HIREDIS

Hiredis is a minimalistic C client library for the Redis database.

It is minimalistic because it just adds minimal support for the protocol, but at the same time it uses an high level printf-alike API in order to make it much higher level than otherwise suggested by its minimal code base and the lack of explicit bindings for every Redis command.

Apart from supporting sending commands and receiving replies, it comes with a reply parser that is decoupled from the I/O layer. It is a stream parser designed for easy reusability, which can for instance be used in higher level language bindings for efficient reply parsing.

Hiredis only supports the binary-safe Redis protocol, so you can use it with any Redis version >= 1.2.0.

The library comes with multiple APIs. There is the synchronous API, the asynchronous API and the reply parsing API.

SYNCHRONOUS API

To consume the synchronous API, there are only a few function calls that need to be introduced:

redisContext *redisConnect(const char *ip, int port);
void *redisCommand(redisContext *c, const char *format, ...);
void freeReplyObject(void *reply);

Connecting

The function redisConnect is used to create a so-called redisContext. The context is where Hiredis holds state for a connection. The redisContext struct has an error field that is non-NULL when the connection is in an error state. It contains a string with a textual representation of the error. After trying to connect to Redis using redisConnect you should check the error field to see if establishing the connection was successful:

redisContext *c = redisConnect("127.0.0.1", 6379);
if (c->error != NULL) {
  printf("Error: %s\n", c->error);
  // handle error
}

Sending commands

There are several ways to issue commands to Redis. The first that will be introduced is redisCommand. This function takes a format similar to printf. In the simplest form, it is used like this:

reply = redisCommand(context, "SET foo bar");

The specifier %s interpolates a string in the command, and uses strlen to determine the length of the string:

reply = redisCommand(context, "SET foo %s", value);

When you need to pass binary safe strings in a command, the %b specifier can be used. Together with a pointer to the string, it requires a size_t length argument of the string:

reply = redisCommand(context, "SET foo %b", value, valuelen);

Internally, Hiredis splits the command in different arguments and will convert it to the protocol used to communicate with Redis. One or more spaces separates arguments, so you can use the specifiers anywhere in an argument:

reply = redisCommand("SET key:%s %s", myid, value);

Using replies

The return value of redisCommand holds a reply when the command was successfully executed. When the return value is NULL, the error field in the context can be used to find out what was the cause of failure. Once an error is returned the context cannot be reused and you should set up a new connection.

The standard replies that redisCommand are of the type redisReply. The type field in the redisReply should be used to test what kind of reply was received:

• REDIS_REPLY_STATUS: The command replied with a status reply. The status string can be accessed using reply->str. The length of this string can be accessed using reply->len.

• REDIS_REPLY_ERROR: The command replied with an error. The error string can be accessed identical to REDIS_REPLY_STATUS.

• REDIS_REPLY_INTEGER: The command replied with an integer. The integer value can be accessed using the reply->integer field of type long long.

• REDIS_REPLY_NIL: The command replied with a nil object. There is no data to access.

• REDIS_REPLY_STRING: A bulk (string) reply. The value of the reply can be accessed using reply->str. The length of this string can be accessed using reply->len.

• REDIS_REPLY_ARRAY: A multi bulk reply. The number of elements in the multi bulk reply is stored in reply->elements. Every element in the multi bulk reply is a redisReply object as well and can be accessed via reply->elements[..index..]. Redis may reply with nested arrays but this is fully supported.

Replies should be freed using the freeReplyObject() function. Note that this function will take care of freeing sub-replies objects contained in arrays and nested arrays, so there is no need for the user to free the sub replies (it is actually harmful and will corrupt the memory).

Cleaning up

To disconnect and free the context the following function can be used:

void redisFree(redisContext *c);

This function immediately closes the socket and then free's the allocations done in creating the context.

Sending commands (cont'd)

Together with redisCommand, the function redisCommandArgv can be used to issue commands. It has the following prototype:

void *redisCommandArgv(redisContext *c, int argc, const char **argv, const size_t *argvlen);

It takes the number of arguments argc, an array of strings argv and the lengths of the arguments argvlen. For convenience, argvlen may be set to NULL and the function will use strlen(3) on every argument to determine its length. Obviously, when any of the arguments need to be binary safe, the entire array of lengths argvlen should be provided.

The return value has the same semantic as redisCommand.

Pipelining

To explain how Hiredis supports pipelining in a blocking connection, there needs to be understanding of the internal execution flow.

