Full example of plugin, which adds a /double command, which displays two times arguments on current channel (ok that's not very useful, but that's just an example!):

#include <stdlib.h>

#include "weechat-plugin.h"

char plugin_name[]        = "Double";
char plugin_version[]     = "0.1";
char plugin_description[] = "Test plugin for WeeChat";

/* "/double" command manager */

int double_cmd (t_weechat_plugin *plugin, int argc, char **argv,
                char *handler_args, void *handler_pointer)
{
    if (argv[2] && (argv[2][0] != '/'))
    {
        plugin->exec_command (plugin, NULL, NULL, argv[2]);
        plugin->exec_command (plugin, NULL, NULL, argv[2]);
    }
    return PLUGIN_RC_OK;
}

int weechat_plugin_init (t_weechat_plugin *plugin)
{
    plugin->cmd_handler_add (plugin, "double",
                             "Display two times a message",
                             "msg",
                             "msg: message to display two times",
                             NULL,
                             &double_cmd,
                             NULL, NULL);
    return PLUGIN_RC_OK;
}

void weechat_plugin_end (t_weechat_plugin *plugin)
{
    /* nothing done here */
}