Thread: buffer contents swapping

I need another pair of eyes, etc. to review my code for logic errors.

Here's what the program should do:
1. read in half of input file to buffer
2. read last half BUT swap each byte read with last byte from buffer back into

file, decrementing buffer end position each time.
3. write back buffer contents to 1st half of file.
in other words, it should reverse the contents of the file. running the program

twice should give you back the original file. AND rules 1-3 must be followed.

Problem for me is my code works OK for 2 byte as this is what I get running

program twice.
as -> sa -> as

But for 4 bytes or more, error.
For 4 bytes, this is what happens to contents
todo -> .dot -> .od.

More bytes = messed up text

I can't seem to find the logic error in my code. Can you?

functions called in this order:
readfirst()
swaphalves() //error in here?
writefirst()

Here's what I have:

Code:

//global vars
FILE *dfile; //file handle
long fsize; //var used to calculate filesize
long readto; //var used to read/write half of file

long back = -1; //for seeking
long forward = 1;

//vars in main()
long byte = 0;

fsize = FileSize(dfile);
readto = fsize/2;

char *buffer;
buffer = (char*) malloc( sizeof(char)*readto );

//this function seems to be the problem
void swaphalves(FILE* afile, long& byte, char* buffer){
	
	unsigned char chr;
	long revpos = byte-1; //counter for reversing buffer contents
	chr = fgetc(afile);
	byte++;
	while(byte < fsize){
	//while(!feof(dfile)){
		//put back last char in buffer to file, seek back 1
		fseek (afile, back, SEEK_CUR);
		fputc(buffer[revpos],afile);
		//put latest char read from file to current buffer pos & 

decrement buffer pos
		buffer[revpos--] = chr;
		//seek forward 1 for get next char
		fseek (afile, forward, SEEK_CUR);
		chr = fgetc(afile); 
		byte++;
	}
	//take care of last char
	//put back last char in buffer to file, seek back 1
	fseek (afile, back, SEEK_CUR);
	fputc(buffer[revpos],afile);
	//put latest char read from file to current buffer pos
	buffer[revpos] = chr;
	//fseek (afile, forward, SEEK_CUR);

	//for debugging
	printf("Buffer: ");
	for(int i = 0; i < readto; i++)
		printf("%c",buffer[i]);
	return;
}

//function works ok in testing
void readfirst(FILE* afile, long& byte, char* buffer){
	buffer[byte] = fgetc(afile);
	byte++;
	while(byte < readto)
		buffer[byte++] = fgetc(afile);

	//for debugging
	printf("Buffer: ");
	for(int i = 0; i < readto; i++){
		printf("%c",buffer[i]);
	}
	putchar('\n');

	return;
}

//function should be ok i believe
void writefirst(FILE* afile, const long& length, char* buffer){
	fseek (afile, 0L, SEEK_SET);
	fwrite(buffer,sizeof(char),length,afile);
	return;
}