How to receive a JPEG image over serial port - c++

Question

So I am trying to send a jpeg image (4Kb) from a raspberry pi to my Mac wirelessly using Xbee Series 1. I have an image on the raspberry pi and can read it into binary format. I've used this binary format to save it into another image file and it creates a copy of the image correctly. That tells me that I am reading it correctly. So I am trying to send that data over a serial port (to be transferred by the xbee's) to my Mac. Side note, Xbee's can only transmit I think 80 bytes of data per packet or something. I don't know how that affects what I'm doing though.
My problem is, I do not know how to read the data and properly store it into a jpeg file itself. Most of the Read() functions I have found require you to enter a length to read and I don't know how to tell how long it is since its just a serial stream coming in.
Here is my code to send the jpeg.
#include "xSerial.hpp"
#include <iostream>
#include <cstdlib>
using namespace std;
int copy_file( const char* srcfilename, const char* dstfilename );
int main(){
copy_file("tylerUseThisImage.jpeg", "copyImage.jpeg");
return 0;
}
int copy_file( const char* srcfilename, const char* dstfilename )
{
long len;
char* buf = NULL;
FILE* fp = NULL;
// Open the source file
fp = fopen( srcfilename, "rb" );
if (!fp) return 0;
// Get its length (in bytes)
if (fseek( fp, 0, SEEK_END ) != 0) // This should typically succeed
{ // (beware the 2Gb limitation, though)
fclose( fp );
return 0;
}
len = ftell( fp );
std::cout << len;
rewind( fp );
// Get a buffer big enough to hold it entirely
buf = (char*)malloc( len );
if (!buf)
{
fclose( fp );
return 0;
}
// Read the entire file into the buffer
if (!fread( buf, len, 1, fp ))
{
free( buf );
fclose( fp );
return 0;
}
fclose( fp );
// Open the destination file
fp = fopen( dstfilename, "wb" );
if (!fp)
{
free( buf );
return 0;
}
// this is where I send data in but over serial port.
//serialWrite() is just the standard write() being used
int fd;
fd = xserialOpen("/dev/ttyUSB0", 9600);
serialWrite(fd, buf, len);
//This is where the file gets copied to another file as a test
// Write the entire buffer to file
if (!fwrite( buf, len, 1, fp ))
{
free( buf );
fclose( fp );
return 0;
}
// All done -- return success
fclose( fp );
free( buf );
return 1;
}
On the receive side I know I need to open up the serial port to read and use some sort of read() but I don't know how that is done. Using a serial library it has some functions to check if serial data is available and return the number of characters available to read.
One question about the number of characters available to read, will that number grow as the serial stream comes over or will it immediately tell the entire length of the data to be read?
But finally, I know after I open the serial port, I need read the data into a buffer and then write that buffer to a file but I have not had any luck. This is what I have tried thus far.
// Loop, getting and printing characters
char temp;
bool readComplete = false;
int bytesRead = 0;
fp = fopen("copyImage11.jpeg", "rwb");
for (;;)
{
if(xserialDataAvail(fd) > 0)
{
bytesRead = serialRead(fd, buf, len);
readComplete = true;
}
if (readComplete)
{
if (!fwrite(buf, bytesRead, 1, fp))
{
free(buf);
fclose(fp);
return 0;
}
fclose(fp);
free(buf);
return 1;
}
}
I don't get errors with my code, it just doesnt create the jpeg file correctly. Maybe I'm not transmitting it right, or maybe I'm not reading/writing to file correctly. Any help would be appreciated. Thanks everyone you rock!

Answer 1

If you are defining your own protocol, then you need to have a method for sending the length first.
I would recommend testing your code by sending short blocks of ascii text to confirm your i/o. Once that is working you can use the ascii to set up the transfer; ie send the length, and have your receiver ready for an expected block.