Turbo C Structures and Writing to file help

PHGDAL · 08-02-2009, 01:31 PM

Hey everyone, I'm basically very new at this and I need to finish a program to finish my C programming course. My main problem is when writing to a file the information of a structure. It works fine, it writes to the file but it terminates the program as soon as the function ends, and it's not supposed to do that. I've been trying to debug for 2 hours now, and still nothing... Can anyone show me some examples of 'fread' and 'fwrite' in use? I don't want to paste my entire current program because it's quite long, but here's the function that I'm having trouble with:

Code:

void f_add_student(void)
{
 int lv_id;
 student_rec s_students;
 char lv_continue;
 int lv_cancel;
 clrscr();
 if((file_student_rec=fopen(c_student_rec,"rb+"))==NULL)
  {
   f_dialog_border();
   gotoxy(13,24);
   printf("First student addition, a file will be created to store your");
   gotoxy(37,27);
   printf("information.");
   getch();
   s_students.id=1;
   clrscr();
   file_student_rec=fopen(c_student_rec,"a+");
  }
 else
  {
   do{
    lv_id=s_students.id;
    fread(&s_students,sizeof(s_students),1,file_student_rec);
   }while(!feof(file_student_rec));
   s_students.id=lv_id+1;
  }
 f_clear_variable(s_students.fname);
 f_clear_variable(s_students.lname);
 f_clear_variable(s_students.address);
 f_clear_variable(s_students.phone);
 s_students.age=0;
 s_students.gender=0x00;
 do{
  clrscr();
  gotoxy(36,14);
  printf("Add Student");
  gotoxy(15,17);
  printf("ID: %05i",s_students.id);
  gotoxy(15,19);
  printf("First Name:");
  gotoxy(15,21);
  printf("Last Name:");
  gotoxy(15,23);
  printf("Address:");
  gotoxy(15,25);
  printf("Age:");
  gotoxy(15,27);
  printf("Gender:");
  gotoxy(15,29);
  printf("Phone Number:");
  f_main_border();
  fflush(stdin);
  gotoxy(28,19);
  gets(s_students.fname);
  gotoxy(27,21);
  gets(s_students.lname);
  gotoxy(24,23);
  gets(s_students.address);
  gotoxy(20,25);
  scanf("%i",&s_students.age);
  fflush(stdin);
  gotoxy(23,27);
  scanf("%c",&s_students.gender);
  fflush(stdin);
  gotoxy(30,29);
  gets(s_students.phone);
  f_dialog_border();
  printf("Are you sure you want to add this student to student records[y/n]?");
  scanf("%c",&lv_continue);
  fflush(stdin);
  if(lv_continue=='n' || lv_continue=='N')
   lv_cancel=1;
  else
   lv_cancel=0;
 }while(lv_cancel==1);
 s_students.s_grades.History=0;
 s_students.s_grades.Geography=0;
 s_students.s_grades.Math=0;
 s_students.s_grades.English=0;
 s_students.s_grades.Humanities=0;
 s_students.s_grades.Science=0;
 s_students.s_grades.Art=0;
 s_students.s_grades.PE=0;
 fwrite(&s_students,sizeof(s_students),1,file_student_rec);
 fclose(file_student_rec);
}

MK27 · 08-02-2009, 02:04 PM

fflush(stdin) will result in undefined behavior:

Quote:

Originally Posted by C Draft Standard 2005

If stream points to an output stream or an update stream in which the most recent operation was not input, the fflush function causes any unwritten data for that stream to be delivered to the host environment to be written to the file; otherwise, the behavior is undefined.

This is a common misuse of fflush(). Get rid of it.

PHGDAL · 08-02-2009, 02:07 PM

How do I clear the input stream without fflush(stdin)?

GL.Sam · 08-02-2009, 02:30 PM

Omg, it looks horrible. First off, you definetely should get rid of unneeded gotoxy() instances. It is non-standard C, there are plenty possibilities of using escape sequences instead, but the main reason is darn uglyness.

