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  • 2 Post By tabstop
  • 3 Post By quzah

Why is there one extra byte when copying file?

This is a discussion on Why is there one extra byte when copying file? within the C Programming forums, part of the General Programming Boards category; Hi guys, am new in C programming. I don't understand the following: In the below code snippet, input.txt is 30 ...

  1. #1
    Registered User
    Join Date
    Jul 2011
    Posts
    10

    Why is there one extra byte when copying file?

    Hi guys, am new in C programming. I don't understand the following:

    In the below code snippet, input.txt is 30 bytes. When I run the program, output.txt is 31 bytes.

    Code:
    FILE *fp, *new_fp;
    int ch;
    
    if((fp = fopen("input.txt", "rb")) == NULL){
    	fprintf(stderr, "Error opening file for reading...");
    	exit(1);
    }
    
    if((new_fp = fopen("output.txt", "wb")) == NULL){
    	fprintf(stderr, "Error opening a new file for writing!");
    	exit(1);
    }
    
    /* I use a while loop to check for FEOF */
    while(!feof(fp)){
    	ch = fgetc(fp);
    	putc(ch, new_fp);
    }
    
    fcloseall();


    In another code snippet as shown below, input.txt and output.txt is both 30 bytes.

    Code:
    FILE *fp, *new_fp;
    int ch;
    
    if((fp = fopen("input.txt", "rb")) == NULL){
    	fprintf(stderr, "Error opening file for reading...");
    	exit(1);
    }
    
    if((new_fp = fopen("output.txt", "wb")) == NULL){
    	fprintf(stderr, "Error opening a new file for writing!");
    	exit(1);
    }
    
    while(1){
    	ch = fgetc(fp);
    	if(!feof(fp)){
    		fputc(ch, new_fp);
    	}else{
    		break;
    	}
    }
    As you can see above, the only different is the while looping to copy data from one file to another.

    Can someone enlighten me why there is a extra one byte in the first code snippet?

    Thanks in advance!

  2. #2
    and the Hat of Guessing tabstop's Avatar
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    Nov 2007
    Posts
    14,185
    Because the loop in your first program is broken. See the FAQ on "why feof shouldn't be used to control a loop".
    Bayint Naung and iMalc like this.

  3. #3
    Registered User
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    10
    Quote Originally Posted by tabstop View Post
    Because the loop in your first program is broken. See the FAQ on "why feof shouldn't be used to control a loop".
    Thanks for your fast reply!

    The link @ Cprogramming.com FAQ > Why it's bad to use feof() to control a loop was very helpful indeed.

    Thanks! Now i know why. LOL!

  4. #4
    ATH0 quzah's Avatar
    Join Date
    Oct 2001
    Posts
    14,826
    Quote Originally Posted by xbfish View Post
    Thanks! Now i know why. LOL!
    You missed a great "...and knowing is half the battle!" opportunity there. But I guess you probably weren't born in time to know. :oldguy:


    Quzah.
    Hope is the first step on the road to disappointment.

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