ZeroMQ

A Minor Note on Strings

ØMQ doesn’t know anything about the data you send except its size in bytes. That means you are responsible for formatting it safely so that applications can read it back. Doing this for objects and complex data types is a job for specialized libraries like protocol buffers. But even for strings, you need to take care.

In C and some other languages, strings are terminated with a null byte. We could send a string like “HELLO” with that extra null byte:

zmq_msg_init_data (&request, "Hello", 6, NULL, NULL);

However, if you send a string from another language, it probably will not include that null byte. For example, when we send that same string in Python, we do this:

socket.send ("Hello")

Then what goes onto the wire is a length (one byte for shorter strings) and the string contents as individual characters (Figure 1-3).

Figure 1-3. A ØMQ string

And if you read this from a C program, you will get something that looks like a string, and might by accident act like a string (if by luck the five bytes find themselves followed by an innocently lurking null), but isn’t a proper string. When your client and server don’t agree on the string format, you will get weird results.

When you receive string data from ØMQ in C, you simply cannot trust that it’s safely terminated. Every single time you read a string, you should allocate a new buffer with space for an extra byte, copy the string, and terminate it properly with a null.

So let’s establish the rule that ØMQ strings are length-specified and are sent on the wire without a trailing null. In the simplest case (and we’ll do this in our examples), a ØMQ string maps neatly to a ØMQ message frame, which looks like the above figure—a length and some bytes.

Here is what we need to do, in C, to receive a ØMQ string and deliver it to the application as a valid C string:

//  Receive 0MQ string from socket and convert into C string
static char *
s_recv (void *socket) {
    zmq_msg_t message;
    zmq_msg_init (&message);
    int size = zmq_msg_recv (&message, socket, 0);
    if (size == -1)
        return NULL;
    char *string = malloc (size + 1);
    memcpy (string, zmq_msg_data (&message), size);
    zmq_msg_close (&message);
    string [size] = 0;
    return (string);
}

This makes a very handy helper function. In the spirit of making things we can reuse profitably, we can write a similar s_send() function that sends strings in the correct ØMQ format and package this into a header file we can reuse.

The result is zhelpers.h, which lets us write sweeter and shorter ØMQ applications in C. It is a fairly long source, and only fun for C developers, so read it at your leisure.