WhY the difference in outputs?

**juice** · 12-26-2011

Code:

#include<stdio.h>

int main(void)
{
    char c[]={26,33};
    char t;
    FILE *fp;
 
    fp=fopen("random.txt","wb");
     
    putc(c[0],fp);
    putc(c[1],fp); 
   
    fclose(fp);

    fp=fopen("random.txt","rb");

    while((t=getc(fp))!=EOF)
	printf("%c(%d)\n",t,t);

	fclose(fp);
}

Code:

#include<stdio.h>
 
int main(void)
{
    char c[]={26,33};
    char t;
    FILE *fp;
 
    fp=fopen("random.txt","w");
     
    putc(c[0],fp);
    putc(c[1],fp); 
   
    fclose(fp);

    fp=fopen("random.txt","r");

    while((t=getc(fp))!=EOF)
	printf("%c(%d)",t,t);

	fclose(fp);
}

The only difference in the above two codes is that in the first code the file random.txt is opened in binary mode, whereas in the second code the file is opened in text mode, but still in the second code the control never enters the while loop because getc encounters ASCII 26 in the first run itself and returns -1 (EOF), but this doesn't happen in the first case and the control enters the while loop in the first case. Why??

**Adak** · 12-27-2011

So what happens if you change the variable t to an int (which it should be)?

Gotta tell you, Juice - that is THE most distracting avatar, ever! The poor guy looks like he's having a fit!

**juice** · 12-27-2011

Originally Posted by Adak

So what happens if you change the variable t to an int (which it should be)?

Still the same results.. Please tell me why this difference in text mode and binary mode.

Originally Posted by Adak

Gotta tell you, Juice - that is THE most distracting avatar, ever!

Don't worry its a cool avatar.

**juice** · 12-27-2011

Am still confused whether -1 (EOF) is returned when fgetc() or fscanf() or any such standard function encounters the ASCII value 26, or whether it is returned when any of these functions reach the end of file...

**Adak** · 12-27-2011

Run this:

Code:

#include <stdio.h>

int main(void) {
   int c, i, j, k;
   FILE *fp;
         
   if((fp=fopen("test1", "wb")) == NULL) {
      printf("Error: unable to open the input file");
      return 0;
   }
   fprintf(fp, "%d %c %d\n",26,'e',35);
   printf("Writing two numbers and a letter, to a binary file\n\n1): 26\t2): e\t3): 35\n");
   fclose(fp);
 
   //get a word from the text file
   printf("\nClosing the file, and reopening it - and reading the data:\n\n");
   if((fp=fopen("test1", "rb")) == NULL) {
      printf("Error: unable to open the input file");
      return 0;
   }
   fscanf(fp,"%d %c %d",&i,&j,&k);
   
   printf("1): %d\t2): %c\t3): %d\n",i,j,k);
   fclose(fp);
   printf("\n\n");
   return 0;

}

In this case, putc() is producing an error, which puts an error onto the stream, and returns an EOF - but that has nothing to do with 26. That's just what putc() was designed to do.

**juice** · 12-27-2011

Originally Posted by Adak

Run this:

Code:

#include <stdio.h>

int main(void) {
   int c, i, j, k;
   FILE *fp;
         
   if((fp=fopen("test1", "wb")) == NULL) {
      printf("Error: unable to open the input file");
      return 0;
   }
   fprintf(fp, "%d %c %d\n",26,'e',35);
   printf("Writing two numbers and a letter, to a binary file\n\n1): 26\t2): e\t3): 35\n");
   fclose(fp);
 
   //get a word from the text file
   printf("\nClosing the file, and reopening it - and reading the data:\n\n");
   if((fp=fopen("test1", "rb")) == NULL) {
      printf("Error: unable to open the input file");
      return 0;
   }
   fscanf(fp,"%d %c %d",&i,&j,&k);
   
   printf("1): %d\t2): %c\t3): %d\n",i,j,k);
   fclose(fp);
   printf("\n\n");
   return 0;

}

In this case, putc() is producing an error, which puts an error onto the stream, and returns an EOF - but that has nothing to do with 26. That's just what putc() was designed to do.

Couldn't understand what ur trying to tell. This code is running just fine.

