Packet Acknowledgment (Debugging with GDB)
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E.11 Packet Acknowledgment
By default, when either the host or the target machine receives a packet, the first response expected is an acknowledgment: either ‘+
’ (to indicate the package was received correctly) or ‘-
’ (to request retransmission). This mechanism allows the GDB remote protocol to operate over unreliable transport mechanisms, such as a serial line.
In cases where the transport mechanism is itself reliable (such as a pipe or TCP connection), the ‘+
’/‘-
’ acknowledgments are redundant. It may be desirable to disable them in that case to reduce communication overhead, or for other reasons. This can be accomplished by means of the ‘QStartNoAckMode
’ packet; see QStartNoAckMode.
When in no-acknowledgment mode, neither the stub nor GDB shall send or expect ‘+
’/‘-
’ protocol acknowledgments. The packet and response format still includes the normal checksum, as described in Overview, but the checksum may be ignored by the receiver.
If the stub supports ‘QStartNoAckMode
’ and prefers to operate in no-acknowledgment mode, it should report that to GDB by including ‘QStartNoAckMode+
’ in its response to ‘qSupported
’; see qSupported. If GDB also supports ‘QStartNoAckMode
’ and it has not been disabled via the set remote noack-packet off
command (see Remote Configuration), GDB may then send a ‘QStartNoAckMode
’ packet to the stub. Only then may the stub actually turn off packet acknowledgments. GDB sends a final ‘+
’ acknowledgment of the stub’s ‘OK
’ response, which can be safely ignored by the stub.
Note that set remote noack-packet
command only affects negotiation between GDB and the stub when subsequent connections are made; it does not affect the protocol acknowledgment state for any current connection. Since ‘+
’/‘-
’ acknowledgments are enabled by default when a new connection is established, there is also no protocol request to re-enable the acknowledgments for the current connection, once disabled.
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