+@node Host I/O Packets
+@section Host I/O Packets
+@cindex Host I/O, remote protocol
+@cindex file transfer, remote protocol
+
+The @dfn{Host I/O} packets allow @value{GDBN} to perform I/O
+operations on the far side of a remote link. For example, Host I/O is
+used to upload and download files to a remote target with its own
+filesystem. Host I/O uses the same constant values and data structure
+layout as the target-initiated File-I/O protocol. However, the
+Host I/O packets are structured differently. The target-initiated
+protocol relies on target memory to store parameters and buffers.
+Host I/O requests are initiated by @value{GDBN}, and the
+target's memory is not involved. @xref{File-I/O Remote Protocol
+Extension}, for more details on the target-initiated protocol.
+
+The Host I/O request packets all encode a single operation along with
+its arguments. They have this format:
+
+@table @samp
+
+@item vFile:@var{operation}: @var{parameter}@dots{}
+@var{operation} is the name of the particular request; the target
+should compare the entire packet name up to the second colon when checking
+for a supported operation. The format of @var{parameter} depends on
+the operation. Numbers are always passed in hexadecimal. Negative
+numbers have an explicit minus sign (i.e.@: two's complement is not
+used). Strings (e.g.@: filenames) are encoded as a series of
+hexadecimal bytes. The last argument to a system call may be a
+buffer of escaped binary data (@pxref{Binary Data}).
+
+@end table
+
+The valid responses to Host I/O packets are:
+
+@table @samp
+
+@item F @var{result} [, @var{errno}] [; @var{attachment}]
+@var{result} is the integer value returned by this operation, usually
+non-negative for success and -1 for errors. If an error has occured,
+@var{errno} will be included in the result. @var{errno} will have a
+value defined by the File-I/O protocol (@pxref{Errno Values}). For
+operations which return data, @var{attachment} supplies the data as a
+binary buffer. Binary buffers in response packets are escaped in the
+normal way (@pxref{Binary Data}). See the individual packet
+documentation for the interpretation of @var{result} and
+@var{attachment}.
+
+@item
+An empty response indicates that this operation is not recognized.
+
+@end table
+
+These are the supported Host I/O operations:
+
+@table @samp
+@item vFile:open: @var{pathname}, @var{flags}, @var{mode}
+Open a file at @var{pathname} and return a file descriptor for it, or
+return -1 if an error occurs. @var{pathname} is a string,
+@var{flags} is an integer indicating a mask of open flags
+(@pxref{Open Flags}), and @var{mode} is an integer indicating a mask
+of mode bits to use if the file is created (@pxref{mode_t Values}).
+@xref{open}, for details of the open flags and mode values.
+
+@item vFile:close: @var{fd}
+Close the open file corresponding to @var{fd} and return 0, or
+-1 if an error occurs.
+
+@item vFile:pread: @var{fd}, @var{count}, @var{offset}
+Read data from the open file corresponding to @var{fd}. Up to
+@var{count} bytes will be read from the file, starting at @var{offset}
+relative to the start of the file. The target may read fewer bytes;
+common reasons include packet size limits and an end-of-file
+condition. The number of bytes read is returned. Zero should only be
+returned for a successful read at the end of the file, or if
+@var{count} was zero.
+
+The data read should be returned as a binary attachment on success.
+If zero bytes were read, the response should include an empty binary
+attachment (i.e.@: a trailing semicolon). The return value is the
+number of target bytes read; the binary attachment may be longer if
+some characters were escaped.
+
+@item vFile:pwrite: @var{fd}, @var{offset}, @var{data}
+Write @var{data} (a binary buffer) to the open file corresponding
+to @var{fd}. Start the write at @var{offset} from the start of the
+file. Unlike many @code{write} system calls, there is no
+separate @var{count} argument; the length of @var{data} in the
+packet is used. @samp{vFile:write} returns the number of bytes written,
+which may be shorter than the length of @var{data}, or -1 if an
+error occurred.
+
+@item vFile:unlink: @var{pathname}
+Delete the file at @var{pathname} on the target. Return 0,
+or -1 if an error occurs. @var{pathname} is a string.
+
+@end table
+