Debugging with GDB: Protocol Basics

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E.13.2 Protocol Basics

The File-I/O protocol uses the F packet as the request as well as reply packet. Since a File-I/O system call can only occur when GDB is waiting for a response from the continuing or stepping target, the File-I/O request is a reply that GDB has to expect as a result of a previous ‘C’, ‘c’, ‘S’ or ‘s’ packet. This F packet contains all information needed to allow GDB to call the appropriate host system call:

• A unique identifier for the requested system call.
• All parameters to the system call. Pointers are given as addresses in the target memory address space. Pointers to strings are given as pointer/length pair. Numerical values are given as they are. Numerical control flags are given in a protocol-specific representation.

At this point, GDB has to perform the following actions.

• If the parameters include pointer values to data needed as input to a system call, GDB requests this data from the target with a standard m packet request. This additional communication has to be expected by the target implementation and is handled as any other m packet.
• GDB translates all value from protocol representation to host representation as needed. Datatypes are coerced into the host types.
• GDB calls the system call.
• It then coerces datatypes back to protocol representation.
• If the system call is expected to return data in buffer space specified by pointer parameters to the call, the data is transmitted to the target using a M or X packet. This packet has to be expected by the target implementation and is handled as any other M or X packet.

Eventually GDB replies with another F packet which contains all necessary information for the target to continue. This at least contains

• Return value.
• errno, if has been changed by the system call.
• “Ctrl-C” flag.

After having done the needed type and value coercion, the target continues the latest continue or step action.

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