What's in a pulse?

Okay, so you receive this message with a receive ID of zero. What does it actually look like?

From the <sys/neutrino.h> header file, here's the definition of the _pulse structure:

struct _pulse {
    uint16_t        type;
    uint16_t        subtype;
    int8_t          code;
    uint8_t         zero [3];
    union sigval    value;
    int32_t         scoid;
};

Both the type and subtype members are zero (a further indication that this is a pulse). The code and value members are set to whatever the sender of the pulse determined. Generally, the code will be an indication of why the pulse was sent; the value will be a 32-bit data value associated with the pulse. Those two fields are where the “40 bits” of content comes from; the other fields aren't user adjustable.

The kernel reserves negative values of code, leaving 128 values for programmers to use as they see fit.

The value member is actually a union:

union sigval {
    int     sival_int;
    void    *sival_ptr;
};

Therefore (expanding on the server example above), you often see code like:

#include <sys/neutrino.h>

    rcvid = MsgReceive (chid, …

    if (rcvid == 0) {   // it's a pulse

        // determine the type of pulse
        switch (msg.pulse.code) {

        case    MY_PULSE_TIMER:
            // One of your timers went off, do something 
            // about it...

            break;

        case    MY_PULSE_HWINT:
            // A hardware interrupt service routine sent 
            // you a pulse.  There's a value in the "value" 
            // member that you need to examine:

            val = msg.pulse.value.sival_int;

            // Do something about it...

            break;

        case    _PULSE_CODE_UNBLOCK:
            // A pulse from the kernel, indicating a client 
            // unblock was received, do something about it...

            break;

        // etc...

    } else {            // it's a regular message

        // determine the type of message
        // handle it

    }

This code assumes, of course, that you've set up your msg structure to contain a struct _pulse pulse; member, and that the manifest constants MY_PULSE_TIMER and MY_PULSE_HWINT are defined. The pulse code _PULSE_CODE_UNBLOCK is one of those negative-numbered kernel pulses mentioned above. You can find a complete list of them in <sys/neutrino.h> along with a brief description of the value field.