>How do I clear the input stream without fflush(stdin)?

The general example goes like this:

Code:

while (getchar() != '\n')
  continue;

Also,

Code:

fread(&s_students,sizeof(s_students),1,file_student_rec);
fwrite(&s_students,sizeof(s_students),1,file_student_rec);

should be

Code:

fread(&s_students,sizeof(student_rec),1,file_student_rec);
fwrite(&s_students,sizeof(student_rec),1,file_student_rec);

MK27 · 08-02-2009, 02:34 PM

Quote:

Originally Posted by PHGDAL

How do I clear the input stream without fflush(stdin)?

This is actually the 2nd time this has come up today. You may want to read this thread from yesturday for more info:

fgetchar() doesnt work

**Bubba** · 08-02-2009, 06:17 PM

First piece of advice I have is get MSVS Express 2008 and dump Turbo C. MSVS can compile C or C++ or both and.

PHGDAL · 08-02-2009, 10:21 PM

Thanks everyone, I'll give it a try. Unfortunately I can't use any other program, my course demands that I use turbo C for this project otherwise I don't pass. For the gotoxy() bits it's to get the writing inside the borders I drew with a different function. I don't know any other way to do it...

**Salem** · 08-02-2009, 11:14 PM

Ask your tutor why they insist on using a compiler which is older than the students, and is obsolete both as a compiler and the OS it was intended to run on.

Who is paying for your course, and will your qualification be worth anything in the real world where NOBODY else uses that fossil?

PHGDAL · 08-02-2009, 11:38 PM

I thought the same thing too. But it's really only a simple 24-hour long course (3 hours a day, 2 days a week) just so I can get the basics of the language. I'll probably take a more useful course on C++ or C# later.

BEN10 · 08-02-2009, 11:39 PM

Quote:

Originally Posted by Salem

Ask your tutor why they insist on using a compiler which is older than the students, and is obsolete both as a compiler and the OS it was intended to run on.

Who is paying for your course, and will your qualification be worth anything in the real world where NOBODY else uses that fossil?

There are a lot of universities who still teach C using TC, and you can't say anything to them like "Why are you using this fossil" coz it's going on for long and nobody cares what you say? So, IMO, the OP should run the code on TC and on any modern compiler for the sake of his/her understanding.

PHGDAL · 08-03-2009, 12:00 AM

Ok, I changed all the fflush(stdin)'s to getchar()!='\n' and I also changed

Code:

fread(&s_students,sizeof(s_students),1,file_student_rec);
fwrite(&s_students,sizeof(s_students),1,file_student_rec);

to

Code:

fread(&s_students,sizeof(student_rec),1,file_student_rec);
fwrite(&s_students,sizeof(student_rec),1,file_student_rec);

but the problem is still there, the program exits after ending the function instead of returning to the last function. I attached the *.cpp file of the whole program just in case it helps. Thanks everyone in advance!

MK27 · 08-03-2009, 01:57 AM

Okay, I don't have windows installed so I cannot test this code, but I looked at it for a while and here's a preliminary observation: You use a flag, "lv_choice", quite a few times, in void() functions with a switch in a while loop to "end" the function:

Code:

  }while(lv_choice==0);   //End of while loop.
}

You can return from a void function. In other words, rather than doing all that with the lv_choice flag, in the cases where you set the flag:

Code:

    case 5:              //Logout.
    {
    f_logout();
    lv_choice=1;
    break;
    }

You should just replace "break" with "return", change the loop to an infinite one:

Code:

while (1) {
       [...code...]
}

and remove all references to lv_choice. This will

simplify the code
reduce the usage of memory by eliminating a superfluous variable
improve efficiency by removing a number of superfluous operations

Those last two are sort of minor, on this scale, but they are indicative of how you should be thinking about programming; the first one is significant here IMO, because it amounts to a lot of clutter. In any case, these two perogatives (simplifying where ever possible, and streamlining the actual functionality of the process) go hand in hand.

