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).

AUTHORS

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

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