Question

If I send plain text there is no problem. Everything is ok.
However If I try to send from the C# client an image, the server receives correct bytes number, but when I save the buffer to a file (in binary mode – wb), it always has 4 bytes.

I send it by the C# client by using the function File.ReadAllBytes().

My saving code looks like

   FILE * pFile;
   char *buf = ReceiveMessage(s);
   pFile = fopen (fileName , "wb");
   fwrite(buf, sizeof(buf[0]), sizeof(buf)/sizeof(buf[0]), pFile);
   fclose (pFile);
   free(buf);

My receiving function looks like

static unsigned char *ReceiveMessage(int s)
{
   int prefix;
   recv(s, &prefix, 4, 0);
   int len = prefix;
   char *buffer= (char*)malloc(len + 1);
   int received = 0, totalReceived = 0;
   buffer[len] = '\0';
   while (totalReceived < len)
   {
      if (len - totalReceived > BUFFER_SIZE)
      {
         received = recv(s, buffer + totalReceived, BUFFER_SIZE, 0);
      }
      else
      {
         received = recv(s, buffer + totalReceived, len - totalReceived, 0);
      }
      totalReceived += received;
   }

   return buffer;
}

Answer 1

Your C code needs to pass len back from the ReceiveMessage() function.

char *buf = ReceiveMessage(s);  // buf is a char*
... sizeof(buff)                // sizeof(char*) is 4 or 8

So you’ll need something like

 static unsigned char *ReceiveMessage(int s, int* lenOut)
 {
     ...
     *lenOut = totalReceived ;
 }