Also, using a tabstop of at least two characters would be nice.

As for your problem, here is a strong possibility (again, I can't test this because of conio.h): it has to do with flushing the input buffer again. Did you read the thread I pointed you to to do with how '\n' gets left in the input buffer by scanf if you use %d %i or %c?

Let's look at the flow of execution with that in mind. You believe f_add_student() is to blame. f_add_student gets called here, in f_student_options():

Code:

  scanf("%i",&lv_choice);
  switch(lv_choice)
   {
    case 1:                        //Add Student.
     f_add_student();
     lv_choice=0;
     break;

Notice, before you even begin, there is an '\n' left in the buffer. This will not get picked up by a %d or %i but it will get into a %c. That will get taken care of by your "while(getchar()!='\n')", altho these all have a strange form:

Code:

  while(getchar()!='\n')
      s_students.gender;

This loop does not need to perform an operation, altho "s_students.gender" is a meaningless no-op anyway. It should just be:

Code:

while(getchar()!='\n');

This will repeat the call in the condition as long as the condition is met, which is all you want. However, I still think this is a better, less confusing way to deal with the '\n':

Code:

scanf("%i%*c",&s_students.age);

which is explained in that thread I pointed you to earlier. You're free to think about that. It sort of short cuts verifying the input, but you are not performing any verification anyway (you presume the user did not accidently go "asdf\n") and that is fine for now.

In any case, I think your problem is here:

Code:

  printf("Are you sure you want to add this student to student records[y/n]?");
  scanf("%c",&lv_continue);
  if(lv_continue=='n' || lv_continue=='N')
   lv_cancel=1;
  else
   lv_cancel=0;
 }while(lv_cancel==1);

You have that one last scanf("%c") call, with no flushing of the buffer, right before you return to f_student_options

Code:

  f_add_student();
  lv_choice=0;

where supposedly lv_choice is set to zero. Except you still have a '\n' in the buffer, so when the f_student_options() loop repeats (too fast to see), you may trigger this

Code:

  printf("Input:");
  f_main_border();
  gotoxy(23,29);
  scanf("%i",&lv_choice);

  switch(lv_choice)

In the wrong way. Kind of unlikely, but worth considering.

Partial moral: Everytime you add a variable, you are adding a complication. Unnecessary complications contribute to -- unnecessary complications....

But my final and most serious criticism is this: you should not have written this with all the conio stuff in initially. First, you should have gotten everything to work in the most simple way; when the essential structure of the program can be demonstrated to work, then you add all the fancy interface stuff. One major advantage to that would be you would then know if the error was actually part of the program flow, or taking place in one of the subroutines and procedures that add the cosmetics. On this scale, with a project like this, there is no excuse for not verifying EVERYTHING works the way you think it should every ten or so lines at most. I suspect you wrote a lot more than that all at once figuring the error checking capacity of the compiler could solve your typos and stuff at the end. And/or: that you were more pre-occupied with making the conio output correct than with the real "engine" of the code. If that is the case, never do it again!

Adak · 08-03-2009, 02:22 AM

I will substitute the letter 'z' for p, since my keyboard has just now lost a few keys!

I believe the zroblem is with fread (which is correct for binary mode), and feof(), which is designed for text mode.

At the end of file, fread() will return a short count of items, (in your case, a zero), but feof() will return a non-zero (usually a -1), on end of file.

So change your feof(), (rezlace it), with simzly while(fread, etc., and the file zroblem may disazzear. Like this:

Code:

    //fread(&s_students,sizeof(student_rec),1,file_student_rec); <=original line
  }while(fread(&s_students,sizeof(student_rec),1,file_student_rec)); //new line of code
//   }while(!feof(file_student_rec)); <=original line

BTW, I use Turbo C/C++ 1.01, a great deal, and the criticism it's received here are both valid, and utter craz. It is horribly out of date, *but* it is simzle to set uz, and have the students szend a minimum of time learning the IDE, and a maximum of time learning C.

