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  • 2022-05-21 10:13
[![Build Status](]( # 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*. ## UPGRADING Version 0.9.0 is a major overhaul of hiredis in every aspect. However, upgrading existing code using hiredis should not be a big pain. The key thing to keep in mind when upgrading is that hiredis >= 0.9.0 uses a `redisContext*` to keep state, in contrast to the stateless 0.0.1 that only has a file descriptor to work with. ## 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 integer `err` field that is non-zero when an the connection is in an error state. The field `errstr` will contain a string with a description of the error. More information on errors can be found in the **Errors** section. After trying to connect to Redis using `redisConnect` you should check the `err` field to see if establishing the connection was successful: redisContext *c = redisConnect("", 6379); if (c != NULL && c->err) { printf("Error: %s\n", c->errstr); // 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, (size_t) 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(context, "SET key:%s %s", myid, value); ### Using replies The return value of `redisCommand` holds a reply when the command was successfully executed. When an error occurs, the return value is `NULL` and the `err` field in the context will be set (see section on **Errors**). 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->element[..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). **Important:** the current version of hiredis (0.10.0) free's replies when the asynchronous API is used. This means you should not call `freeReplyObject` when you use this API. The reply is cleaned up by hiredis _after_ the callback returns. This behavior will probably change in future releases, so make sure to keep an eye on the changelog when upgrading (see issue #39). ### 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
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