FWRITE()

functions directly return the number of objects processed rather than return the value in an argument.

#include <stdio.h>
#include <stddef.h>
#include <errno.h>

#define BUFFER_SIZE 256

int main(int argc, char *argv[])
{
  FILE *inFile = NULL;
  FILE *outFile = NULL;
  size_t bytesIn, bytesOut;
  char received[BUFFER_SIZE];

  if(argc != 3){
    printf("Usage: copyFile source, destination\n");
	return 1;
  }
  
  inFile = fopen(argv[1], "rb");
  if (inFile == NULL){
    perror(argv[1]);
	return 2;
  }
  
  outFile = fopen(argv[2], "wb");
  if (outFile == NULL){
    perror(argv[2]);
	return 3;
  }
  
  //process input file a record at a time.
  while ((bytesIn = fread(received, 1, BUFFER_SIZE, inFile)) > 0){   //the 1 represents the size of each item to be read (or written), in bytes
    bytesOut = fwrite(received, 1, bytesIn, outFile);
	if(bytesOut != bytesIn){
	  perror("Fatal write error.");
	  return 4;
	}
  }
  fclose(inFile);
  fclose(outFile);
  return 0;
}

 * fread(rec, 1, BUFFER_SIZE, inFile)
    - received: The buffer where the data read from the file will be stored.
    - 1: Means "read 1 byte at a time"
    - BUFFER_SIZE: Means "read up to BUFFER_SIZE such 1-byte items".
	- size_t fread(void *ptr, size_t size, size_t nmemb, FILE *stream);
	   - size: The size of each item (or "record") to be read/written, in bytes. This is the 1 in the code.
	   - nmemb: The maximum number of items (each of size size) to read/write. This is BUFFER_SIZE in the code.

 * fwrite(rec, 1, bytesIn, outFile)
    - received: The buffer containing the data to write to the file.
	- 1: Means "write 1 byte at a time".
	- bytesIn: Means "write bytesIn such 1-byte items" (which is the number of bytes successfully read from the input file in the previous fread call).
    - size_t fwrite(const void *ptr, size_t size, size_t nmemb, FILE *stream);
 	   - size: The size of each item (or "record") to be read/written, in bytes. This is the 1 in the code.
	   - nmemb: The maximum number of items (each of size size) to read/write. This is BUFFER_SIZE in the code.