Integrate hiredis with a libevent app

The libevent library provides an implementation of an event loop that lets you call functions asynchronously in response to events. This guide explains how to use hiredis to connect to a Redis server from a libevent app.

Install libevent

The libevent home page has links to download all versions of the library, but you should use the latest version unless there is a specific version you need to target.

When you have downloaded libevent, follow the instructions in the README file to compile and install the library.

Create a simple app

For a real project, you would build your code with a makefile, but for this simple test, you can just place it in a file called main.c and build it with the following command (assuming you used make install to install the libhiredis and libevent libraries):

cc main.c -L/usr/local/lib -lhiredis -levent

See [Build and install]({{< relref "/develop/clients/hiredis#build-and-install" >}}) to learn how to build hiredis, if you have not already done so.

Now, add the following code in main.c. An explanation follows the code example:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <signal.h>

#include <hiredis/hiredis.h>
#include <hiredis/async.h>
#include <hiredis/adapters/libevent.h>

// Callback for the `GET` command. 
void getCallback(redisAsyncContext *c, void *r, void *privdata) {
    redisReply *reply = r;
    char *key = privdata;

    if (reply == NULL) {
        if (c->errstr) {
            printf("errstr: %s\n", c->errstr);
        }
        return;
    }

    printf("Key: %s, value: %s\n", key, reply->str);

    /* Disconnect after receiving the reply to GET */
    redisAsyncDisconnect(c);
}

// Callback to respond to successful or unsuccessful connection.
void connectCallback(const redisAsyncContext *c, int status) {
    if (status != REDIS_OK) {
        printf("Error: %s\n", c->errstr);
        return;
    }
    printf("Connected...\n");
}

// Callback to respond to intentional or unexpected disconnection.
void disconnectCallback(const redisAsyncContext *c, int status) {
    if (status != REDIS_OK) {
        printf("Error: %s\n", c->errstr);
        return;
    }
    printf("Disconnected...\n");
}


int main (int argc, char **argv) {
#ifndef _WIN32
    signal(SIGPIPE, SIG_IGN);
#endif

    // Create the libevent `event_base` object to track all
    // events.
    struct event_base *base = event_base_new();

    redisAsyncContext *c = redisAsyncConnect("127.0.0.1", 6379);

    if (c->err) {
        printf("Error: %s\n", c->errstr);
        return 1;
    }

    // Use the Redis libevent adapter to attach the Redis connection
    // to the libevent main loop.
    redisLibeventAttach(c,base);
    
    redisAsyncSetConnectCallback(c, connectCallback);
    redisAsyncSetDisconnectCallback(c, disconnectCallback);
    
    char *key = "testkey";
    char *value = "testvalue";

    redisAsyncCommand(c, NULL, NULL, "SET %s %s", key, value);
    redisAsyncCommand(c, getCallback, key, "GET %s", key);
    
    // Run the event loop.
    event_base_dispatch(base);

    return 0;
}

The code calls event_base_new() to initialize the core event_base object that manages the event loop. It then creates a standard [asynchronous connection]({{< relref "/develop/clients/hiredis/connect#asynchronous-connection" >}}) to Redis and uses the libevent adapter function redisLibeventAttach() to attach the connection to the event loop.

After setting the [connection callbacks]({{< relref "/develop/clients/hiredis/connect#asynchronous-connection" >}}), the code issues two asynchronous Redis commands (see [Construct asynchronous commands]({{< relref "/develop/clients/hiredis/issue-commands#construct-asynchronous-commands" >}}) for more information). The final step is to call event_base_dispatch() to start the event loop. This will wait for the commands to be processed and then exit when the Redis connection is closed in the getCallback() function.

Run the code

If you compile and run the code, you will see the following output, showing that the callbacks executed correctly:

Connected...
Key: testkey, value: testvalue
Disconnected...

You can use the [KEYS]({{< relref "/commands/keys" >}}) command from [redis-cli]({{< relref "/develop/tools/cli" >}}) or [Redis Insight]({{< relref "/develop/tools/insight" >}}) to check that the "testkey" string key was added to the Redis database.