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This Readme reflects the latest changed in the master branch. See v0.13.3 for the Readme and documentation for the latest release.
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 a 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.
1.0.0
Version 1.0.0 marks a stable release of hiredis.
It includes some minor breaking changes, mostly to make the exposed API more uniform and self-explanatory.
It also bundles the updated sds
library, to sync up with upstream and Redis.
For most applications a recompile against the new hiredis should be enough.
For code changes see the Changelog.
<0.9.0
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.
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);
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 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("127.0.0.1", 6379);
if (c == NULL || c->err) {
if (c) {
printf("Error: %s\n", c->errstr);
// handle error
} else {
printf("Can't allocate redis context\n");
}
}
Note: A redisContext
is not thread-safe.
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);
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
:
reply->str
.
The length of this string can be accessed using reply->len
.REDIS_REPLY_ERROR
:
REDIS_REPLY_STATUS
.REDIS_REPLY_INTEGER
:
reply->integer
field of type long long
.REDIS_REPLY_NIL
:
REDIS_REPLY_STRING
:
reply->str
.
The length of this string can be accessed using reply->len
.REDIS_REPLY_ARRAY
:
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-reply 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) frees 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).
To disconnect and free the context the following function can be used:
void redisFree(redisContext *c);
This function immediately closes the socket and then frees the allocations done in creating the context.
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
.
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:
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 err
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 read(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);
}
When a function call is not successful, depending on the function either NULL
or REDIS_ERR
is
returned. The err
field inside the context will be non-zero and set to one of the
following constants:
REDIS_ERR_IO
:
There was an I/O error while creating the connection, trying to write
to the socket or read from the socket. If you included errno.h
in your
application, you can use the global errno
variable to find out what is
wrong.
REDIS_ERR_EOF
:
The server closed the connection which resulted in an empty read.
REDIS_ERR_PROTOCOL
:
There was an error while parsing the protocol.
REDIS_ERR_OTHER
:
Any other error. Currently, it is only used when a specified hostname to connect
to cannot be resolved.
In every case, the errstr
field in the context will be set to hold a string representation
of the error.
Hiredis comes with an asynchronous API that works easily with any event library. Examples are bundled that show using Hiredis with libev and libevent.
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 err
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.
Note: A redisAsyncContext
is not thread-safe.
redisAsyncContext *c = redisAsyncConnect("127.0.0.1", 6379);
if (c->err) {
printf("Error: %s\n", c->errstr);
// 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 err
field in the context can be accessed to find out the cause of the error.
The context object is always freed 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);
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 freed. When the callback
for a command is non-NULL
, the memory is freed immediately following the callback: the reply is only
valid for the duration of the callback.
All pending callbacks are called with a NULL
reply when the context encountered an error.
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 freed.
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.
Hiredis comes with a reply parsing API that makes it easy for writing higher level language bindings.
The reply parsing API consists of the following functions:
redisReader *redisReaderCreate(void);
void redisReaderFree(redisReader *reader);
int redisReaderFeed(redisReader *reader, const char *buf, size_t len);
int redisReaderGetReply(redisReader *reader, void **reply);
The same set of functions are used internally by hiredis when creating a normal Redis context, the above API just exposes it to the user for a direct usage.
The function redisReaderCreate
creates a redisReader
structure that holds a
buffer with unparsed data and state for the protocol parser.
Incoming data -- most likely from a socket -- can be placed in the internal
buffer of the redisReader
using redisReaderFeed
. This function will make a
copy of the buffer pointed to by buf
for len
bytes. This data is parsed
when redisReaderGetReply
is called. This function returns an integer status
and a reply object (as described above) via void **reply
. The returned status
can be either REDIS_OK
or REDIS_ERR
, where the latter means something went
wrong (either a protocol error, or an out of memory error).
The parser limits the level of nesting for multi bulk payloads to 7. If the multi bulk nesting level is higher than this, the parser returns an error.
The function redisReaderGetReply
creates redisReply
and makes the function
argument reply
point to the created redisReply
variable. For instance, if
the response of type REDIS_REPLY_STATUS
then the str
field of redisReply
will hold the status as a vanilla C string. However, the functions that are
responsible for creating instances of the redisReply
can be customized by
setting the fn
field on the redisReader
struct. This should be done
immediately after creating the redisReader
.
For example, hiredis-rb uses customized reply object functions to create Ruby objects.
Both when using the Reader API directly or when using it indirectly via a normal Redis context, the redisReader structure uses a buffer in order to accumulate data from the server. Usually this buffer is destroyed when it is empty and is larger than 16 KiB in order to avoid wasting memory in unused buffers
However when working with very big payloads destroying the buffer may slow
down performances considerably, so it is possible to modify the max size of
an idle buffer changing the value of the maxbuf
field of the reader structure
to the desired value. The special value of 0 means that there is no maximum
value for an idle buffer, so the buffer will never get freed.
For instance if you have a normal Redis context you can set the maximum idle buffer to zero (unlimited) just with:
context->reader->maxbuf = 0;
This should be done only in order to maximize performances when working with
large payloads. The context should be set back to REDIS_READER_MAX_BUF
again
as soon as possible in order to prevent allocation of useless memory.
Hiredis was written by Salvatore Sanfilippo (antirez at gmail) and
Pieter Noordhuis (pcnoordhuis at gmail) and is released under the BSD license.
Hiredis is currently maintained by Matt Stancliff (matt at genges dot com) and
Jan-Erik Rediger (janerik at fnordig dot com)