Setup complex mach message header

Question

I'm trying to setup a mach message for sending a string out of line using mach IPC. However I get an (ipc/send) invalid msg-header error when trying to send the out of line message.

Here's the message structure.

struct ool_msg
{
    mach_msg_header_t header;
    mach_msg_body_t body;
    mach_msg_ool_descriptor_t data;
    mach_msg_type_number_t count;
};

I'm setting up the header like this. The variable msg below is a struct ool_msg.

hdr = &(msg.header);
hdr->msgh_bits = MACH_MSGH_BITS_LOCAL(MACH_MSG_TYPE_COPY_SEND);
hdr->msgh_bits |= MACH_MSGH_BITS_COMPLEX;
hdr->msgh_size = sizeof(msg);
hdr->msgh_local_port = MACH_PORT_NULL;
hdr->msgh_remote_port = port;

This is how I setup up the rest of the Mach message.

msg.body.msgh_descriptor_count = 1;
msg.data.address = string;
msg.data.size = strlen(string) + 1;
msg.data.deallocate = FALSE;
msg.data.copy = MACH_MSG_VIRTUAL_COPY;
msg.data.type = MACH_MSG_OOL_DESCRIPTOR;
msg.count = msg.data.size;

So how do I correctly setup the message header for OOL Mach IPC?

Edit

Here's a program that reproduces my problem.

#include <stdio.h>
#include <mach/mach.h>

static char *string = "123456789";

struct ool_msg
{
    mach_msg_header_t header;
    mach_msg_body_t body;
    mach_msg_ool_descriptor_t data;
    mach_msg_type_number_t count;
};

int main(void)
{
    kern_return_t err;
    struct ool_msg msg;
    mach_msg_header_t *hdr;
    mach_port_t port = MACH_PORT_NULL;

    err = mach_port_allocate(mach_task_self(), MACH_PORT_RIGHT_RECEIVE, &port);
    if(err != KERN_SUCCESS)
    {
        mach_error("Can't allocate mach port\n", err);
        return (-1);
    }

    err = mach_port_insert_right(mach_task_self(), port, port,
                                 MACH_MSG_TYPE_MAKE_SEND);
    if(err != KERN_SUCCESS)
    {
        mach_error("Can't insert port right\n", err);
        return (-1);
    }

    hdr = &(msg.header);
    hdr->msgh_bits = MACH_MSGH_BITS_LOCAL(MACH_MSG_TYPE_COPY_SEND);
    hdr->msgh_bits |= MACH_MSGH_BITS_COMPLEX;
    hdr->msgh_size = sizeof(msg);
    hdr->msgh_local_port = MACH_PORT_NULL;
    hdr->msgh_remote_port = port;

    msg.body.msgh_descriptor_count = 1;
    msg.data.address = string;
    msg.data.size = strlen(string) + 1;
    msg.data.deallocate = FALSE;
    msg.data.copy = MACH_MSG_VIRTUAL_COPY;
    msg.data.type = MACH_MSG_OOL_DESCRIPTOR;
    msg.count = msg.data.size;

    err = mach_msg(hdr,
                  MACH_SEND_MSG,
                  hdr->msgh_size,
                  0,
                  MACH_PORT_NULL,
                  MACH_MSG_TIMEOUT_NONE,
                  MACH_PORT_NULL);
    if(err != KERN_SUCCESS)
    {
        mach_error("Can't send mach msg\n", err);
        return (-1);
    }

    return (0);
}

Answer 1

As the docs state:

MACH_MSGH_BITS_REMOTE and MACH_MSGH_BITS_LOCAL macros return the appropriate mach_msg_type_name_t values, given a msgh_bits value. The MACH_MSGH_BITS macro constructs a value for msgh_bits, given two mach_msg_type_name_t values.

So you need to use the MACH_MSGH_BITS macro rather than the MACH_MSGH_BITS_LOCAL macro when setting the mach_msg_header_t.msgh_bits. Otherwise you are setting it to a mach_msg_type_name_t as opposed to the msgh_bits which is the one you want.

To clarify, you need something like the following piece of code to get your msgh_bits:

hdr->msgh_bits = MACH_MSGH_BITS(MACH_MSG_TYPE_COPY_SEND, MACH_MSG_TYPE_MAKE_SEND)