gdb.info: The F reply packet

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The `F' reply packet

The `F' reply packet has the following format:
          `F'RETCODE`,'ERRNO`,'CTRL-C FLAG`;'CALL SPECIFIC ATTACHMENT
     RETCODE is the return code of the system call as hexadecimal value.
     ERRNO is the errno set by the call, in protocol specific
     representation.  This parameter can be omitted if the call was
     successful.
     CTRL-C FLAG is only send if the user requested a break.  In this
     case, ERRNO must be send as well, even if the call was successful.
     The CTRL-C FLAG itself consists of the character 'C':

F0,0,C

     or, if the call was interupted before the host call has been
     performed:

F-1,4,C

     assuming 4 is the protocol specific representation of `EINTR'.