一直以来，个人概念中只有非阻塞socket才会产生EAGAIN的错误，意思是当前不可读写，只要继续重试就好。当最近我们redis模块的一个报错纠正我的这个概念错误。

事件回顾：hiredis的redisConnectWithTimeout和redisContextSetTimeout接口会设置与redis-server连接的socket为阻塞模式，并且设置读写超时，我们项目中设置超时为50ms。接着在下面几天的日志中发现hiredis报错“Resource temporarily unavailable”，一开始非常奇怪，因为这个错误对应的就是EAGAIN，而这种情况在我知识概念中只

有非阻塞模式下才会报。下面介绍下我理清这个概念的过程：

1.查看redis的读写接口

/* Use this function to handle a read event on the descriptor. It will try
 * and read some bytes from the socket and feed them to the reply parser.
 *
 * After this function is called, you may use redisContextReadReply to
 * see if there is a reply available. */
int redisBufferRead(redisContext *c) {
    char buf[1024*16];
    int nread;

    /* Return early when the context has seen an error. */
    if (c->err)
        return REDIS_ERR;

    nread = read(c->fd,buf,sizeof(buf));
    if (nread == -1) {
        if ((errno == EAGAIN && !(c->flags & REDIS_BLOCK)) || (errno == EINTR)) {
            /* Try again later */
        } else {
            __redisSetError(c,REDIS_ERR_IO,NULL);
            return REDIS_ERR; </span>                                  //分支A1</span>
        }
    } else if (nread == 0) {
            __redisSetError(c,REDIS_ERR_EOF,"Server closed the connection");
            return REDIS_ERR;  </span>                                 //分支A2</span>
    } else {
        if (redisReaderFeed(c->reader,buf,nread) != REDIS_OK) {
            __redisSetError(c,c->reader->err,c->reader->errstr);
            return REDIS_ERR;
        }
    }
    return REDIS_OK;
}

/* Write the output buffer to the socket.
 *
 * Returns REDIS_OK when the buffer is empty, or (a part of) the buffer was
 * succesfully written to the socket. When the buffer is empty after the
 * write operation, "done" is set to 1 (if given).
 *
 * Returns REDIS_ERR if an error occured trying to write and sets
 * c->errstr to hold the appropriate error string.
 */
int redisBufferWrite(redisContext *c, int *done) {
    int nwritten;

    /* Return early when the context has seen an error. */
    if (c->err)
        return REDIS_ERR;

    if (sdslen(c->obuf) > 0) {
        nwritten = write(c->fd,c->obuf,sdslen(c->obuf));
        if (nwritten == -1) {
            if ((errno == EAGAIN && !(c->flags & REDIS_BLOCK)) || (errno == EINTR)) {
                /* Try again later */
            } else {
                __redisSetError(c,REDIS_ERR_IO,NULL);       //分支B1
                return REDIS_ERR;
            }
        } else if (nwritten > 0) {
            if (nwritten == (signed)sdslen(c->obuf)) {
                sdsfree(c->obuf);
                c->obuf = sdsempty();
            } else {
                sdsrange(c->obuf,nwritten,-1);
            }
        }
    }
    if (done != NULL) *done = (sdslen(c->obuf) == 0);
    return REDIS_OK;
}

2.查看错误设置过程

void __redisSetError(redisContext *c, int type, const char *str) {
    size_t len;

    c->err = type;
    if (str != NULL) {
        len = strlen(str);
        len = len < (sizeof(c->errstr)-1) ? len : (sizeof(c->errstr)-1);
        memcpy(c->errstr,str,len);
        c->errstr[len] = '\0';
    } else {
<span style="color:#ff0000;">        /* Only REDIS_ERR_IO may lack a description! */
        assert(type == REDIS_ERR_IO);
        strerror_r(errno,c->errstr,sizeof(c->errstr));</span>
    }
}

3.为什么blocking的read和write会导致errno为EAGAIN？

1）我们对socket做了什么？ 设置了超时时间

int redisContextSetTimeout(redisContext *c, const struct timeval tv) {
    if (setsockopt(c->fd,SOL_SOCKET,SO_RCVTIMEO,&tv,sizeof(tv)) == -1) {
        __redisSetErrorFromErrno(c,REDIS_ERR_IO,"setsockopt(SO_RCVTIMEO)");
        return REDIS_ERR;
    }
    if (setsockopt(c->fd,SOL_SOCKET,SO_SNDTIMEO,&tv,sizeof(tv)) == -1) {
        __redisSetErrorFromErrno(c,REDIS_ERR_IO,"setsockopt(SO_SNDTIMEO)");
        return REDIS_ERR;
    }
    return REDIS_OK;
}

阻塞socket上read/write出现errno为EAGAIN的原因解密