Serving large files (>2GB) with libevent on 32-bit system - c

Question

Preamble: lightweight http server written in C based on libevent v2 (evhttp), Linux, ARM, glibc2.3.4
I'm trying to serve big files (over 2GB) using evbuffer_add_file() on 32 bit system.
The libevent was compiled with -D_FILE_OFFSET_BITS=64 flag. Here is the simplified code:
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 has correct value, in this case 4913809524, but response header Content-Length has value of 618842228. Even if i set Content-Length header to appropriate value the file transfer stops at 618842228 ...
Do i miss or do something wrong? Is it possible at all?
Thanks in advance

Answer 1

As i said in comment obviously the problem is not in libevent, but in system's sendfile implementation. So with a little workaround i found the way to solve this problem using
evhttp_send_reply_(start|chunk|end) functions family:
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);