--- name: "aio_readv" section: "2" platform: "freebsd" package: "base" version: "16.0-CURRENT-20261027-61c70dc16a79" release: "FreeBSD 16.0-CURRENT (2026-10-27)" platforms: ["freebsd"] url: "https://new.carta.tech/aio_read" --- # aio\_readv(2) asynchronous read from a file (REALTIME) Part of [aio\_read(2)](/aio_read/2.md). ## NAME **aio\_read**, **aio\_read2**, **aio\_readv** – asynchronous read from a file (REALTIME) ## LIBRARY Standard C Library (libc, -lc) ## SYNOPSIS **#include \** *int*\ **aio\_read**(*struct aiocb \*iocb*); *int*\ **aio\_read2**(*struct aiocb \*iocb*, *int flags*); **#include \** *int*\ **aio\_readv**(*struct aiocb \*iocb*); ## DESCRIPTION The **aio\_read**(), **aio\_read2**() and **aio\_readv**() system calls allow the calling process to read from the descriptor *iocb-\>aio\_fildes*. The syscalls return immediately after the read request has been enqueued to the descriptor; the read may or may not have completed at the time the call returns. For the **aio\_read**() and **aio\_readv**() calls, the read begins at the offset *iocb-\>aio\_offset*. The **aio\_read**() call will read *iocb-\>aio\_nbytes* into the buffer pointed to by *iocb-\>aio\_buf*, whereas **aio\_readv**() reads the data into the *iocb-\>aio\_iovcnt* buffers specified by the members of the *iocb-\>aio\_iov* array. For **aio\_readv**() the *iovec* structure is defined in [readv(2)](/readv/2.md). The **aio\_read2**() call takes the *flags* argument. If *flags* is passed as zero, the call behaves identically to **aio\_read**(). The following flags can be specified by logical or: - AIO\_OP2\_FOFFSET The read occurs at the file descriptor offset, which is advanced by the operation as done by the [read(2)](/read/2.md) syscall. The *iocb-\>aio\_offset* field is ignored. - AIO\_OP2\_VECTORED Similar to **aio\_readv**(), the read buffers are specified by the *aiocb-\>aio\_iov* array. The *iocb* pointer may be subsequently used as an argument to **aio\_return**() and **aio\_error**() in order to determine return or error status for the enqueued operation while it is in progress. If the request could not be enqueued (generally due to invalid arguments), then the call returns without having enqueued the request. If the request is successfully enqueued, the value of *iocb-\>aio\_offset* can be modified during the request as context, so this value must not be referenced after the request is enqueued. The *iocb-\>aio\_sigevent* structure can be used to request notification of the operation's completion as described in [aio(4)](/aio/4.md). ## RESTRICTIONS The Asynchronous I/O Control Block structure pointed to by *iocb* and the buffer that the *iocb-\>aio\_buf* member of that structure references must remain valid until the operation has completed. The asynchronous I/O control buffer *iocb* should be zeroed before the system calls to avoid passing bogus context information to the kernel. Modifications of the Asynchronous I/O Control Block structure or the buffer contents are not allowed while the request is queued. If the file offset in *iocb-\>aio\_offset* is past the offset maximum for *iocb-\>aio\_fildes*, no I/O will occur. ## RETURN VALUES The **aio\_read()**, **aio\_read2()**, and **aio\_readv()** functions return the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error. ## DIAGNOSTICS None. ## ERRORS The **aio\_read**(), **aio\_read2**(), and **aio\_readv**() system calls will fail if: - \[EAGAIN\] The request was not queued because of system resource limitations. - \[EFAULT\] Part of *aio\_iov* points outside the process's allocated address space. - \[EINVAL\] The asynchronous notification method in *iocb-\>aio\_sigevent.sigev\_notify* is invalid or not supported. - \[EOPNOTSUPP\] Asynchronous read operations on the file descriptor *iocb-\>aio\_fildes* are unsafe and unsafe asynchronous I/O operations are disabled. The following conditions may be synchronously detected when the **aio\_read**() or **aio\_readv**() system call is made, or asynchronously, at any time thereafter. If they are detected at call time, The calls return -1 and set *errno* appropriately; otherwise the **aio\_return**() system call must be called, and will return -1, and **aio\_error**() must be called to determine the actual value that would have been returned in *errno*. - \[EBADF\] The *iocb-\>aio\_fildes* argument is invalid. - \[EINVAL\] The offset *iocb-\>aio\_offset* is not valid, the priority specified by *iocb-\>aio\_reqprio* is not a valid priority, or the number of bytes specified by *iocb-\>aio\_nbytes* is not valid. - \[EOVERFLOW\] The file is a regular file, *iocb-\>aio\_nbytes* is greater than zero, the starting offset in *iocb-\>aio\_offset* is before the end of the file, but is at or beyond the *iocb-\>aio\_fildes* offset maximum. If the request is successfully enqueued, but subsequently cancelled or an error occurs, the value returned by the **aio\_return**() system call is per the [read(2)](/read/2.md) system call, and the value returned by the **aio\_error**() system call is either one of the error returns from the [read(2)](/read/2.md) system call, or one of: - \[EBADF\] The *iocb-\>aio\_fildes* argument is invalid for reading. - \[ECANCELED\] The request was explicitly cancelled via a call to **aio\_cancel**(). - \[EINVAL\] The offset *iocb-\>aio\_offset* would be invalid. ## SEE ALSO [aio\_cancel(2)](/aio_cancel/2.md), [aio\_error(2)](/aio_error/2.md), [aio\_return(2)](/aio_return/2.md), [aio\_suspend(2)](/aio_suspend/2.md), [aio\_waitcomplete(2)](/aio_waitcomplete/2.md), [aio\_write(2)](/aio_write/2.md), [sigevent(3)](/sigevent/3.md), [siginfo(3)](/siginfo/3.md), [aio(4)](/aio/4.md) ## STANDARDS The **aio\_read**() system call is expected to conform to the IEEE Std 1003.1 (“POSIX.1”) standard. The **aio\_read2**() and **aio\_readv**() system calls are FreeBSD extensions, and should not be used in portable code. ## HISTORY The **aio\_read**() system call first appeared in FreeBSD 3.0. The **aio\_readv**() system call first appeared in FreeBSD 13.0. The **aio\_read2**() system call first appeared in FreeBSD 14.1. ## AUTHORS This manual page was written by Terry Lambert ⟨*terry@whistle.com*⟩. ## BUGS Invalid information in *iocb-\>\_aiocb\_private* may confuse the kernel.