A hello world for complex mach messages

ipc_poc.c

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

#define plog(str,args...) printf("[%6d] "str"\n",getpid(), ##args)
#define HandleError(kr) if (kr != KERN_SUCCESS) {\
                printf("error: line %d in PID: %d, (%d) 0x%x, %s\n", __LINE__,\
                getpid(), kr, kr, mach_error_string(kr));\
                exit(1);}

struct favorite_animal_msg {
    mach_msg_header_t header;
    mach_msg_size_t count; // will be 1
    mach_msg_ool_descriptor_t question; // is type MACH_MSG_OOL_DESCRIPTOR
};

mach_port_t create_port(void) {
  mach_port_t port = MACH_PORT_NULL;
    // There's also mach_port_construct for a one liner but you will see more
    // examples of going this way
  HandleError(mach_port_allocate(mach_task_self(), MACH_PORT_RIGHT_RECEIVE, &port));
  HandleError(mach_port_insert_right(mach_task_self(), port, port, MACH_MSG_TYPE_MAKE_SEND));
  plog("port is 0x%x", port);
    
  return port;
}

void send_message(mach_port_t remotePort) {
    
    mach_port_t sendPort = MACH_PORT_NULL;
      mach_msg_type_name_t acquiredType = 0;
      task_t parentTask = TASK_NULL;
      HandleError(task_for_pid(mach_task_self(), getppid(), &parentTask));
      HandleError(mach_port_extract_right(parentTask,
                                           remotePort,
                         MACH_MSG_TYPE_MAKE_SEND_ONCE,
                                         &sendPort,
                                      &acquiredType));
      
      struct favorite_animal_msg msg = {};

      mach_port_t localPort = create_port(); // 1
      msg.header.msgh_bits = MACH_MSGH_BITS_SET(MACH_MSG_TYPE_MOVE_SEND_ONCE,
                                                MACH_MSG_TYPE_MAKE_SEND_ONCE, // 2
                                                                           0,
                                                      MACH_MSGH_BITS_COMPLEX);
      msg.header.msgh_local_port = localPort; // 3

      msg.header.msgh_id = 12345;
      msg.header.msgh_remote_port = sendPort;
      msg.header.msgh_size = sizeof(msg);
      msg.header.msgh_voucher_port = MACH_PORT_NULL;
      
      char questionStr[1024] = {};
      snprintf(questionStr, 1024, "Yo, what's your favorite animal? [Sent from %d]", getpid());
      msg.count = 1;

      msg.question.address = questionStr;
      msg.question.type = MACH_MSG_OOL_DESCRIPTOR;
      msg.question.size = (mach_msg_size_t)strlen(questionStr);
      msg.question.copy = MACH_MSG_PHYSICAL_COPY;
      msg.question.deallocate = 0;
      
      plog("Sending message from child");
      HandleError(mach_msg(&msg.header,
                  MACH_RCV_MSG|MACH_SEND_MSG, // 4
                                             sizeof(struct favorite_animal_msg),
                          sizeof(struct favorite_animal_msg) + MAX_TRAILER_SIZE,
                                                                      localPort,
                           0000000,
                                                                MACH_PORT_NULL));

      plog("!!!! Resp is \"%s\" !!!!!!", (char*)msg.question.address); // 5
}

void recv_message(mach_port_t localPort) {
    struct favorite_animal_msg msg = {};
    msg.header.msgh_local_port = localPort;
    msg.header.msgh_size = sizeof(msg) + MAX_TRAILER_SIZE;
    HandleError(mach_msg_receive(&msg.header));
    plog("msgh_id: %d", msg.header.msgh_id);
    plog("%s", (char*)msg.question.address);

    char respStr[1024];
    snprintf(respStr, 1024, "Koaka [Sent from %d]", getpid());
    msg.question.address = respStr;
    msg.question.size = (mach_msg_size_t)strlen(respStr);
    msg.question.copy = MACH_MSG_PHYSICAL_COPY;

    msg.header.msgh_local_port = MACH_PORT_NULL;
    HandleError(mach_msg_send(&msg.header));
}


int main() {
  mach_port_t port = create_port();
  pid_t pid = fork();
  if (pid == 0) {
    plog("I am the child");
    sleep(2);
    send_message(port);
  } else if (pid > 0) {
    plog("I am the parent");
    sleep(2);
      
    recv_message(port);
    wait(&pid);
  }
    
  return 0;
}