cat_uring.c
#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <sys/ioctl.h>
#include <sys/syscall.h>
#include <sys/mman.h>
#include <sys/uio.h>
#include <linux/fs.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>
/* If your compilation fails because the header file below is missing,
* your kernel is probably too old to support io_uring.
* */
#include <linux/io_uring.h>
#define QUEUE_DEPTH 1
#define BLOCK_SZ 1024
/* This is x86 specific */
#define read_barrier() __asm__ __volatile__("":::"memory")
#define write_barrier() __asm__ __volatile__("":::"memory")
struct app_io_sq_ring {
unsigned *head;
unsigned *tail;
unsigned *ring_mask;
unsigned *ring_entries;
unsigned *flags;
unsigned *array;
};
struct app_io_cq_ring {
unsigned *head;
unsigned *tail;
unsigned *ring_mask;
unsigned *ring_entries;
struct io_uring_cqe *cqes;
};
struct submitter {
int ring_fd;
struct app_io_sq_ring sq_ring;
struct io_uring_sqe *sqes;
struct app_io_cq_ring cq_ring;
};
struct file_info {
off_t file_sz;
struct iovec iovecs[]; /* Referred by readv/writev */
};
/*
* This code is written in the days when io_uring-related system calls are not
* part of standard C libraries. So, we roll our own system call wrapper
* functions.
* */
int io_uring_setup(unsigned entries, struct io_uring_params *p)
{
return (int) syscall(__NR_io_uring_setup, entries, p);
}
int io_uring_enter(int ring_fd, unsigned int to_submit,
unsigned int min_complete, unsigned int flags)
{
return (int) syscall(__NR_io_uring_enter, ring_fd, to_submit, min_complete,
flags, NULL, 0);
}
/*
* Returns the size of the file whose open file descriptor is passed in.
* Properly handles regular file and block devices as well. Pretty.
* */
off_t get_file_size(int fd) {
struct stat st;
if(fstat(fd, &st) < 0) {
perror("fstat");
return -1;
}
if (S_ISBLK(st.st_mode)) {
unsigned long long bytes;
if (ioctl(fd, BLKGETSIZE64, &bytes) != 0) {
perror("ioctl");
return -1;
}
return bytes;
} else if (S_ISREG(st.st_mode))
return st.st_size;
return -1;
}
/*
* io_uring requires a lot of setup which looks pretty hairy, but isn't all
* that difficult to understand. Because of all this boilerplate code,
* io_uring's author has created liburing, which is relatively easy to use.
* However, you should take your time and understand this code. It is always
* good to know how it all works underneath. Apart from bragging rights,
* it does offer you a certain strange geeky peace.
* */
int app_setup_uring(struct submitter *s) {
struct app_io_sq_ring *sring = &s->sq_ring;
struct app_io_cq_ring *cring = &s->cq_ring;
struct io_uring_params p;
void *sq_ptr, *cq_ptr;
/*
* We need to pass in the io_uring_params structure to the io_uring_setup()
* call zeroed out. We could set any flags if we need to, but for this
* example, we don't.
* */
memset(&p, 0, sizeof(p));
s->ring_fd = io_uring_setup(QUEUE_DEPTH, &p);
if (s->ring_fd < 0) {
perror("io_uring_setup");
return 1;
}
/*
* io_uring communication happens via 2 shared kernel-user space ring buffers,
* which can be jointly mapped with a single mmap() call in recent kernels.
* While the completion queue is directly manipulated, the submission queue
* has an indirection array in between. We map that in as well.
* */
int sring_sz = p.sq_off.array + p.sq_entries * sizeof(unsigned);
int cring_sz = p.cq_off.cqes + p.cq_entries * sizeof(struct io_uring_cqe);
/* In kernel version 5.4 and above, it is possible to map the submission and
* completion buffers with a single mmap() call. Rather than check for kernel
* versions, the recommended way is to just check the features field of the
* io_uring_params structure, which is a bit mask. If the
* IORING_FEAT_SINGLE_MMAP is set, then we can do away with the second mmap()
* call to map the completion ring.
* */
if (p.features & IORING_FEAT_SINGLE_MMAP) {
if (cring_sz > sring_sz) {
sring_sz = cring_sz;
}
cring_sz = sring_sz;
}
/* Map in the submission and completion queue ring buffers.
* Older kernels only map in the submission queue, though.
* */
sq_ptr = mmap(0, sring_sz, PROT_READ | PROT_WRITE,
MAP_SHARED | MAP_POPULATE,
s->ring_fd, IORING_OFF_SQ_RING);
if (sq_ptr == MAP_FAILED) {
perror("mmap");
return 1;
}
if (p.features & IORING_FEAT_SINGLE_MMAP) {
cq_ptr = sq_ptr;
} else {
/* Map in the completion queue ring buffer in older kernels separately */
cq_ptr = mmap(0, cring_sz, PROT_READ | PROT_WRITE,
MAP_SHARED | MAP_POPULATE,
s->ring_fd, IORING_OFF_CQ_RING);
if (cq_ptr == MAP_FAILED) {
perror("mmap");
return 1;
}
}
/* Save useful fields in a global app_io_sq_ring struct for later
* easy reference */
sring->head = sq_ptr + p.sq_off.head;
sring->tail = sq_ptr + p.sq_off.tail;
sring->ring_mask = sq_ptr + p.sq_off.ring_mask;
sring->ring_entries = sq_ptr + p.sq_off.ring_entries;
sring->flags = sq_ptr + p.sq_off.flags;
sring->array = sq_ptr + p.sq_off.array;
/* Map in the submission queue entries array */
s->sqes = mmap(0, p.sq_entries * sizeof(struct io_uring_sqe),
PROT_READ | PROT_WRITE, MAP_SHARED | MAP_POPULATE,
s->ring_fd, IORING_OFF_SQES);
if (s->sqes == MAP_FAILED) {
perror("mmap");
return 1;
}
/* Save useful fields in a global app_io_cq_ring struct for later
* easy reference */
cring->head = cq_ptr + p.cq_off.head;
cring->tail = cq_ptr + p.cq_off.tail;
cring->ring_mask = cq_ptr + p.cq_off.ring_mask;
cring->ring_entries = cq_ptr + p.cq_off.ring_entries;
cring->cqes = cq_ptr + p.cq_off.cqes;
return 0;
}
/*
* Output a string of characters of len length to stdout.
