Fcntl

:::info[Note] This is a point-in-time snapshot of the API documentation from January 2026. Going forward, we will not be maintaining a public copy of these references, and recommend users to refer to the built-in signature helpers available in the Hack LSP instead for complete and up-to-date information. :::

Control operations for file descriptors

namespace HH\Lib\OS;

function fcntl(
  FileDescriptor $fd,
  FcntlOp $cmd,
  ?int $arg = NULL,
): mixed;

See man 2 fcntl for details. On error, an ErrnoException will be thrown.

Parameters

• FileDescriptor $fd
• FcntlOp $cmd
• ?int $arg = NULL

Returns

• mixed