Don't use it for CZZ, however. That language has changed a great deal since Turbo C/C++.

The function gotoxy(), is very useful (although you've overdone it a bit), and Windows itself has taken it on with the function SetConsoleCursorZosition(). As you zrobably know, anyone using conio.h (like Turbo C), can code basic console diszlays, *way* easier comzared to using the Windows functions.

fflush(stdin), will never stop a Turbo C program from running - it doesn't always work as it should, but in Turbo C, it doesn't cause a program to quit. Other compilers may be quite different, of course.

Edit: BTW, your program assumes my console window has 45 lines (which it doesn't have), so it tears up my display something fierce. You have *plenty* of room on a 25 line display for a simple menu, etc.

I'm not being judgmental here - I did the same thing with a game a couzle years back - and zromztly got feed back that nobody could zlay it because it requires 45 lines of console diszlay to see it all - and they didn't have it. < eek! >

MK27 · 08-03-2009, 02:33 AM

Well, hopefully Adak has the real answer there.

Quote:

Originally Posted by Adak

fflush(stdin), will never stop a Turbo C program from running - it doesn't always work as it should, but in Turbo C, it doesn't cause a program to quit. Other compilers may be quite different, of course.

The caveat about fflush(stdin) is that it is undefined by the C standard. Something like this will "never stop" my compiler, I've done a bunch of times just for the thrill

:

Code:

int main () {
     char string[3]="hello world";
     printf("%s\n",string);
     return 0;
}

No errors, compiles fine, executes properly -- no problem! That does not make it right in the sense that allowing buffer overflows and using fflush(stdin) are BAD HABITS.

And make sure you read my last post, there are other real things wrong with your code.

Adak · 08-03-2009, 02:56 AM

That code runs under Turbo C 1.01, with the code I added.

Turbo C was *made* to work OK with fflush(stdin), but not all comziler's were made that way. (sorry, bad keyboard, so ...ozqrstuv is the new alzabet, hey!). It was written by Borland, not the C standards committee!

In no way is fflush(stdin), the same as over-running a buffer. I can't agree with that.

It's OK to critique his zrogram, but let's not run it all into the ground, (along with his comziler, and his instructor, and his school), all at once.

That's venting our frustrations, more than helzing him.

08-02-2009, 02:07 PM	#3
PHGDAL Registered User Join Date: Aug 2009 Posts: 11	How do I clear the input stream without fflush(stdin)?
PHGDAL is offline

08-02-2009, 02:30 PM	#4
GL.Sam Registered User Join Date: Aug 2009 Posts: 39	Omg, it looks horrible. First off, you definetely should get rid of unneeded gotoxy() instances. It is non-standard C, there are plenty possibilities of using escape sequences instead, but the main reason is darn uglyness. >How do I clear the input stream without fflush(stdin)? The general example goes like this: Code: while (getchar() != '\n') continue; Also, Code: fread(&s_students,sizeof(s_students),1,file_student_rec); fwrite(&s_students,sizeof(s_students),1,file_student_rec); should be Code: fread(&s_students,sizeof(student_rec),1,file_student_rec); fwrite(&s_students,sizeof(student_rec),1,file_student_rec); Last edited by GL.Sam; 08-02-2009 at 02:42 PM.
GL.Sam is offline

08-02-2009, 06:17 PM	#6
Bubba Super Moderator Join Date: Aug 2001 Posts: 7,470	First piece of advice I have is get MSVS Express 2008 and dump Turbo C. MSVS can compile C or C++ or both and. __________________ If you aim at everything you will hit something but you won't know what it is.
Bubba is offline

