nrOfAccounts)
{
int choice = 0;
do
{
printf("(1) Create New Account \n(2) Print All Accounts\n(3) Increment Account Balance\n");
printf("\n\nEnter Number: ");
scanf("%d", &choice);
switch (choice)
{
case 1:
reg_new_acc(account_reg, nrOfAccounts);
break;
case 2:
print_account(account_reg, nrOfAccounts);
break;
case 3:
printf("Please Enter the Account Number: ");
char accountNumber21[ACCOUNT_NUM_LEN];
scanf("%126s", accountNumber21);
printf("Please Enter the Amount: ");
int amount=0;
scanf("%d", &amount);
increaseBalance(account_reg, accountNumber21,amount, nrOfAccounts);
break;
}
} while ( choice != 9);
}
void reg_new_acc(Account account_reg[], int *nrOfAccounts)
{
char owner_first_name[NAME_LEN], owner_last_name[NAME_LEN], account_num[ACCOUNT_NUM_LEN];
while (*nrOfAccounts < MAX_ACCOUNT_NUM)
{
printf("\nRegistering User\n");
printf("Please enter (6-dig) ID number (q for quiting): ");
scanf("%s%*c", account_num);
if (strcmp(account_num, "q") == 0)
{
printf("Avslutar\n");
return ;
}
while(checkAccountNum(account_reg, account_num, nrOfAccounts)==0)
{
printf("This Already Exists\n");
printf("Please Enter Again: ");
scanf("%s%*c", account_num);
}
printf("Enter Name and Lastname: ");
scanf(" %s",owner_first_name);
scanf(" %s", owner_last_name);
account_reg[*nrOfAccounts] = create_user(owner_first_name, owner_last_name, account_num);
(*nrOfAccounts)++;
}
}
int checkAccountNum(Account account_reg[],char account_num[], int *nrOfAccounts)
{
if(strcmp(account_num, "q") == 0)
{
accountManagement(account_reg , nrOfAccounts);
}
for(int i = 0; i < *nrOfAccounts; i++)
{
if(strcmp(account_num,account_reg[i].account_num) == 0)
{
return 0;
}
}
return 1;
}
Account create_user(char owner_first_name[], char owner_last_name[], char account_num[])
{
Account account;
strcpy(account.account_num, account_num);
strcpy(account.owner_first_name, owner_first_name);
strcpy(account.owner_last_name, owner_last_name);
account.balance=0;
return account;
}
void print_account(Account account_reg[], int *pNrOfAccounts)
{
int i;
printf("\nPrinting All Accounts\n");
printf("ID\tFull Name\t\tBalance (kr)\n");
printf("____________________________________________\n");
for(i = 0; i < *pNrOfAccounts; i++)
{
printf("%s\t\t%s\t%s\t\t%d\n\n", account_reg[i].account_num, account_reg[i].owner_first_name, account_reg[i].owner_last_name, account_reg[i].balance);
}
}
void increaseBalance(Account account_reg[], char accountNum[], int amount, int *nrOfAccounts)
{
for(int i = 0; i < *nrOfAccounts; i++)
{
if(strcmp(accountNum,account_reg[i].account_num) == 0)
{
printf("In Increase");
account_reg[i].balance+=amount;
}
}
}
If you run this code and after adding a user, try to increment the user account balance and then print out the user details, you will see that something strange has happened to the user ID number. I don't know why this is happening. Thank you for the time.

Answer 1

From the code, change the value for ACCOUNT_NUM_LEN to 7 to take care of the null terminator. This should fix your problem.
Remember that c string are null terminated. The null terminator also takes up some space and should be accounted for.

Answer 2

"(6-dig) ID number" need 'char[7]' minimum, but ACCOUNT_NUM_LEN == 6
In function reg_new_acc(), after enter "123456" for account and "last" for owner_last_name, account variable contains "123456last".
In function create_user() strcpy(account.account_num, account_num) write "123456" to account.account_num, "last" to account.balance and '\0' cause 1-byte buffer overflow.
But operator 'account.balance=0' reset balance to 0 and account.account_num to "123456", because next byte write to '\0'.
After increaseBalance() account_reg[i].balance not 0. Therefore, the string account. account_num can get non-zero bytes after "123456" and will be printed as " 123456???".
P.S. Strictly speaking, in this program there is not a single correct operation with strings and not only with strings.
P.P.S. A good idea would be to use the latest versions of gcc or clang with stack-protector, mudflap, and _FORTIFY_SOURCE enabled, which provide some control over the use of arrays for storing strings.