iocb

An I/O control block (iocb) is a data structure used to describe an asynchronous I/O request in Linux’s asynchronous I/O (AIO) subsystem. It allows a process to specify an I/O operation without blocking, enabling overlap of computation and I/O.

In userspace, an iocb typically encodes the target file descriptor, the memory buffer for input or output, the number of bytes to transfer, and the file offset. It also includes an operation code that identifies the type of request (for example, read or write), and a field to carry user data that can be used to identify the request when it completes. The kernel maintains a separate I/O context (an io_context_t) to track in-flight iocbs for a given submission.

Requests are submitted to the kernel in batches via the io_submit system call (or through a library such as libaio). The kernel schedules the I/O and, upon completion, returns completion events through io_getevents or via other notification mechanisms. Each completion event includes a result code and, if a user data field was provided, the corresponding identifier so the application can correlate completions with their original requests.

The iocb mechanism is the core of Linux’s asynchronous I/O interface, suitable for high-throughput or latency-sensitive workloads such as databases and file servers. It contrasts with synchronous I/O by allowing a thread to issue multiple operations and proceed with other work while the kernel completes them in the background. Related concepts include io_setup, io_submit, io_getevents, and io_destroy.