memcached iov msglist

最近研究memcached 的iov部分，把理解的相关东西做个笔记，防止忘了，有错误之处也欢迎各位指摘。微笑

struct msghdr
{
void *msg_name; /* Socket name */
int msg_namelen; /* Length of name */
struct iovec *msg_iov; /* Data blocks */
int msg_iovlen; /* Number of blocks */有几个iov块
void *msg_accrights; /* Per protocol magic (eg BSD file descriptor passing) */
int msg_accrightslen; /* Length of rights list */
};

telnet localhost 11211后

set key 0 0 4

key1

get key

...分析1

VALUE key 0 4
key1

分析1：

gdb(如何用gdb见http://blog.csdn.net/yxnyxnyxnyxnyxn/article/details/7955259)跟踪：

3995	            switch (transmit(c)) {
(gdb) s
transmit (fd=<value optimized out>, which=<value optimized out>, arg=0x7fffe8013560) at memcached.c:3691
3691	    if (c->msgcurr < c->msgused &&
(gdb) n
3692	            c->msglist[c->msgcurr].msg_iovlen == 0) {
(gdb) 
3691	    if (c->msgcurr < c->msgused &&
(gdb) 
3698	        struct msghdr *m = &c->msglist[c->msgcurr];
(gdb) 
3700	        res = sendmsg(c->sfd, m, 0);
(gdb) 
3701	        if (res > 0) {
(gdb) p res
$20 = <value optimized out>
(gdb) n
3700	        res = sendmsg(c->sfd, m, 0);
(gdb) p res
$21 = <value optimized out>
(gdb) n
3701	        if (res > 0) {
(gdb) p res
$22 = 26
(gdb) l
3696	    if (c->msgcurr < c->msgused) {
3697	        ssize_t res;
3698	        struct msghdr *m = &c->msglist[c->msgcurr];
3699	
3700	        res = sendmsg(c->sfd, m, 0);
3701	        if (res > 0) {
3702	            pthread_mutex_lock(&c->thread->stats.mutex);
3703	            c->thread->stats.bytes_written += res;
3704	            pthread_mutex_unlock(&c->thread->stats.mutex);
3705	
(gdb) p c->msglist[c->msgcurr]
$23 = {msg_name = 0x7fffe80136d8, msg_namelen = 16, msg_iov = 0x7fffe8014e80, msg_iovlen = 4, msg_control = 0x0, msg_controllen = 0, msg_flags = 0}
(gdb) p c->msglist[c->msgcurr].msg_iov
msg_iov     msg_iovlen  
(gdb) p c->msglist[c->msgcurr].msg_iov
$24 = (struct iovec *) 0x7fffe8014e80
(gdb) p c->msglist[c->msgcurr].msg_iov.buf
没有名为 buf 的成员
(gdb) p c->msglist[c->msgcurr].msg_iov[0].iov_base
$25 = (void *) 0x412097
(gdb) p (char *)c->msglist[c->msgcurr].msg_iov[0].iov_base
$26 = 0x412097 "VALUE "
(gdb) p (char *)c->msglist[c->msgcurr].msg_iov[1].iov_base
$27 = 0x7ffff7e2b048 "key"
(gdb) p (char *)c->msglist[c->msgcurr].msg_iov[2].iov_base
$28 = 0x7ffff7e2b04c " 0 4\r\nkey1\r\n"
(gdb) p (char *)c->msglist[c->msgcurr].msg_iov[3].iov_base
$29 = 0x411944 "END\r\n"
(gdb) p (char *)c->msglist[c->msgcurr].msg_iov[4].iov_base
$30 = 0x0

msg_iovlen = 4 //（3+1）

if (add_iov(c, “VALUE “, 6) ||
add_iov(c, ITEM_key(it), it->nkey) ||
add_iov(c, ITEM_suffix(it), it->nsuffix + it->nbytes))

用了3个iov结构体，

add_iov() {
    c->msgbytes += len;
    c->iovused++;
    m->msg_iovlen++;
}

调用完了之后执行下面的语句：add_iov(c, "END\r\n", 5)
add_iov() { c->msgbytes += len; c->iovused++; m->msg_iovlen++;// 加到了4}

res：26
发送成功的字节数VALUE key 0 4 key1\r\nEND\r\n



set key 0 0 4

key1

set key1 0 0 4

key2

get key key2                
VALUE key 0 4
key1
VALUE key2 0 4
key2

(gdb) p c->msglist[c->msgcurr]
$52 = {msg_name = 0x7fffe80136d8, msg_namelen = 16, msg_iov = 0x7fffe8014e80, msg_iovlen = 7, msg_control = 0x0, msg_controllen = 0, msg_flags = 0}
(gdb) n
3708	            while (m->msg_iovlen > 0 && res >= m->msg_iov->iov_len) {
(gdb) p (char *)c->msglist[c->msgcurr].msg_iov[0].iov_base
$53 = 0x412097 "VALUE "
(gdb) p (char *)c->msglist[c->msgcurr].msg_iov[1].iov_base
$54 = 0x7ffff7e2b048 "key"
(gdb) p (char *)c->msglist[c->msgcurr].msg_iov[2].iov_base
$55 = 0x7ffff7e2b04c " 0 4\r\nkey1\r\n"
(gdb) p (char *)c->msglist[c->msgcurr].msg_iov[3].iov_base
$56 = 0x412097 "VALUE "
(gdb) p (char *)c->msglist[c->msgcurr].msg_iov[4].iov_base
$57 = 0x7ffff7e2b0a8 "key2"
(gdb) p (char *)c->msglist[c->msgcurr].msg_iov[5].iov_base
$58 = 0x7ffff7e2b0ad " 0 4\r\nkey2\r\n"
(gdb) p (char *)c->msglist[c->msgcurr].msg_iov[6].iov_base
$59 = 0x411944 "END\r\n"
(gdb) p (char *)c->msglist[c->msgcurr].msg_iov[7].iov_base
$60 = 0x0