Beginner question, user_data not getting filled in as expected

Beginner question, user_data not getting filled in as expected

[David Butler]

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?

Thanks :)
--Dave

Re: Beginner question, user_data not getting filled in as expected

[Jens Axboe]

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

Re: Beginner question, user_data not getting filled in as expected

[David Butler]

>
> 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

Oh! Thanks!