* We use buffered output here to be efficient,
* since we need to output character-by-character.
* */
void output_to_console(char *buf, int len) {
while (len--) {
fputc(*buf++, stdout);
}
}
/*
* Read from completion queue.
* In this function, we read completion events from the completion queue, get
* the data buffer that will have the file data and print it to the console.
* */
void read_from_cq(struct submitter *s) {
struct file_info *fi;
struct app_io_cq_ring *cring = &s->cq_ring;
struct io_uring_cqe *cqe;
unsigned head, reaped = 0;
head = *cring->head;
do {
read_barrier();
/*
* Remember, this is a ring buffer. If head == tail, it means that the
* buffer is empty.
* */
if (head == *cring->tail)
break;
/* Get the entry */
cqe = &cring->cqes[head & *s->cq_ring.ring_mask];
fi = (struct file_info*) cqe->user_data;
if (cqe->res < 0)
fprintf(stderr, "Error: %s\n", strerror(abs(cqe->res)));
int blocks = (int) fi->file_sz / BLOCK_SZ;
if (fi->file_sz % BLOCK_SZ) blocks++;
for (int i = 0; i < blocks; i++)
output_to_console(fi->iovecs[i].iov_base, fi->iovecs[i].iov_len);
head++;
} while (1);
*cring->head = head;
write_barrier();
}
/*
* Submit to submission queue.
* In this function, we submit requests to the submission queue. You can submit
* many types of requests. Ours is going to be the readv() request, which we
* specify via IORING_OP_READV.
*
* */
int submit_to_sq(char *file_path, struct submitter *s) {
struct file_info *fi;
int file_fd = open(file_path, O_RDONLY);
if (file_fd < 0 ) {
perror("open");
return 1;
}
struct app_io_sq_ring *sring = &s->sq_ring;
unsigned index = 0, current_block = 0, tail = 0, next_tail = 0;
off_t file_sz = get_file_size(file_fd);
if (file_sz < 0)
return 1;
off_t bytes_remaining = file_sz;
int blocks = (int) file_sz / BLOCK_SZ;
if (file_sz % BLOCK_SZ) blocks++;
fi = malloc(sizeof(*fi) + sizeof(struct iovec) * blocks);
if (!fi) {
fprintf(stderr, "Unable to allocate memory\n");
return 1;
}
fi->file_sz = file_sz;
/*
* For each block of the file we need to read, we allocate an iovec struct
* which is indexed into the iovecs array. This array is passed in as part
* of the submission. If you don't understand this, then you need to look
* up how the readv() and writev() system calls work.
* */
while (bytes_remaining) {
off_t bytes_to_read = bytes_remaining;
if (bytes_to_read > BLOCK_SZ)
bytes_to_read = BLOCK_SZ;
fi->iovecs[current_block].iov_len = bytes_to_read;
void *buf;
if( posix_memalign(&buf, BLOCK_SZ, BLOCK_SZ)) {
perror("posix_memalign");
return 1;
}
fi->iovecs[current_block].iov_base = buf;
current_block++;
bytes_remaining -= bytes_to_read;
}
/* Add our submission queue entry to the tail of the SQE ring buffer */
next_tail = tail = *sring->tail;
next_tail++;
read_barrier();
index = tail & *s->sq_ring.ring_mask;
struct io_uring_sqe *sqe = &s->sqes[index];
sqe->fd = file_fd;
sqe->flags = 0;
sqe->opcode = IORING_OP_READV;
sqe->addr = (unsigned long) fi->iovecs;
sqe->len = blocks;
sqe->off = 0;
sqe->user_data = (unsigned long long) fi;
sring->array[index] = index;
tail = next_tail;
/* Update the tail so the kernel can see it. */
if(*sring->tail != tail) {
*sring->tail = tail;
write_barrier();
}
/*
* Tell the kernel we have submitted events with the io_uring_enter() system
* call. We also pass in the IOURING_ENTER_GETEVENTS flag which causes the
* io_uring_enter() call to wait until min_complete events (the 3rd param)
* complete.
* */
int ret = io_uring_enter(s->ring_fd, 1,1,
IORING_ENTER_GETEVENTS);
if(ret < 0) {
perror("io_uring_enter");
return 1;
}
return 0;
}
int main(int argc, char *argv[]) {
struct submitter *s;
if (argc < 2) {
fprintf(stderr, "Usage: %s <filename>\n", argv[0]);
return 1;
}
s = malloc(sizeof(*s));
if (!s) {
perror("malloc");
return 1;
}
memset(s, 0, sizeof(*s));
if(app_setup_uring(s)) {
fprintf(stderr, "Unable to setup uring!\n");
return 1;
}
for (int i = 1; i < argc; i++) {
if(submit_to_sq(argv[i], s)) {
fprintf(stderr, "Error reading file\n");
return 1;
}
read_from_cq(s);
}
return 0;
}