Prepare I/O read request
# DESCRIPTION
The [io_uring_prep_read] prepares an IO read request. The
submission queue entry *sqe* is setup to use the file descriptor *fd* to
start reading *nbytes* into the buffer *buf* at the specified *offset*.
On files that support seeking, if the offset is set to **-1**, the read
operation commences at the file offset, and the file offset is
incremented by the number of bytes read. See [read](https://man7.org/linux/man-pages/man2/read.2.html) for more
details. Note that for an async API, reading and updating the current
file offset may result in unpredictable behavior, unless access to the
file is serialized. It is not encouraged to use this feature, if it's
possible to provide the desired IO offset from the application or
library.
On files that are not capable of seeking, the offset must be 0 or -1.
After the read has been prepared it can be submitted with one of the
submit functions.
# RETURN VALUE
None
# ERRORS
The CQE *res* field will contain the result of the operation. See the
related man page for details on possible values. Note that where
synchronous system calls will return **-1** on failure and set *errno*
to the actual error value, io_uring never uses *errno*. Instead it
returns the negated *errno* directly in the CQE *res* field.
# NOTES
This function accepts an unsigned number of bytes, but io_uring_cqe's
result code is an \_\_s32 value, so in theory a short read with a large
enough nbytes value could generate an ambiguous return. But the number
of bytes actually transferred has the same limit as [read](https://man7.org/linux/man-pages/man2/read.2.html) so this
cannot happen in practice.
# SEE ALSO
[io_uring_get_sqe], [io_uring_prep_readv],
[io_uring_prep_readv2], [io_uring_submit]