Hook mach_msg and print message contents

mach_msg_hook.c

/*
 * View the contents of messages sent/received via `mach_msg`.
 *
 * Compile with:
 * clang -arch x86_64 -arch i386 -Wall -o mach_msg_hook.dylib -dynamiclib mach_msg_hook.c
 *
 * and run as:
 * DYLD_FORCE_FLAT_NAMESPACE=1 DYLD_INSERT_LIBRARIES=mach_msg_hook.dylib [COMMAND]
 *
 * Have fun.
 */
#include <stdbool.h>
#include <stdio.h>
#include <stdint.h>
#include <ctype.h>
#include <dlfcn.h>
#include <xpc/xpc.h>

void print_hexdump(const void *buf, size_t len)
{
    /* Embrace the DIY paradigm. */
}

mach_msg_return_t (*orig_mach_msg)(mach_msg_header_t *,
    mach_msg_option_t, mach_msg_size_t,
    mach_msg_size_t,
    mach_port_t,
    mach_msg_timeout_t,
    mach_port_t);

mach_msg_return_t mach_msg(mach_msg_header_t *msg,
    mach_msg_option_t option,
    mach_msg_size_t send_size,
    mach_msg_size_t rcv_size,
    mach_port_t rcv_name,
    mach_msg_timeout_t timeout,
    mach_port_t notify){

    /* We will call the original `mach_msg` after we print out the contents of each
     * message:
     */
    if(!orig_mach_msg){
        orig_mach_msg = dlsym(RTLD_NEXT, "mach_msg");
    }

    /* The mach message's local_port is how a response will be sent
     * back. If this is set, we assume there will be a response.
     */
    bool response = msg->msgh_local_port > 0;

    /* Request:*/
    print_hexdump(msg, send_size);
    mach_msg_return_t ret = mach_msg(msg,
        option,
        send_size,
        rcv_size,
        rcv_name,
        timeout,
        notify);

    /* Response: */
    if(response){
        print_hexdump(msg, rcv_size);
    }

    return(ret);
}