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 otherm
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
orX
packet. This packet has to be expected by the target implementation and is handled as any otherM
orX
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.
Next: The F Request Packet, Previous: File-I/O Overview, Up: File-I/O Remote Protocol Extension [Contents][Index]