user.h

#include <stdio.h>
#include <string.h>
struct user
{
    char user_name[21];
    char password[21];
    char security_answer[20];
   
};
struct User_node
{
    struct user u;
    struct User_node *Next; 

};

//funtions on list
struct User_node * create_user(void);
struct User_node* add_user(struct User_node **Head,struct User_node **Tail);
struct user fill_user(void);
struct User_node * user_search(struct User_node *Head,struct User_node *Tail,char user[]);
void print_user(struct user u);
int login(struct User_node *Head,struct User_node *Tail);


int readUsersFileIntoList(struct User_node** Head , struct User_node** Tail,char U_Path[]);
int writeUserIntoFile(struct user* user ,char U_Path[]  );



//create user
struct User_node * create_user()
{
    struct User_node *ptr;
    ptr=(struct User_node *)malloc(sizeof(struct User_node));
    if(ptr)
    {
        ptr-> u=fill_user();
        ptr-> Next =NULL;
    }
    return ptr;
}

struct User_node* add_user(struct User_node **Head,struct User_node **Tail)
{
    struct User_node *retval=NULL;
    struct User_node *ptr= create_user();
    struct User_node *isExist;

    if(ptr)
    {
        if(!*Head) //no list
        {
            *Head=*Tail=ptr;
        }
        else //there is a list
        {   
            //search for the same user is exist or not
            isExist=user_search(*Head,*Tail,ptr->u.user_name);
            while(isExist)
            {
                system("clear");
                printf("sorry user name already exist try again..\n\n");
                free(ptr);
                ptr=create_user();
				isExist=user_search(*Head,*Tail,ptr->u.user_name);
            }

			if(*Head==*Tail)//1 user only
			{
				(*Head)->Next=ptr; //error is here
				*Tail=ptr;

			}
			else // more than 1 user
			{
				 (*Tail)->Next=ptr;
				 *Tail=ptr;
			}

		}
        retval=ptr;
    }
    return retval;
}

//fill user
struct user fill_user(void)
{
    
    struct user u;
	char c,pass[21],*ptr;
	int l,done=0;
	strcpy(pass,"");
    printf("please enter user name: ");
    scanf("%s",u.user_name);
    printf("please enter password (max 20 chars): ");
    ptr=&c;
    while(!done)
    {
        l=0;
        do
        {
            c=getch();
            pass[l]=c;
            l++;
            // printf("%s\n",pass);
        }while(c!=10&&l<=20);
         if(l>20)
        {
            printf("\nsorry password is 20 chars max...\n enter your pass word again\n");
            strcpy(pass,"");
        }
        else
        {
             
              strcpy(u.password,pass);
              u.password[l-1]=0;
              done=1;
              
        }

    }
    // scanf("%s",u.password);
    return u;
}


//search for user
struct User_node * user_search(struct User_node *Head,struct User_node *Tail,char user[])
{
    struct User_node *ptr, *retval;
    ptr=Head;
    while(ptr)
    {
        if(!strcmp(ptr->u.user_name,user))
        {
            retval=ptr;
        }
        ptr=ptr->Next;
    }
    return retval;
}

//print user
void print_user(struct user u)
{
    printf("user: %s \t pass: %s\n",u.user_name,u.password);
}

int login(struct User_node *Head,struct User_node *Tail)
{
    char user[21],pass[21];
    struct User_node *isExist;
    int retval=0;
    printf("user name: ");
    scanf("%s",user);
    printf("\npassword: ");
    scanf("%s",pass);
    isExist=user_search(Head,Tail,user);
    if(isExist)
    {  
        if(!strcmp(pass,isExist->u.password))
        {
            
            retval=1;
        }
    }
    return retval;
}

int writeUserIntoFile(struct user* user ,char U_Path[]  )
{
    int retval=0;
    FILE *outfile;
    outfile=fopen(U_Path,"a");
    //check if the file opened
    if(outfile==NULL)
    {
        fprintf(stderr, "\nError opening the file .. data may not be saved \n"); 
    }
    else
    {
         // writing the content of user struct into the file 
         fwrite(user,sizeof(struct user),1,outfile);
         fclose(outfile);
         retval=1;
    }
    return retval;
}
int readUsersFileIntoList(struct User_node** Head , struct User_node** Tail,char U_Path[])
{
    int retval=0;
    struct user input;
    FILE *inFile;
    inFile=fopen(U_Path,"r");
    if(inFile!=NULL)
    {
        while(fread(&input,sizeof(struct user),1,inFile))
        {
            struct User_node *ptr;
              ptr=(struct User_node *)malloc(sizeof(struct User_node));
             if(ptr)
             {
                ptr-> u=input;
                 ptr-> Next =NULL;
             }
             if(!*Head)
             {
                 *Head=*Tail=ptr;
             }
             else
             {
                 if(*Head==*Tail)//1 user only
			    {
				    (*Head)->Next=ptr; 
				    *Tail=ptr;

			    }
		    	else // more than 1 user
			    {
				     (*Tail)->Next=ptr;
				     *Tail=ptr;
			    }
             }

        }
        retval=1;
    }
    else
    {
        fopen(U_Path,"w");//create file if first use
        retval=2;
    }
    return retval;
}