Creating C++ Redis Module - “does not export Redis

问题:

I am having some trouble loading my Redis module. I am just copying the example from https://redis.io/topics/modules-intro, but I stripped it down.

#include "redismodule.h"
#include <stdlib.h>

int RedisModule_OnLoad(RedisModuleCtx *ctx, RedisModuleString **argv, int argc) {
    if (RedisModule_Init(ctx,"avromodule",1,REDISMODULE_APIVER_1)
        == REDISMODULE_ERR) return REDISMODULE_ERR;


    return REDISMODULE_OK;
}

This is saved in avromodule.cpp. I compile it using the following:

g++ -shared -fPIC -o avromodule.so avromodule.cpp

Then I go over to the Redis CLI and try to load the module.

10.XXX.XXX.XXX:7004> module load /path/to/module/avromodule.so
(error) ERR Error loading the extension. Please check the server logs.

The server logs give me the following error:

159392:M 17 May 10:21:19.773 # Module /path/to/module/avromodule.so does not export RedisModule_OnLoad() symbol. Module not loaded.

The above error makes no sense to me, because I get the following output using the 'nm' command:

$ nm -CD avromodule.so | grep " T "
0000000000003622 T RedisModule_OnLoad(RedisModuleCtx*, RedisModuleString**, int)
000000000000366c T _fini
0000000000002878 T _init

Does anyone have a clue what could be going wrong here? I know that I am using C++ as opposed to the recommended C, but this should still work AFAIK.

回答1:

This is happening because RedisModule_OnLoad is getting name mangled by your C++ compiler.

RedisModule_OnLoad is getting re-named to to __Z18RedisModule_OnLoadP14RedisModuleCtxPP17RedisModuleStringi by GCC, so Redis is unable to find the exported symbol it is looking for.

$ nm avromodule.so | grep OnLoad                   
0000000000000970 T __Z18RedisModule_OnLoadP14RedisModuleCtxPP17RedisModuleStringi

You can use the extern "C" directive to ensure your exported symbols remain un-mangled.

#include "redismodule.h"
#include <stdlib.h>

#ifdef __cplusplus
extern "C" {
#endif

int RedisModule_OnLoad(RedisModuleCtx *ctx, RedisModuleString **argv, int argc) {
    if (RedisModule_Init(ctx,"avromodule",1,REDISMODULE_APIVER_1)
        == REDISMODULE_ERR) return REDISMODULE_ERR;


    return REDISMODULE_OK;
}

#ifdef __cplusplus
}
#endif

Which results in an un-mangled symbol getting exported

nm avromodule.so | grep OnLoad                   
0000000000000970 T _RedisModule_OnLoad