**Adak** · 12-27-2011

I'm trying to say that when you reached the end of the file in the one example above, it was caused by putc() or getc(), returning an error - had nothing to do with 26 being in the file. That's just what putc() and getc() do on error - they return EOF.

Although I have to say, I use either of these functions very much.

**laserlight** · 12-27-2011

Originally Posted by juice

Am still confused whether -1 (EOF) is returned when fgetc() or fscanf() or any such standard function encounters the ASCII value 26, or whether it is returned when any of these functions reach the end of file...

Well...

Originally Posted by C99 Clause 7.19.1 Paragraph 3 (part)

EOF
which expands to an integer constant expression, with type int and a negative value, that is returned by several functions to indicate end-of-file, that is, no more input from a stream;

Originally Posted by C99 Clause 7.19.7.1 Paragraph 3

If the end-of-file indicator for the stream is set, or if the stream is at end-of-file, the end-of-file indicator for the stream is set and the fgetc function returns EOF. Otherwise, the fgetc function returns the next character from the input stream pointed to by stream. If a read error occurs, the error indicator for the stream is set and the fgetc function returns EOF.

Originally Posted by C99 Clause 7.19.6.2 Paragraph 16

The fscanf function returns the value of the macro EOF if an input failure occurs before any conversion. Otherwise, the function returns the number of input items assigned, which can be fewer than provided for, or even zero, in the event of an early matching failure.

Therefore:

EOF is a negative integer value. Don't get hung up about it being -1, because that does not matter anyway.
EOF is returned by certain input related functions in the C standard library to denote end of file or some kind of input error.

Originally Posted by juice

WhY the difference in outputs?

This business about the ASCII value 26 is something implementation specific: I believe that you took up this investigation from your other thread, especially my post #57. Read my conclusion: "even if you get a count of 1, it does not mean that such a value is appended to files; it would mean that such a value is treated as an end-of-file marker". So that would be my answer: the difference in outputs is because in binary mode, 26 is not special, but in text mode, it is treated specially as an end-of-file marker.

**whiteflags** · 12-27-2011

What makes you think EOF belongs in a text file? You can't type EOF, which makes the whole issue contrived. Of course if you use putc or getc in weird ways, like reading what should be a binary file, since you are writing non-human readable data, expect interpretations to be weird.

I believe the failure case for the binary function fread or fwrite is just any number smaller than the number of things you told it to write or read. At least, it indicates a problem. If you want to know about EOF, specifically, after reading binary file, you have to call feof(stream) appropriately.

**andrew89** · 12-27-2011

So something like the following might work?:

Code:

fp=fopen("test1","wb") //write to file in binary
     foo write data to file
fclose(fp);
fp=fopen("test1","a+") // write EOF if it exists
foo write EOF to file

Would this possibly work?

**juice** · 12-27-2011

Originally Posted by laserlight

If the end-of-file indicator for the stream is set, or if the stream is at end-of-file, the end-of-file indicator for the stream is set and the fgetc function returns EOF

What is this end-of-file indicator? What is the difference between EOF and end-of-file indicator...

**whiteflags** · 12-27-2011

So something like the following might work?:

Code:

fp=fopen("test1","wb") //write to file in binary
     foo write data to file
fclose(fp);
fp=fopen("test1","a+") // write EOF if it exists
foo write EOF to file

Would this possibly work?

No. You can do it, but that would be wrong.

What is the difference between EOF and end-of-file indicator...

EOF indicates an error state, and using a specfic value helps us know what kind of error it is.

You still can't type EOF, so it doesn't belong in a text file.

**laserlight** · 12-27-2011

Originally Posted by andrew89

So something like the following might work?

I don't understand what you are suggesting in the first place. You can certainly write EOF to file, but then if EOF is -1, what you would be doing is writing -1 to file. It would lose its meaning as the EOF macro.

**juice** · 12-27-2011

Wat the hell is going on.....

**laserlight** · 12-27-2011

Originally Posted by juice

What is this end-of-file indicator?

Read up on the feof function.

Originally Posted by juice

What is the difference between EOF and end-of-file indicator...

The end-of-file indicator is opaque to you: it is a hidden value set such that feof will tell you that end-of-file condition for the file stream has been set.

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