Authentication in C (read and check from given file) - c

Question

I have created the code below. What i need to do is that I need to get and input from the user(username and password) and I need to check with a file what I entered. The Problem forces me not to copy the whole .txt file in once. The .txt file is converted like this:
root
jfl34jgf
ben
12b43
stella
ldfo421
kate
jfd45g
bill
iu3556
guest
1234
test
1234
So I was thinking of getting all the user names, putting them into a list and then checking each one of them with the input. I created a struct that includes the username name, the position at which username name ends (using ftell()) and the next pointer that points to the next element. As you can see the text is formatted in the way that one like is username and the other is the password. so the password comes right after the username. using ftell() for each username if the comparison of name gives me true than I can check for the password check.
This is my code until now.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct names
{
char name[20];
int pos;
struct names *next;
};
typedef struct names Names;
void create_list(Names *l, char *temp1, int pos)
{
Names *temp;
temp=(Names*)malloc(1*sizeof(Names));
temp->next=NULL;
strcpy(temp->name, temp1);
temp->pos=pos;
temp->next=l;
printf("%s ", temp->name);
l=temp;
printf("%s\n", l->name);
}
void create_list_from_File(FILE *fp, Names *l)
{
char ch;
int i=0, check=0, pos;
char temp1[20];
temp1[0]='\0';
while(1)
{
if(check==1)
break;
while((ch=fgetc(fp))!='\n')
{
temp1[i]=ch;
i++;
}
temp1[i]='\0';
pos=ftell(fp)+1;
create_list(l,temp1, pos);
while((ch=fgetc(fp))!='\n')
{
if(ch==EOF)
{
printf("EOF ");
check=1;
break;
}
}
i=0;
}
}
void display(Names *l)
{
Names *cursor;
cursor=l;
while (cursor!=NULL)
{
printf("%s\n", cursor->name);
cursor=cursor->next;
}
}
int main()
{
char usern[20][20];
char passw[20];
FILE *fp;
Names l;
l.next=NULL;
char c;
int i=0;
fp=fopen("users.txt", "r");
if(fp==NULL)
{
printf("File not opened!\n");
exit(1);
}
create_list_from_File(fp, &l);
display(&l);
/* fgets(usern, 20, stdin);
usern[strlen(usern)-1]='\0';
while(strcmp(usern, "exit")!=0)
{
fgets(passw,20, stdin);
passw[strlen(passw)-1]='\0';
check(fp, usern, passw);
}
*/
return 0;
}
Right now the I do not see stuff inside the linked list. I am getting the strings from the file correctly ( the username are printed) but when I try to print the list it just gives me some weird values. Help would be much appreciated.

Answer 1

The provided source code presents a classical error when managing linked-list. Modifying the content of a pointer inside a function when passed as value as only a local effect and doesn't affect the value in the calling function.
Problem - Modifying the value of Names *l in the function create_list().
The function create_list() adds a new node Names *temp; in front of the list to increase performance by modifying only the first node.
The new node is pointing to the first temp->next=l;,
Then the new node becomes the first l=temp;.
When printing locally values, seems no problem.
But, the assignment l=temp; is only available inside the function (assignment to a local Names *l pointer) and when return to the calling function create_list_from_File() the l doesn't change.
Solution 1 - Modify the value of the l->next is the fastest solution.
In the main(), a starting Names l; has been declared with an assignment l.next=NULL; meaning (list empty).
In the create_list() function, the assignment is:
strcpy(temp->name, temp1);
temp->pos=pos;
// assigning to the next pointer
temp->next=l->next;
printf("%s ", temp->name);
// modify the next pointer
l->next=temp;
printf("%s\n", l->next->name);
Instead of:
strcpy(temp->name, temp1);
temp->pos=pos;
temp->next=l;
printf("%s ", temp->name);
l=temp;
printf("%s\n", l->name);
To be complete, for that shortest solution, the function display() shall start from the l->next which is the first node.
void display(Names *l)
{
Names *cursor;
cursor=l->next;
while (cursor!=NULL)
{
printf("%s\n", cursor->name);
cursor=cursor->next;
}
}

Answer 2

Thank you #J.Piquard for the help given above. I managed to solve this problem. I am posting the code below and the code can be tested in the above .txt file. Firstly an user enters the input and after that the username followed by a password. It is assumed that the username input will be always correct, but to check username is not that hard.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct names
{
char name[20];
int pos;
struct names *next;
};
typedef struct names Names;
Names *create_list(Names *l, char *temp1, int pos)
{
Names *temp;
temp=(Names*)malloc(1*sizeof(Names));
strcpy(temp->name, temp1);
temp->pos=pos;
temp->next=l;
return temp;
}
Names * create_list_from_File(FILE *fp)
{
char ch;
int i=0, check=0, pos;
Names *l=NULL;
char temp1[20];
temp1[0]='\0';
while(1)
{
if(check==1)
break;
while((ch=fgetc(fp))!='\n')
{
temp1[i]=ch;
i++;
}
temp1[i]='\0';
pos=ftell(fp)+1;
l=create_list(l,temp1, pos);
while((ch=fgetc(fp))!='\n')
{
if(ch==EOF)
{
check=1;
break;
}
}
i=0;
}
return l;
}
int check_usrn(char *s, Names *l, int *i)
{
int flag=0;
Names *cursor=l;
*i=0;
while (cursor!=NULL)
{
if(strcmp(cursor->name, s)==0)
{
flag=1;
break;
}
cursor=cursor->next;
(*i)++;
}
return flag;
}
int check_passw(FILE *fp, Names *l, char *passw, int *i)
{
Names * cursor;
cursor=l;
int m=0, flag=0;
char c, temp[20];
while(m!=*i)
{
cursor=cursor->next;
m++;
}
m=0;
fseek(fp, cursor->pos-1, SEEK_SET);
while((c=fgetc(fp))!='\n')
{
if(c==EOF)
break;
temp[m]=c;
m++;
}
temp[m]='\0';
if(strcmp(passw, temp)==0)
flag=1;
return flag;
}
int main()
{
char usern[20];
char passw[20];
char file_name[20];
FILE *fp;
Names *l=NULL;
int i=0;
fgets(file_name, 20, stdin);
file_name[strlen(file_name)-1]='\0';
fp=fopen(file_name, "r");
if(fp==NULL)
{
printf("File not opened!\n");
exit(1);
}
l=create_list_from_File(fp);
while(strcmp(usern, "exit")!=0)
{
fgets(usern, 20, stdin);
usern[strlen(usern)-1]='\0';
if(check_usrn(usern, l, &i)==1)
{
fgets(passw, 20, stdin);
passw[strlen(passw)-1]='\0';
if(check_passw(fp ,l, passw, &i)==1)
printf("Access to user %s is granted.\n", usern);
else
printf("Access to user %s is denied.\n", usern);
}
}
printf("Exiting ...\n");
fclose(fp);
return 0;
}