c语言 - 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 的C语言编写的轻量级http服务器

我正在尝试在32位系统上使用evbuffer_add_file（）来提供大文件（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.strongize的值正确，在本例中为4913809524，但响应标头Content-Length的值为618842228。即使我将Content-Length标头设置为适当的值，文件传输也会在618842228处停止。。。

我是想念还是做错了什么？有可能吗？

提前感谢

Answer 1

正如我在评论中所说，问题显然不在libevent中，而是在系统的sendfile实现中。因此，通过一个小的变通方法，我找到了解决这个问题的方法evhttp_send_reply_（start|chunk|end）函数族：

    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);