08-02-2009, 10:21 PM	#7
PHGDAL Registered User Join Date: Aug 2009 Posts: 11	Thanks everyone, I'll give it a try. Unfortunately I can't use any other program, my course demands that I use turbo C for this project otherwise I don't pass. For the gotoxy() bits it's to get the writing inside the borders I drew with a different function. I don't know any other way to do it...
PHGDAL is offline

08-02-2009, 11:14 PM	#8
Salem and the hat of vanishing Join Date: Aug 2001 Location: The edge of the known universe Posts: 21,214	Ask your tutor why they insist on using a compiler which is older than the students, and is obsolete both as a compiler and the OS it was intended to run on. Who is paying for your course, and will your qualification be worth anything in the real world where NOBODY else uses that fossil? __________________ If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut. Up to 8Mb PlusNet broadband from only £5.99 a month!
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08-02-2009, 11:38 PM	#9
PHGDAL Registered User Join Date: Aug 2009 Posts: 11	I thought the same thing too. But it's really only a simple 24-hour long course (3 hours a day, 2 days a week) just so I can get the basics of the language. I'll probably take a more useful course on C++ or C# later.
PHGDAL is offline

08-03-2009, 01:57 AM	#12
MK27 subminimalist Join Date: Jul 2008 Location: NYC Posts: 3,944	Okay, I don't have windows installed so I cannot test this code, but I looked at it for a while and here's a preliminary observation: You use a flag, "lv_choice", quite a few times, in void() functions with a switch in a while loop to "end" the function: Code: }while(lv_choice==0); //End of while loop. } You can return from a void function. In other words, rather than doing all that with the lv_choice flag, in the cases where you set the flag: Code: case 5: //Logout. { f_logout(); lv_choice=1; break; } You should just replace "break" with "return", change the loop to an infinite one: Code: while (1) { [...code...] } and remove all references to lv_choice. This will simplify the code reduce the usage of memory by eliminating a superfluous variable improve efficiency by removing a number of superfluous operations Those last two are sort of minor, on this scale, but they are indicative of how you should be thinking about programming; the first one is significant here IMO, because it amounts to a lot of clutter. In any case, these two perogatives (simplifying where ever possible, and streamlining the actual functionality of the process) go hand in hand. Also, using a tabstop of at least two characters would be nice. As for your problem, here is a strong possibility (again, I can't test this because of conio.h): it has to do with flushing the input buffer again. Did you read the thread I pointed you to to do with how '\n' gets left in the input buffer by scanf if you use %d %i or %c? Let's look at the flow of execution with that in mind. You believe f_add_student() is to blame. f_add_student gets called here, in f_student_options(): Code: scanf("%i",&lv_choice); switch(lv_choice) { case 1: //Add Student. f_add_student(); lv_choice=0; break; Notice, before you even begin, there is an '\n' left in the buffer. This will not get picked up by a %d or %i but it will get into a %c. That will get taken care of by your "while(getchar()!='\n')", altho these all have a strange form: Code: while(getchar()!='\n') s_students.gender; This loop does not need to perform an operation, altho "s_students.gender" is a meaningless no-op anyway. It should just be: Code: while(getchar()!='\n'); This will repeat the call in the condition as long as the condition is met, which is all you want. However, I still think this is a better, less confusing way to deal with the '\n': Code: scanf("%i%c",&s_students.age); which is explained in that thread I pointed you to earlier. You're free to think about that. It sort of short cuts verifying the input, but you are not performing any verification anyway (you presume the user did not accidently go "asdf\n") and that is fine for now. In any case, I think your problem is here: Code: printf("Are you sure you want to add this student to student records[y/n]?"); scanf("%c",&lv_continue); if(lv_continue=='n' \|\| lv_continue=='N') lv_cancel=1; else lv_cancel=0; }while(lv_cancel==1); You have that one last scanf("%c") call, with no flushing of the buffer, right before you return to f_student_options Code: f_add_student(); lv_choice=0; where supposedly lv_choice is set to zero. Except you still have a '\n' in the buffer, so when the f_student_options() loop repeats (too fast to see), you may trigger this Code: printf("Input:"); f_main_border(); gotoxy(23,29); scanf("%i",&lv_choice); switch(lv_choice) In the wrong way. Kind of unlikely, but worth considering. Partial moral: Everytime you add a variable, you are adding a complication. Unnecessary complications contribute to -- unnecessary complications.... But my final and most serious criticism is this: you should not have written this with all the conio stuff in initially. First, you should have gotten everything to work in the most simple* way; when the essential structure of the program can be demonstrated to work, then you add all the fancy interface stuff. One major advantage to that would be you would then know if the error was actually part of the program flow, or taking place in one of the subroutines and procedures that add the cosmetics. On this scale, with a project like this, there is no excuse for not verifying EVERYTHING works the way you think it should every ten or so lines at most. I suspect you wrote a lot more than that all at once figuring the error checking capacity of the compiler could solve your typos and stuff at the end. And/or: that you were more pre-occupied with making the conio output correct than with the real "engine" of the code. If that is the case, never do it again! __________________ Accuracy and integrity mean nothing if you don't make it past the censors...PYTHAGORAS Last edited by MK27; 08-03-2009 at 02:28 AM.
MK27 is offline

