On 12/22/21 7:48 PM, David Butler wrote:
> Hello,
>
> I'm trying to learn the basics of io_uring. I have this code `hello_uring.cpp':
>
> // clang-13 -O0 -glldb -fsanitize=address -fno-exceptions -Wall
> -Werror -luring hello_uring.cpp -o build/hello_uring.exec
> #include <fcntl.h>
> #include <stdio.h>
> #include <assert.h>
> #include <liburing.h>
>
> #define error_check(v) if ((u_int64_t)v == -1) {perror(#v);
> assert((u_int64_t)v != -1);}
> static int const queue_depth = 4;
> static int const buf_size = 1<<10;
>
> char buffers[queue_depth][buf_size];
>
> int main () { int r;
>
> {
> // setup test
> auto f = fopen("build/testfile2", "w");
> for (unsigned long i = 0; i< 1024; i++) {
> fwrite(&i, sizeof i, 1, f);
> }
> fclose(f);
> }
>
> auto file_fd = open("build/testfile2", O_RDONLY);
>
> io_uring ring;
> r = io_uring_queue_init(queue_depth, &ring, 0);
> error_check(r);
>
> {
> struct iovec vecs[queue_depth];
> for (int veci = 0; veci < queue_depth; veci++) {
> auto sqe = io_uring_get_sqe(&ring);
> assert(sqe);
> sqe->user_data = veci;
> printf("submit: %d\n", veci);
> vecs[veci] = { .iov_base = buffers[veci], .iov_len = buf_size};
> io_uring_prep_readv(sqe, file_fd, &vecs[veci], 1, veci * buf_size);
> }
> r = io_uring_submit(&ring);
> error_check(r);
> assert(r == queue_depth);
> }
>
> for (int done_count = 0; done_count < queue_depth; done_count++) {
>
> struct io_uring_cqe *cqe;
> r = io_uring_wait_cqe(&ring, &cqe);
> error_check(r);
>
> printf("got_completion: %lld, %d, %d\n", cqe->user_data,
> cqe->res, cqe->flags);
> io_uring_cqe_seen(&ring, cqe);
> }
>
> {
> unsigned long next_value = 0;
> for (int buf_i = 0; buf_i < queue_depth; buf_i++) {
> for (auto buf_values = (unsigned long *)buffers[buf_i];
> (char*)buf_values < buffers[buf_i] + buf_size; buf_values++) {
> assert(*buf_values == next_value++);
> }
> }
> assert(next_value == (1024/8) * 4);
> }
> }
>
> On execution, I get all zeros for user_data on the `got_completion'
> lines... I was expecting those to be 0, 1, 2, 3.... What am I missing
> here?
You're doing a prep command after filling in the user_data, the former
clears the sqe. Move the user_data assignment after
io_uring_prep_readv().
--
Jens Axboe
