layout: post title: 使用pahole進行c struct優化 categories:
本來想找一個可以爬c struct的工具，進行自動轉換
結果發現pahole，不但可以秀出c struct，還可以秀出它的layout
在linux man page裡面有它的說明
pahole shows data structure layouts encoded in debugging information formats, DWARF and CTF being supported. This is useful for, among other things: optimizing important data structures by reducing its size, figuring out what is the field sitting at an offset from the start of a data structure, investigating ABI changes and more generally understanding a new codebase you have to work with.
主要是分析加入debug information(-g)的執行檔或object檔來顯示資訊
但是ubuntu apt竟然找不到pahole..翻了一下原來在etfutils裡
以xdmq裡的某個struct分析出的結果如下：
struct cli_ctx_s {
char host[255]; /* 0 255 */
/* XXX 1 byte hole, try to pack */
/* --- cacheline 4 boundary (256 bytes) --- */
int port; /* 256 4 */
/* XXX 4 bytes hole, try to pack */
uv_tcp_t client; /* 264 248 */
/* --- cacheline 8 boundary (512 bytes) --- */
uv_connect_t conn; /* 512 96 */
/* --- cacheline 9 boundary (576 bytes) was 32 bytes ago --- */
buffer_t buff; /* 608 8 */
int node_id; /* 616 4 */
int self_id; /* 620 4 */
read_cb r_cb; /* 624 8 */
msgpack_unpacker * unp; /* 632 8 */
/* --- cacheline 10 boundary (640 bytes) --- */
int status; /* 640 4 */
/* XXX 4 bytes hole, try to pack */
raft_node_t node; /* 648 8 */
/* size: 656, cachelines: 11, members: 11 */
/* sum members: 647, holes: 3, sum holes: 9 */
/* last cacheline: 16 bytes */
};