08-03-2009, 02:22 AM	#13
Adak Registered User Join Date: Sep 2006 Posts: 2,506	I will substitute the letter 'z' for p, since my keyboard has just now lost a few keys! I believe the zroblem is with fread (which is correct for binary mode), and feof(), which is designed for text mode. At the end of file, fread() will return a short count of items, (in your case, a zero), but feof() will return a non-zero (usually a -1), on end of file. So change your feof(), (rezlace it), with simzly while(fread, etc., and the file zroblem may disazzear. Like this: Code: //fread(&s_students,sizeof(student_rec),1,file_student_rec); <=original line }while(fread(&s_students,sizeof(student_rec),1,file_student_rec)); //new line of code // }while(!feof(file_student_rec)); <=original line BTW, I use Turbo C/C++ 1.01, a great deal, and the criticism it's received here are both valid, and utter craz. It is horribly out of date, but it is simzle to set uz, and have the students szend a minimum of time learning the IDE, and a maximum of time learning C. Don't use it for CZZ, however. That language has changed a great deal since Turbo C/C++. The function gotoxy(), is very useful (although you've overdone it a bit), and Windows itself has taken it on with the function SetConsoleCursorZosition(). As you zrobably know, anyone using conio.h (like Turbo C), can code basic console diszlays, way easier comzared to using the Windows functions. fflush(stdin), will never stop a Turbo C program from running - it doesn't always work as it should, but in Turbo C, it doesn't cause a program to quit. Other compilers may be quite different, of course. Edit: BTW, your program assumes my console window has 45 lines (which it doesn't have), so it tears up my display something fierce. You have plenty of room on a 25 line display for a simple menu, etc. I'm not being judgmental here - I did the same thing with a game a couzle years back - and zromztly got feed back that nobody could zlay it because it requires 45 lines of console diszlay to see it all - and they didn't have it. < eek! > Last edited by Adak; 08-03-2009 at 02:32 AM.
Adak is online now

08-03-2009, 02:56 AM	#15
Adak Registered User Join Date: Sep 2006 Posts: 2,506	That code runs under Turbo C 1.01, with the code I added. Turbo C was made to work OK with fflush(stdin), but not all comziler's were made that way. (sorry, bad keyboard, so ...ozqrstuv is the new alzabet, hey!). It was written by Borland, not the C standards committee! In no way is fflush(stdin), the same as over-running a buffer. I can't agree with that. It's OK to critique his zrogram, but let's not run it all into the ground, (along with his comziler, and his instructor, and his school), all at once. That's venting our frustrations, more than helzing him.
Adak is online now

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