File transfer with arduino - file

Question

My final goal is to send a 30 KB file over XBEE to another arduino. But for now i am just trying to duplicate a 4KB file on SD connected to first arduino. First i tried to send the data one byte by one byte.it worked and file duplicated successfully . but i have to have a buffer and then send data on 64 byte packets to XBEE so i should be able to read and write file in 64 byte packets. this is what i have done:
#include <SD.h>
#include <SPI.h>
void setup() {
Serial.begin(115200);
while (!Serial) {
; // wait for serial port to connect. Needed for native USB port only
}
if (!SD.begin(4)) {
Serial.println("begin failed");
return;
}
File file = SD.open("student.jpg",FILE_READ);
File endFile = SD.open("cop.jpg",FILE_WRITE);
Serial.flush();
char buf[64];
if(file) {
while (file.position() < file.size())
{
while (file.read(buf, sizeof(buf)) == sizeof(buf)) // read chunk of 64bytes
{
Serial.println(((float)file.position()/(float)file.size())*100);//progress %
endFile.write(buf); // Send to xbee via serial
delay(50);
}
}
file.close();
}
}
void loop() {
}
It successfully finish its progress until 100% but when i open the SD on laptop the file is created but it shown as 0 KB file.
whats the problem?

Answer 1

You are not telling .write what's the length of your buffer, so it will think it's a null-terminated string (which it isn't).
Plus, the inner loop appears to be not only unnecessary but even harmful because it would skip the last chunk if it's less than 64 bytes.
Check this out:
while(file.position() < file.size()) {
// The docs tell me this should be file.readBytes... but then I wonder why file.read even compiled for you?
// So if readBytes doesn't work, go back to "read".
int bytesRead = file.readBytes(buf, sizeof(buf));
Serial.println(((float)file.position()/(float)file.size())*100);//progress %
// We have to specify the length! Otherwise it will stop when encountering a null byte...
endFile.write(buf, bytesRead); // Send to xbee via serial
delay(50);
}