Question

我试图在C中为collectd编写一个glusterfs插件，它使用glusterfs库来进行实际的RPC调用。我设法让一个独立的C程序工作，基本上只是拼凑了部分gluster CLI。

但是，当我将该代码移植到collectd库时，我在进行使用gluster库中的符号的函数调用时会遇到一些奇怪的行为。在函数调用之外，内存位置X指的是具有非null成员的结构。在函数调用内部，内存位置X指的是具有null成员的结构。这导致函数失败并返回NULL而不是指向结构的指针，这当然会导致RPC失败。

以下是GDB通过程序执行的一些输出。您可以看到生成在0x7fffec0231b0中的结构的成员在进入函数时会发生变化。请注意，我已从打印件中删除了部分成员，以使其更加清晰：

(gdb) next
718                 frame = create_frame (THIS, state.ctx->pool);
(gdb) print state.ctx.pool
$22 = (struct call_pool *) 0x7fffdc022740
(gdb) print *state.ctx.pool
$23 = {..., frame_mem_pool = 0x7fffdc0227e0, stack_mem_pool = 0x7fffdc0586d0}
(gdb) print *{struct call_pool *}0x7fffdc022740
$24 = {..., frame_mem_pool = 0x7fffdc0227e0, stack_mem_pool = 0x7fffdc0586d0}
(gdb) step
create_frame (xl=0x7ffff4fc96e0 <global_xlator>, pool=pool@entry=0x7fffdc022740) at stack.c:17
17      {
(gdb) print pool
$26 = (call_pool_t *) 0x7fffdc022740
(gdb) print *pool
$27 = {..., frame_mem_pool = 0x0, stack_mem_pool = 0x0}
(gdb) print *{struct call_pool*}0x7fffdc022740
$28 = {..., frame_mem_pool = 0x0, stack_mem_pool = 0x0}
(gdb) finish
Run till exit from #0  create_frame (xl=0x7ffff4fc96e0 <global_xlator>, pool=pool@entry=0x7fffdc022740) at stack.c:17
0x00007ffff5400492 in profiler_callback (d=<optimized out>) at src/glusterfs.c:718
718                 frame = create_frame (THIS, state.ctx->pool);
Value returned is $29 = (call_frame_t *) 0x0
(gdb) print state.ctx.pool
$30 = (struct call_pool *) 0x7fffdc022740
(gdb) print *{struct call_pool*}0x7fffdc022740
$31 = {..., frame_mem_pool = 0x7fffdc0227e0, stack_mem_pool = 0x7fffdc0586d0}

正如您所看到的，函数内部的内存位置似乎是指具有不同值的结构，因为其成员在函数内部为null，但不在其外部。

也许值得注意的是，create_frame来自一个gluster库，它与我的collectd库分开。我描述的行为也发生在gluster库中的另一个函数中。

编辑：

可以看到完整的来源here

这里有一段简化的片段：

// globals
struct cli_state state = {0, };
struct rpc_clnt *global_rpc;
rpc_clnt_prog_t *cli_rpc_prog;

int
glusterfs_ctx_defaults_init ()
{
        call_pool_t   *pool = NULL;
        glusterfs_ctx_t *ctx = state.ctx;

        pool = GF_CALLOC (1, sizeof (call_pool_t),
                          mt_call_pool_t);

        /* stack_mem_pool size 256 * 128 */
        pool->stack_mem_pool = mem_pool_new (call_stack_t, 16);
        if (!pool->stack_mem_pool)
                return 1;

        ctx->pool = pool;

        return 0;

}

void * profiler_callback (void* d) {
    int ret = 0;
        call_frame_t            *frame = NULL;
        // function from gluster library returns
        // pthread_getspecific (this_xlator_key);
        // so essentially a thread-specific variable
        xlator_t                *xlator = (*__glusterfs_this_location());

        // here, state.ctx->pool->stack_mem_pool is not null
        frame = create_frame (xlator, xlator->ctx->pool);
        // it is also not null here. however, frame is null
        // since state.ctx->pool->stack_mem_pool *is* null
        // inside create_frame
        if (!frame)
                goto out;

out:
        INFO("Exiting with: %d", ret);
        kill_self();

        return NULL;

}

int
read_stats (void)
{
    int ret = -1;
    glusterfs_ctx_t   *ctx = NULL;
    xlator_t * xlator = NULL;

        ctx = glusterfs_ctx_new ();
        if (!ctx)
                return ENOMEM;

        ret = glusterfs_globals_init (ctx);
        if (ret)
                return ret;

        // function from gluster library returns
        // pthread_getspecific (this_xlator_key);
        // so essentially a thread-specific variable
        xlator = (*__glusterfs_this_location());

        // Sets ctx.pool.stack_mem_pool
        ret = glusterfs_ctx_defaults_init (xlator);
        if (ret) {
                return ret;
        }

        state.ctx = ctx;

        pthread_mutex_init (&cond_mutex, NULL);
        pthread_cond_init (&cond, NULL);
        pthread_mutex_init (&conn_mutex, NULL);
        pthread_cond_init (&conn, NULL);

        ret = pthread_create ((&(state.input)), NULL, profiler_callback, &state);
        if (ret) {
            ERROR("problem! %d", ret);
        }

        ret = event_dispatch (state.ctx->event_pool);
out:

        return ret;
}

void module_register(void) {
  plugin_register_read("glusterfs", read_stats);
}

module_register向collectd注册read_stats函数，然后在每个间隔期间调用它。

内存位置在函数内部和外部具有不同的值

0 个答案: