Protocol Basics (Debugging with GDB)

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.