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Diffstat (limited to 'doc')
| -rw-r--r-- | doc/mach.texi | 19 |
1 files changed, 19 insertions, 0 deletions
diff --git a/doc/mach.texi b/doc/mach.texi index b187888..c57e607 100644 --- a/doc/mach.texi +++ b/doc/mach.texi @@ -1949,6 +1949,25 @@ loses the receive right after the message was dequeued from it, then right still exists, but isn't held by the caller, then @code{msgh_local_port} specifies @code{MACH_PORT_NULL}. +Servers usually associate some state with a receive right. To that +end, they might use a hash table to look up the state for the port a +message was sent to. To optimize this, a task may associate an opaque +@var{payload} with a receive right using the +@code{mach_port_set_protected_payload} function. Once this is done, +the kernel will set the @code{msgh_protected_payload} field to +@var{payload} when delivering a message to this right and indicate +this by setting the local part of @code{msgh_bits} to +@code{MACH_MSG_TYPE_PROTECTED_PAYLOAD}. + +The support for protected payloads was added to GNU Mach. To preserve +binary compatibility, the @code{msgh_local_port} and +@code{msgh_local_port} share the same location. This makes it +possible to add the payload information without increasing the size of +@code{mach_msg_header_t}. This is an implementation detail. Which +field is valid is determined by the local part of the +@code{msgh_bits}. Existing software is not affected. When a receive +right is transferred to another task, its payload is cleared. + Received messages are stamped with a sequence number, taken from the port from which the message was received. (Messages received from a port set are stamped with a sequence number from the appropriate member |