Protocol Basics - Debugging with GDB
Description: Debugging with GDB

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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.