When any of the functions in the redisCommand family is called, Hiredis first formats the command according to the Redis protocol. The formatted command is then put in the output buffer of the context. This output buffer is dynamic, so it can hold any number of commands. After the command is put in the output buffer, redisGetReply is called. This function has the following two execution paths:

1. The input buffer is non-empty:

• Try to parse a single reply from the input buffer and return it
• If no reply could be parsed, continue at 2

2. The input buffer is empty:

• Write the entire output buffer to the socket
• Read from the socket until a single reply could be parsed

The function redisGetReply is exported as part of the Hiredis API and can be used when a reply is expected on the socket. To pipeline commands, the only things that needs to be done is filling up the output buffer. For this cause, two commands can be used that are identical to the redisCommand family, apart from not returning a reply:

void redisAppendCommand(redisContext *c, const char *format, ...);
void redisAppendCommandArgv(redisContext *c, int argc, const char **argv, const size_t *argvlen);

After calling either function one or more times, redisGetReply can be used to receive the subsequent replies. The return value for this function is either REDIS_OK or REDIS_ERR, where the latter means an error occurred while reading a reply. Just as with the other commands, the error field in the context can be used to find out what the cause of this error is.

The following examples shows a simple pipeline (resulting in only a single call to write(2) and a single call to write(2)):

redisReply *reply;
redisAppendCommand(context,"SET foo bar");
redisAppendCommand(context,"GET foo");
redisGetReply(context,&reply); // reply for SET
freeReplyObject(reply);
redisGetReply(context,&reply); // reply for GET
freeReplyObject(reply);

This API can also be used to implement a blocking subscriber:

reply = redisCommand(context,"SUBSCRIBE foo");
freeReplyObject(reply);
while(redisGetReply(context,&reply) == REDIS_OK) {
  // consume message
  freeReplyObject(reply);
}

Asynchronous API

Hiredis comes with an asynchronous API that works easily with any event library. Examples are bundled that show using Hiredis with libev and libevent.

Connecting

The function redisAsyncConnect can be used to establish a non-blocking connection to Redis. It returns a pointer to the newly created redisAsyncContext struct. The error field should be checked after creation to see if there were errors creating the connection. Because the connection that will be created is non-blocking, the kernel is not able to instantly return if the specified host and port is able to accept a connection.

redisAsyncContext *c = redisAsyncConnect("127.0.0.1", 6379);
if (c->error != NULL) {
  printf("Error: %s\n", c->error);
  // handle error
}

The asynchronous context can hold a disconnect callback function that is called when the connection is disconnected (either because of an error or per user request). This function should have the following prototype:

void(const redisAsyncContext *c, int status);

On a disconnect, the status argument is set to REDIS_OK when disconnection was initiated by the user, or REDIS_ERR when the disconnection was caused by an error. When it is REDIS_ERR, the error field in the context can be accessed to find out the cause of the error.

The context object is always free'd after the disconnect callback fired. When a reconnect is needed, the disconnect callback is a good point to do so.

Setting the disconnect callback can only be done once per context. For subsequent calls it will return REDIS_ERR. The function to set the disconnect callback has the following prototype:

int redisAsyncSetDisconnectCallback(redisAsyncContext *ac, redisDisconnectCallback *fn);

Sending commands and their callbacks

In an asynchronous context, commands are automatically pipelined due to the nature of an event loop. Therefore, unlike the synchronous API, there is only a single way to send commands. Because commands are sent to Redis asynchronously, issuing a command requires a callback function that is called when the reply is received. Reply callbacks should have the following prototype:

void(redisAsyncContext *c, void *reply, void *privdata);

The privdata argument can be used to curry arbitrary data to the callback from the point where the command is initially queued for execution.

The functions that can be used to issue commands in an asynchronous context are:

int redisAsyncCommand(
  redisAsyncContext *ac, redisCallbackFn *fn, void *privdata,
  const char *format, ...);
int redisAsyncCommandArgv(
  redisAsyncContext *ac, redisCallbackFn *fn, void *privdata,
  int argc, const char **argv, const size_t *argvlen);

Both functions work like their blocking counterparts. The return value is REDIS_OK when the command was successfully added to the output buffer and REDIS_ERR otherwise. Example: when the connection is being disconnected per user-request, no new commands may be added to the output buffer and REDIS_ERR is returned on calls to the redisAsyncCommand family.

If the reply for a command with a NULL callback is read, it is immediately free'd. When the callback for a command is non-NULL, it is responsible for cleaning up the reply.

All pending callbacks are called with a NULL reply when the context encountered an error.

Disconnecting

An asynchronous connection can be terminated using:

void redisAsyncDisconnect(redisAsyncContext *ac);

When this function is called, the connection is not immediately terminated. Instead, new commands are no longer accepted and the connection is only terminated when all pending commands have been written to the socket, their respective replies have been read and their respective callbacks have been executed. After this, the disconnection callback is executed with the REDIS_OK status and the context object is free'd.

Hooking it up to event library X

There are a few hooks that need to be set on the context object after it is created. See the adapters/ directory for bindings to libev and libevent.

Reply parsing API

To be done.

AUTHORS

Hiredis was written by Salvatore Sanfilippo (antirez at gmail) and Pieter Noordhuis (pcnoordhuis at gmail) and is released under the BSD license.