# nng_ctx(5) Manual Page

## NAME

nng_ctx - protocol context

## SYNOPSIS

    #include <nng/nng.h>

    typedef struct nng_ctx_s nng_ctx

## DESCRIPTION

An `nng_ctx` is a handle to an underlying context object, which keeps the protocol state for some stateful protocols. The purpose of a separate context object is to permit applications to share a single socket, with its various underlying dialers, listeners, and pipes, while still benefiting from separate state tracking.

For example, a req context will contain the request ID of any sent request, a timer to retry the request on failure, and so forth. A separate context on the same socket can have similar data, but corresponding to a completely different request.

| The `nng_ctx` structure is always passed by value (both for input parameters and return values), and should be treated opaquely. Passing structures this way gives the compiler a chance to perform accurate type checks in functions passing values of this type.

All contexts share the same socket, and so some options, as well as the underlying transport details, will be common to all contexts on that socket.

| Not every protocol supports separate contexts. See the protocol-specific documentation for further details about whether contexts are supported, and details about what options are supported for contexts.

Protocols that make use of contexts will also have a default context that is used when the socket global operations are used. Operations using the global context will generally not interfere with any other contexts, except that certain socket options may affect socket global behavior.

Historically, applications wanting to use a stateful protocol concurrently would have to resort to raw mode sockets, which bypasses much of the various protocol handling, leaving it to up to the application to do so. Contexts make it possible to still benefit from advanced protocol handling, including timeouts, retries, and matching requests to responses, while doing so concurrently.

| Raw mode sockets do not support contexts, since there is generally no state tracked for them, and thus contexts make no sense.

| Contexts are an excellent mechanism to use when building concurrent applications, and should be used in lieu of raw mode sockets when possible.

| Use of file descriptor polling (with descriptors obtained using the `NNG_OPT_RECVFD` or `NNG_OPT_SENDFD` options) while contexts are in use on the same socket is not supported, and may lead to unpredictable behavior. These asynchronous methods should not be mixed on the same socket.

### Initialization

A context may be initialized using the macro `NNG_CTX_INITIALIZER` before it is opened, to prevent confusion with valid open contexts.

## EXAMPLE

The following program fragment demonstrates the use of contexts to implement a concurrent rep service that simply echos messages back to the sender.

    struct echo_context {
        nng_ctx ctx;
        nng_aio *aio;
        enum { INIT, RECV, SEND } state;
    };

    void
    echo(void *arg)
    {
        struct echo_context *ec = arg;

        switch (ec->state) {
        case INIT:
            ec->state = RECV;
            nng_ctx_recv(ec->ctx, ec->aio);
            return;
        case RECV:
            if (nng_aio_result(ec->aio) != 0) {
                // ... handle error
            }
            // We reuse the message on the ec->aio
            ec->state = SEND;
            nng_ctx_send(ec->ctx, ec->aio);
            return;
        case SEND:
            if (nng_aio_result(ec->aio) != 0) {
                // ... handle error
            }
            ec->state = RECV;
            nng_ctx_recv(ec->ctx, ec->aio);
            return;
        }
    }

Given the above fragment, the following example shows setting up the service. It assumes that the socket has already been created and any transports set up as well with functions such as `nng_dial()` or `nng_listen()`.

    #define CONCURRENCY 1024

    static struct echo_context ecs[CONCURRENCY];

    void
    start_echo_service(nng_socket rep_socket)
    {
        for (int i = 0; i < CONCURRENCY; i++) {
            // error checks elided for clarity
            nng_ctx_open(&ecs[i].ctx, rep_socket);
            nng_aio_alloc(&ecs[i].aio, echo, ecs+i);
            ecs[i].state = INIT;
            echo(ecs+i); // start it running
        }
    }

## SEE ALSO

libnng(3), nng_ctx_close(3), nng_ctx_open(3), nng_ctx_get(3), nng_ctx_id(3), nng_ctx_recv(3), nng_ctx_recvmsg(3), nng_ctx_send(3), nng_ctx_sendmsg(3), nng_ctx_set(3), nng_dialer(5), nng_listener(5), nng_socket(5), nng_options(5), nng(7)