c - 在 32 位系统上使用 libevent 提供大文件 (>2GB)

Question

序言:基于 libevent v2 (evhttp)、Linux、ARM、glibc2.3.4 用 C 编写的轻量级 http 服务器

我正在尝试在 32 位系统上使用 evbuffer_add_file() 提供大文件(超过 2GB)。libevent 是使用 -D_FILE_OFFSET_BITS=64 标志编译的。这是简化的代码:

int fd = -1;
if ((fd = open(path, O_RDONLY)) < 0) {
    // error handling
}

struct stat st;
if (fstat(fd, &st) < 0) {
    // error handling
}

struct evbuffer *buffer = evbuffer_new();
evbuffer_set_flags(buffer, EVBUFFER_FLAG_DRAINS_TO_FD); // force using system's sendfile
evbuffer_add_file(buffer, fd, 0, st.st_size);
evhttp_send_reply(req, 200, NULL, buffer);
evbuffer_free(buffer);

st.st_size 具有正确的值，在本例中为 4913809524，但响应 header Content-Length 的值为 618842228。即使我将 Content-Length header 设置为适当的值，文件传输也会停止在 618842228 ...

我是错过了还是做错了什么？有可能吗？

提前致谢

Answer 1

正如我在评论中所说，显然问题不在 libevent 中，而在系统的 sendfile 实现中。所以通过一些解决方法，我找到了解决这个问题的方法evhttp_send_reply_(start|chunk|end) 函数系列:

    struct chunk_req_state {
        struct evhttp_request *req;
        int fd;
        long chunksize;
        off_t filesize;
        off_t offset;
    };

    static void
    chunked_trickle_cb(evutil_socket_t fd, short events, void *arg)
    {
        struct evbuffer *evb = evbuffer_new();
        struct chunk_req_state *state = arg;
        struct timeval when = { 0, 0 };
        ev_ssize_t read;

        if (lseek(state->fd, state->offset, SEEK_SET) == -1) {
                    evbuffer_free(evb);
                    close(state->fd);
                    free(state);
                    return;
        }

        read = evbuffer_read(evb, state->fd, (ev_ssize_t) state->chunksize);
        if (read == -1) {
            evbuffer_free(evb);
            evhttp_send_reply_end(state->req);
            close(state->fd);
            free(state);
            return;
        }

        evhttp_send_reply_chunk(state->req, evb);
        evbuffer_free(evb);

        state->offset += read;

        if (state->offset < state->filesize) {
              // there's more data to send
            event_base_once(ebase, -1, EV_TIMEOUT, chunked_trickle_cb, state, &when);
        } else {
                // reached the end
            evhttp_send_reply_end(state->req);
            close(state->fd);
            free(state);
        }
    }

    int fd = -1;
    if ((fd = open(path, O_RDONLY)) < 0) {
        // error handling
    }

    struct stat st;
    if (fstat(fd, &st) < 0) {
        // error handling
    }

    struct timeval when = { 0, 0 };
    struct chunk_req_state *state = malloc(sizeof(struct chunk_req_state));
    memset(state, 0, sizeof(struct chunk_req_state));
    state->req = req;
    state->fd = fd;
    state->chunksize = 10*1024*1024;
    state->filesize = st.st_size;
    state->offset = 0;
    // set Content-Length to prevent chunked transfer
    char *length = NULL;
    spprintf(&length, 0, "%lld", st.st_size);
    evhttp_add_header(evhttp_request_get_output_headers(request->req), "Content-Length", length);
    free(length);
    evhttp_send_reply_start(request->req, 200, NULL);
    event_base_once(ebase, -1, EV_TIMEOUT, chunked_trickle_cb, state, &when);