Compare password to a given set of passwords - c

Question

I have got an assignment from uni which mentions that I have to compare a given set of passwords to the password given by the user. The set of passwords are predetermined in the question as follows
const char *passwd[NUM_PASSWDS] = {
"123foo", "bar456", "bla_blubb"
};
and has to be compared with input from user...
So I have written my code as follows;
#include <stdio.h>
#include <string.h>
#define NUM_PASSWDS 3
const char *passwd[NUM_PASSWDS] = {
"123foo", "bar456", "bla_blubb"
};
int pwdquery(char pass[]) {
for (int i = 0; i < 2; i++) {
if (passwd[i] == pass[i]) {
return printf("correct");
}
}
}
int main() {
char a[100];
for (int i = 0; i < 3; i++) {
printf("Please enter password");
scanf("%s", a);
}
pwdquery(a);
}
When I tried running my code, it shows an error...
Thank you for your time

Answer 1

To compare string you need to strcmp function.
printf returns number of character printed. You should return fixed value on success or failure of password match. In my example I'm returning 1 in case of password match or 0 in case of no-password match.
In main function scanf function with %s argument can read entire string. You don't need for-loop to read input string.
#include<stdio.h>
#include<string.h>
#define NUM_PASSWDS 3
const char* passwd[NUM_PASSWDS] = {
"123foo", "bar456", "bla_blubb"
};
int pwdquery(char pass[]) {
for (int i = 0; i < NUM_PASSWDS; i++)
{
if (strcmp(pass, passwd[i]) == 0)
{
return 1; // One means currect password
}
}
return 0; // Zero means incorrect password password
}
int main() {
char a[100];
printf("Please enter password");
scanf("%s", a);
if(pwdquery(a) == 1)
{
printf("Password entered is correct");
}
else
{
printf("Password entered is incorrect");
}
}
With custom strcmp
#include<stdio.h>
#include<string.h>
#define NUM_PASSWDS 3
const char* passwd[NUM_PASSWDS] = {
"123foo", "bar456", "bla_blubb"
};
//Returns 0 if string don't match else returns 1
int user_strcmp(const char *str1, const char *str2)
{
size_t str1_size = strlen(str1);
size_t str2_size = strlen(str2);
if(str1_size != str2_size)
{
return 0; //String are not of same size. The strings won't match.
}
for(size_t i = 0; i < str1_size; i++)
{
if(str1[i] != str2[i])
{
return 0;
}
}
return 1;
}
int pwdquery(char pass[]) {
for (int i = 0; i < NUM_PASSWDS; i++)
{
if (user_strcmp(pass, passwd[i]) == 1)
{
return 1; // One means currect password
}
}
return 0; // Zero means incorrect password password
}
int main() {
char a[100];
printf("Please enter password");
scanf("%99s", a);
if(pwdquery(a) == 1)
{
printf("Password entered is correct");
}
else
{
printf("Password entered is incorrect");
}
}

Answer 2

There are multiple problems in your code:
the password comparison function pwdquery compares a character and a pointer: this is incorrect. You should use strcmp to compare the strings and return a non zero value if one of these comparisons returns 0.
in the loop, you ask for the password 3 times, but you only check the password after this loop, hence only test the last one entered.
scanf("%s", a) may cause a buffer overflow if the user enters more than 99 characters without white space. Use scanf("%99s", a) to tell scanf the maximum number of characters to store into a before the null terminator.
Here is a modified version:
#include <stdio.h>
#include <string.h>
#define NUM_PASSWDS 3
const char *passwd[NUM_PASSWDS] = {
"123foo", "bar456", "bla_blubb",
};
int pwd_check(const char *pass) {
for (size_t i = 0; i < NUM_PASSWDS; i++) {
if (passwd[i] && strcmp(passwd[i], pass) == 0)
return 1;
}
return 0;
}
int main() {
char a[100];
for (int i = 0; i < 3; i++) {
printf("Please enter password: ");
if (scanf("%99s", a) != 1) {
printf("missing input, aborting\n");
return 1;
}
if (pwd_check(a)) {
printf("correct!\n");
return 0;
}
printf("incorrect password\n");
}
printf("too many errors, aborting\n");
return 1;
}