Thread: read and write to file

Code:

 /*very very simple encrytion*/
fp=fopen("message.txt","r+");
do
{
    c=fgetc(fp);
    xy=(c+1)%256;
    fputc(xy, fp);
}while(c!=EOF);
fclose(fp);

Im trying to read from a file and then shift it by +1 and write to the same file again.
but it wont work. I don´t get any error.

example: message.txt
hello what is your name bla bla bla

I get:
an empty file

I don't get what you're trying to do within the program but you are using the wrong mode to open the file. You used "w" wich is for creating a file with the name "message.txt" or overwriting it if it already exist. (w = write)

You should use the mode "r+" to read from an existing file and write to it (update it).

Code:

int ch;
while ( (ch=fgetc(fp)) != EOF ) {
    xy=(ch+1)%256;
    fseek( fp, -1, SEEK_CUR );  // step back a char
    fputc(xy, fp);
}

Never use do while to read a file - it doesn't handle empty files

Originally Posted by sand_man

you cant read and write to the same file without closing the file first and re-opening it with a different attribute.
best to have an input file and then an output file.

Bet me.

man 3 fopen

DESCRIPTION

The fopen function opens the file whose name is the string
pointed to by path and associates a stream with it.

The argument mode points to a string beginning with one of
the following sequences (Additional characters may follow
these sequences.):

r Open text file for reading. The stream is posi-
tioned at the beginning of the file.

r+ Open for reading and writing. The stream is posi-
tioned at the beginning of the file.

w Truncate file to zero length or create text file
for writing. The stream is positioned at the
beginning of the file.

w+ Open for reading and writing. The file is created
if it does not exist, otherwise it is truncated.
The stream is positioned at the beginning of the
file.

a Open for writing. The file is created if it does
not exist. The stream is positioned at the end of
the file.

a+ Open for reading and writing. The file is created
if it does not exist. The stream is positioned at
the end of the file.

All you had to do was read up on fopen to see that.

Quzah.

I found a thread with posting on a similar topic here. I think the upshot was to do something like this.

Code:

#include <stdio.h>
#include <limits.h>

int main (void)
{
   int ch, key = 1;
   const char filename[] = "file.txt";
   FILE *file = fopen(filename, "r+b");
   if ( file )
   {
      while ( (ch = fgetc(file)) != EOF )
      {
         int xy = (ch + key) % UCHAR_MAX;
         fseek(file, -1, SEEK_CUR);
         fputc(xy, file);
         fflush(file);
         fseek(file, 0, SEEK_CUR);
      }
      fclose(file);
   }
   return 0;
}

i didn't read the modified code but i noticed that you commited the same error again when opening the stream at the line :

Code:

fp=fopen("message.txt","r");   /*read file, show on screen*/

you opened it with a "read only" mode then you tried to write to the stream with :

Code:

fputc(xy, fp);

use the "r+" mode to read AND write.

Originally Posted by staticalloc

thanks to everyone for your reply, but this is still really frustrating. I dont get any error nor warnings. Whats wrong with my algorithm?

Just because you don't get "any error nor warnings", there's no guarantee that the program performs as you intended.

Salem's fix made the program work "OK" with my Windows XP gcc compiler, but not with Borland bcc32 or with Visual C++.

Dave_Sinkula's solution worked with all compilers (actually we don't need the fflush(), but the second fseek() is required).


Here's a passage from The Standard C Library by P. J. Plauger, copyright 1992, based on the 1998 ISO C standard:

When a file is opened with update mode, both input and output may be performed on the associated stream. However, output may not be directly followed by input without an intervening call to the fflush function or to a file positioning function (fseek, fsetpos, or rewind), and input may not be directly followed by output without an intervening call to a file positioning function...

So, it turns out that my gcc doesn't require the extra fseek(), but bcc32 and Visual C++ do, consistent with the above quote.

By the way, if you want to do error checking on fseek(), note that its returned value is zero for success and non-zero for failure.

I hadn't run across this in my own work, because I always create a new file for writing (therefore saving the original file unchanged). If it is required for the modified file to have the same name as the original, the files can be renamed by the program after they are closed.

Regards,

Dave

Originally Posted by sand_man

I seem to be wrong a lot dont I?

It happens to all of us now and again.

Originally Posted by sand_man

I guess I'm learning something new everyday.

That's the important thing.

Quzah.

I'm just now starting to learn files and it seems You might also want to make sure you close the file properly.

Code:

if( fclose(file) == 0)
  
       printf("whatever you want here");
       
    else
    	puts("CLOSING FILE FAILED!!");

    return 0;

fclose returns a value of 0 if succesful and EOF if not. Anything can happen. ex: disk is full, diskette removed
or even an i/o error.

edit: thanks for those links you posted quazah. They seem to include some details my book failed to mention.

>You might also want to make sure you close the file properly.
It certainly couldn't hurt if you really need robust code or if you don't flush the stream before closing it and a write error occurs. At that point you're looking at a loss of data situation, and recovery is critical. For toy programs like we write here, you just need to think of a call to fclose as a placeholder for something more solid.