Found that USBcommands from the device are silently overwritten if the host does not quickly enough read them, which causes problems in several places. Implemented a circular buffer which stores up to 50 commands (still tiny, memorywise) and prints out a message if the buffer wraps around (overwrites).

client/cmdmain.c

@@ -33,6 +33,14 @@ UsbCommand current_response_user;
 static int CmdHelp(const char *Cmd);
 static int CmdQuit(const char *Cmd);
 
+//For storing command that are received from the device
+#define CMD_BUFFER_SIZE 50
+static UsbCommand cmdBuffer[CMD_BUFFER_SIZE];
+//Points to the next empty position to write to
+static int cmd_head;//Starts as 0
+//Points to the position of the last unread command
+static int cmd_tail = -1;//Starts as -1
+
 static command_t CommandTable[] = 
 {
   {"help",  CmdHelp,  1, "This help. Use '<command> help' for details of the following commands:\n"},
@@ -56,30 +64,31 @@ int CmdQuit(const char *Cmd)
   exit(0);
   return 0;
 }
-
+/**
+ * Waits for a certain response type. This method waits for a maximum of
+ * ms_timeout milliseconds for a specified response command.
+ *@brief WaitForResponseTimeout
+ * @param cmd command to wait for
+ * @param response struct to copy received command into.
+ * @param ms_timeout
+ * @return true if command was returned, otherwise false
+ */
 bool WaitForResponseTimeout(uint32_t cmd, UsbCommand* response, size_t ms_timeout) {
 
-	// Wait until the command is received
-  for(size_t i=0; received_command != cmd && i < ms_timeout/10; i++) {
-		msleep(10); // XXX ugh
-    if (i == 200) { // Two seconds elapsed
-      PrintAndLog("Waiting for a response from the proxmark...");
-      PrintAndLog("Don't forget to cancel its operation first by pressing on the button");
-    }
+  // Wait until the command is received
+  for(size_t dm_seconds=0; dm_seconds < ms_timeout/10; dm_seconds++) {
+
+      if(getCommand(response) && response->cmd == cmd){
+          //We got what we expected
+          return true;
+      }
+        msleep(10); // XXX ugh
+        if (dm_seconds == 200) { // Two seconds elapsed
+          PrintAndLog("Waiting for a response from the proxmark...");
+          PrintAndLog("Don't forget to cancel its operation first by pressing on the button");
+        }
 	}
-	
-  // Check if timeout occured
-  if(received_command != cmd) return false;
-
-	// Copy the received response (if supplied)
-  if (response) {
-    memcpy(response, &current_response, sizeof(UsbCommand));
-  }
-
-	// Reset the received command
-  received_command = CMD_UNKNOWN;
-
-	return true;
+    return false;
 }
 
 bool WaitForResponse(uint32_t cmd, UsbCommand* response) {
@@ -183,37 +192,61 @@ void UsbCommandReceived(UsbCommand *UC)
         default: {
         } break;
       }
-//      // Store the last received command
-//      memcpy(&current_response, UC, sizeof(UsbCommand));
-//      received_command = UC->cmd;
     } break;
   }
-  // Store the last received command
-  memcpy(&current_response, UC, sizeof(UsbCommand));
-  received_command = UC->cmd;
-/*
-  // Maybe it's a response:
-  switch(current_command) {
-    case CMD_DOWNLOAD_RAW_ADC_SAMPLES_125K:
-      if (UC->cmd != CMD_DOWNLOADED_RAW_ADC_SAMPLES_125K) goto unexpected_response;
-      int i;
-      for(i=0; i<48; i++) sample_buf[i] = UC->d.asBytes[i];
-      received_command = UC->cmd;
-      return;
-    case CMD_ACQUIRE_RAW_ADC_SAMPLES_125K:
-    case CMD_DOWNLOADED_SIM_SAMPLES_125K:
-      if (UC->cmd != CMD_ACK) goto unexpected_response;
-      // got ACK
-      received_command = UC->cmd;
-      return;
-    default:
-    unexpected_response:
 
-	if(UC->cmd != CMD_ACK)
-		PrintAndLog("unrecognized command %08x       \n", UC->cmd);
-	else
-		memcpy(&current_response, UC, sizeof(UsbCommand));
-	received_command = UC->cmd;
-  }
- */
+  storeCommand(UC);
+
+}
+/**
+ * @brief storeCommand stores a USB command in a circular buffer
+ * @param UC
+ */
+void storeCommand(UsbCommand *command)
+{
+    if(cmd_head == cmd_tail)
+    {
+        //If these two are equal, we're about to overwrite in the
+        // circular buffer.
+        PrintAndLog("WARNING: Command buffer about to overwrite command! This needs to be fixed!");
+    }
+    //Store the command at the 'head' location
+    UsbCommand* destination = &cmdBuffer[cmd_head];
+    memcpy(destination, command, sizeof(UsbCommand));
+
+    //Also, if cmd_tail is still -1 because the buffer was
+    // previously empty, set it to head
+    if(cmd_tail < 0) {
+        cmd_tail = cmd_head;
+    }
+
+    cmd_head++; //increment head
+    cmd_head %= CMD_BUFFER_SIZE;//wrap around
+
+}
+/**
+ * @brief getCommand gets a command from an internal circular buffer.
+ * @param response location to write command
+ * @return 1 if response was returned, 0 if nothing has been received
+ */
+int getCommand(UsbCommand* response)
+{
+    //If head == tail, there's nothing to read, or if we just got initialized
+    if(cmd_head == cmd_tail || cmd_tail == -1){
+        return 0;
+    }
+    //Pick out the next unread command
+    UsbCommand* last_unread = &cmdBuffer[cmd_tail];
+    memcpy(response, last_unread, sizeof(UsbCommand));
+    //Increment tail - this is a circular buffer, so modulo buffer size
+    cmd_tail = (cmd_tail +1 ) % CMD_BUFFER_SIZE;
+    //In order to detect when the buffer overwrites itself, we set the
+    // tail to -1 whenever it 'catches up' with head : this means the buffer is empty.
+    // Otherwise, head==tail could mean both: either empty or full.
+    if(cmd_tail == cmd_head){
+        cmd_tail = -1 ;
+    }
+
+    return 1;
+
 }