When sending non-complex (serialize = false) messages to a program in C, you need to add the null character to the string (\0). Otherwise the previous message will be partially visible if it is longer than the current message. Took some kind help from comp.lang.php for me to figure that out. While it seems so obvious now, I thought I'd share it here.
(PHP 4 >= 4.3.0, PHP 5)
msg_send — Send a message to a message queue
$serialize= true [, bool
$blocking= true [, int
msg_send() sends a
message of type
msgtype (which MUST be greater than 0) to
the message queue specified by
serializecontrols how the
TRUEwhich means that the
messageis serialized using the same mechanism as the session module before being sent to the queue. This allows complex arrays and objects to be sent to other PHP scripts, or if you are using the WDDX serializer, to any WDDX compatible client.
If the message is too large to fit in the queue, your script will wait until another process reads messages from the queue and frees enough space for your message to be sent. This is called blocking; you can prevent blocking by setting the optional
FALSE, in which case msg_send() will immediately return
FALSEif the message is too big for the queue, and set the optional
MSG_EAGAIN, indicating that you should try to send your message again a little later on.
TRUE on success or
FALSE on failure.
Upon successful completion the message queue data structure is updated as
msg_lspid is set to the process-ID of the
msg_qnum is incremented by 1 and
msg_stime is set to the current time.
After about an hour of debugging I've discovered the meaning of the undocumented "PHP Warning: msg_send(): msgsnd failed: Invalid argument" ($errorcode = 13).
This occurred when the size of $message was larger than msg_qbytes (see msg_stat_queue() for how to determine and change msg_qbytes).
$msgtype used in msg_send function can be any positive integer.
I think it is kinda lame why I cannot find out (easily) explicitly which types of messages are allowed and which ones are not. Maybe we can start our own little list. I know that strings work, and arrays do not.