File problem

I have this code, but it doesn't work. It should read a number from a file and print it back, in the same file, multiplied by ten.

Code:

#include <stdio.h>

int main()
{
	int  num;
	FILE *in;

	in=fopen("test.txt","r+");
	fscanf( in, "%d", &num );
	fprintf( in, "%d", num * 10 );

	fclose( in );

	return 0;
}

Can you help me find the error?

Also, if a program that reads charcter by character from a file, prints back that character changed in the same file, will the next characters of the file be lost.? For example a file contains 10 characters. You read the first one, you change it, and you print it back in the same file( and you want to procceed in the nect character ). The next 9 characters of the file will be lost?

Thanks

Verify that you are actually opening the file by checking the value of in (initialize it to NULL or I'll kill you) The file "test.txt" will have to exist and be in the same directory as the .exe. Screw it, it's almost 3:00 am, goodnight.

The file exists and is in the same directory with the .exe and it opens.
Any better ides please?

Salem, i compile you'r code and it compiles fine. However when i run it makes a problem and tells me: "Unhandled Exception".

??

Also:
fflush(in);
fseek(in,readpos,SEEK_SET);

I think that these are not needed. Do they?

Also:
readpos = ftell(in); // where we need to get back to for reading
fseek(in,writepos,SEEK_SET);

rewind( in ) would so the same work, isn't it?

Also:
What about the second part of my initiall question?
Thanks again.

1)Many thanks, i just forgot to change the name of the file.. It works fine only if in the file there is just the number. When i wrote the file, i wrote the number and i wrote and an ENTER. When the programm works, it prints the right number and a box in black at the position of the enter. Why that ( if there isn't enter it doesn't print that box ) ?

2) I tried to make it in a more general form. This code should print all the characters uppercase:

Code:

#include <stdio.h>
#include <ctype.h>

int main()
{
	int a;
	long readpos, writepos;
	FILE *in;

	in=fopen("test.txt","r+");
	writepos = ftell(in);
	while( ( a = getc( in ) ) != feof( in )){
		readpos = ftell(in);    // where we need to get back to for reading
		fseek(in,writepos,SEEK_SET);   // go back to the writepos
		putc( toupper( a ), in );
		fflush(in);  
		fseek(in,readpos,SEEK_SET);     // go back to the readpos
	}
	fclose( in );

	return 0;
}

But it doesn't work. Why?

Also, can you explain me why we need the fflush ( in ) please?
Thanks

1)Thanks for your comments

2) Does this code ( the one about characters ) with these changes run fine in your computer?
Not in mine.

3) When should i use EOF and when feof( stream ) ?

4) I don't know if i have i hex editor, but, can you run that program and put an Enter after the number in the file and see the results and tell me what you think please?

5) About the fflush you said "make sure it happens before another file reading function". Can you explain that a little bit more please?

6) Sorry if i am getting tiring

&)Thanks in advance.

A last the code works!!
Many thanks.

But please, would you mind answering my question 3 ?

Thanks again.

Ok, thanks, i got it.

However, i just thought that we don't need the fflush(in) because the bufeer will be always empty since we get one-by one the characters and we don't use scanf(). But even if we used scanf we would use the %c so it would return fine without the need to read the next characcter.
Am i right?

So, because i don't really understad the use of fflush(), when and after what statement ( after the printf()statement ?? ) do you recommend me to put it? ( If you could give me a link that explains very good how fflush() works and how to use it, i would be greatfull )

Thanks again.

>>So, because i don't really understad the use of fflush(),
I'll try and reword Salem's answer for you. When you write to a file, the data being written isn't sent directly to the file, instead it goes via a buffer which is out of your control. All you can tell is that the data is enroute to the file.

As you want to read back from the same file, using the same FILE* variable, you *must* ensure that the writing of any data stored in the output buffer completes first. This is when the fflush() function comes in, as calling it will tell the OS to complete the write process.

Here's the
man page for fflush().

Try this, Notice the do while statement while scaning and printing into the file~


#Include <stdio.h>



int main()
{
int num;
FILE *in;

in=fopen("test.txt","r+");
do
{
fscanf( in, "%d", &num );
fprintf( in, "%d", num * 10 );
}while (getchar(in))!=EOF);
fclose( in );

return 0;
}

try this,,notice that I have included the do ...while function, while reading and printing your file!

#include <stdio.h>

int main()
{
int num;
FILE *in;

in=fopen("test.txt","r+");
do
{
fscanf( in, "%d", &num );
fprintf( in, "%d", num * 10 );
}while (getchar(in))!=EOF);
fclose( in );

return 0;
}

That's completely wrong