Whats wrong with fread in this program?

Question

I\'m intermediate student of C. I\'m trying to make a bank management program but first I need to make a login program, so I created one of the following. As I\'ve recently lear

Answer 1

The problem is that you have an ASCII/text file but you are trying to use fread to read directly into a structure; this requires a binary formatted file. fread cannot do format conversion. Use fscanf instead:

fscanf(fp, "%s %d", &log_in.u_name, &log_in.u_pin);

Answer 2

a) fread is used to read fixed sized (in bytes) records. While that is an option, you might find that fscanf is more suited to your use case.

b) goto has its uses, arguably, but in your particular case a while loop is far more appropriate as it make the intention clearer and is less susceptible to misuse.

c) The crux of your issue is reading the file. You only read in and check against a single record from your file per pass and once all records are read you will be getting EOF rather than additional records.

You have a few options. You might read the entire user table into a list prior to your login loop. When a user logs in you must iterate through the list until a match or end of list is found. Alternatively you might loop through the entire file on each user attempt looking for a match and seek to the back to the beginning of the file when you are done. In either case you must check for and handle read errors.

d) Finally when a non-digit is entered you will probably find that backspace isn't what you had in mind. Also you might consider allowing the user to press backspace while entering the pin.

Answer 3

Problems that I see:

1. The following line is incorrect.

   scanf("%s",&t_name);
   

   You need to use:

   scanf("%29s",t_name);
   

2. fread is not the right solution given the file format. fread works when the file is in binary format.

   Given the format of your input file, you need to use:

    scanf("%29s %d", log_in.uname, &log_in.u_pin); 
   

3. Comparing the pins using log_in.u_pin == t_pin should produce a compiler error. The type of log_in.u_pin is int while the type of t_pin is int [4]. You will have to change your strategy for getting the integer value from t_pin.

   You can use something like:

   int pin = 0; 
   for (i = 0; i < 4; ++i )
   {
      pin = 10*pin + t_pin[i];
   }
   

   However, for that to work, you have store the proper integer values in t_pin. When the character you read is '1', you have to store 1. Either that, or you will have to change the above for loop to:

      pin = 10*pin + t_pin[i]-'0';
   

4. The way are using goto start to loop in your function is not correct. You haven't read all the user ID and pins to from the input file and compared them. The way you have it, you will end up doing:

   • Read the user name and pin

   • Check against the first entry in the file. If they don't match...

   • Read the user name and pin again.

   • Check against the next entry in the file. If they don't match...

   etc.

   You want to:

   • Read the user name and pin

   • Check against the first entry in the file. If they don't match...

   • Check against the next entry in the file. If they don't match...
   • Check against the next entry in the file. If they don't match...

   etc.