File Handling in C(using fread & fwrite)

Question

doing this simple program. This is just the firt part of a banking service program. Somehow, I am stuck with this fread() . I cant compare the input of the user and my database. When my program starts, after I input user and pass it kinda 'hang' or 'freeze' then a popup will appear and says "end program". BTW I am using Dev C++.

#include<stdio.h>
#include<string.h>

struct client
{
    char accnum[9];
    char accode[5];
    char fname[20];
    char lname[20];
}s;




main()
{
    FILE *fp;

    char user[9];
    char pass[5];

    fp=fopen("account.txt","a");
    if(fp!=NULL)
    {
        /**
        strcpy(s.accnum,"abcd1234");
        strcpy(s.accode,"1234");
        strcpy(s.fname,"john");
        strcpy(s.lname,"doe");      
        fwrite(&s,sizeof(s),1,fp);
        **/

        printf("BANKING SERVICE");
        printf("\nInput User: ");
        gets(user);

        printf("\nInput Pass: ");
        gets(pass);


        while(!feof(fp))
        {
                while(fread(&s,sizeof(s),1,fp)==1);
                {


                    if(ferror(fp))
                    {
                        printf("error");
                    }
                    if (strcmp(user,s.accnum) == 0 && strcmp(pass,s.accode) == 0)
                    {
                        printf("\n\nsuccess!");
                    }

                    else 
                    {
                                printf("\n\nerror!");
                    }

                }
        }

        fclose(fp);
    }






    fclose(fp);
    getch();
}

score 1 · Accepted Answer · answered Dec 21 '14 at 12:02

In the loop check for successful match. If no match is found, the found flag will still be zero. Then report the problem after the while loop has completed.
fgets will include the newline so it has to be removed before comparing to the contents of the file as the commented section does not show any newlines.

#include<stdio.h>
#include<string.h>

struct client
{
    char accnum[9];
    char accode[5];
    char fname[20];
    char lname[20];
}s;

int main()
{
    FILE *fp;
    int found = 0;
    char user[20];//allow extra
    char pass[20];

    fp=fopen("account.txt","a");
    if(fp==NULL)
    {
        perror ("could not open file ");
        return 1;
    }
    /**
    strcpy(s.accnum,"abcd1234");
    strcpy(s.accode,"1234");
    strcpy(s.fname,"john");
    strcpy(s.lname,"doe");
    fwrite(&s,sizeof(s),1,fp);
    **/
    printf("BANKING SERVICE");
    printf("\nInput User: ");
    fgets(user, sizeof ( user), stdin);
    if ( user[strlen(user)-1] == '\n') {
        user[strlen(user)-1] = '\0';//remove newline
    }
    printf("\nInput Pass: ");
    fgets(pass, sizeof ( pass), stdin);
    if ( pass[strlen(pass)-1] == '\n') {
        pass[strlen(pass)-1] = '\0';//remove newline
    }
    while(fread(&s,sizeof(s),1,fp)==1)
    {
        if (strcmp(user,s.accnum) == 0 && strcmp(pass,s.accode) == 0)
        {
            printf("\n\nsuccess!");
            found = 1;
            break;
        }
    }
    if ( found == 0)
    {// after entire file has been read, report problem
        printf("\n\nno match for that account and passcode!");
    }
    fclose(fp);
    getchar();
    return 0;
}

Midhun MP · Answer 2 · 2014-12-21T08:17:09.910

0

You need to check for end of file.

while( !feof( fp ) )
{
   fread(&s,sizeof(s),1,fp);
   if( ferror( fp ) )
   {
     // Error occurred
     break;
   }

   if (strcmp(user,s.accnum) == 0 && strcmp(pass,s.accode) == 0)
   {
       printf("\n\nsuccess!");
   }
   else 
   {
       printf("\n\nerror!");
   }
}

edited Dec 21 '14 at 08:17

answered Dec 21 '14 at 08:10

Midhun MP

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3

`feof()` should not be used Check this link: http://stackoverflow.com/questions/5431941/while-feof-file-is-always-wrong – Gopi Dec 21 '14 at 08:14
1

@Gopi: read his answer carefully. ` If fread() returns 0, you must use feof/ferror to decide.` – Midhun MP Dec 21 '14 at 08:19
@MidhunMP: You need to test the return value of `fread()` (or `feof()`) before using `s`. The end-of-file indicator will not be set until **after** a failed attempt to read from the file. – Nisse Engström Dec 21 '14 at 11:08

File Handling in C(using fread & fwrite)

2 Answers2