D.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.
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
mpacket request. This additional communication has to be expected by the target implementation and is handled as any other
- 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
Xpacket. This packet has to be expected by the target implementation and is handled as any other
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.