Thread: 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!

Because the loop in your first program is broken. See the FAQ on "why feof shouldn't be used to control a loop".

Originally Posted by tabstop

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!