Open

:::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. :::

Open the specified path

namespace HH\Lib\OS;

function open(
  string $path,
  int $flags,
  ?int $mode = NULL,
): FileDescriptor;

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

Parameters

• string $path
• int $flags a bitmask of O_ flags; one out of O_RDONLY, O_WRONLY, and O_RDWR must be specified. O_CLOEXEC is implicit, so that standalone CLI mode is consistent with server modes. If needed, this can be removed with OS\fcntl().
• ?int $mode = NULL specify the mode of the file to create if O_CREAT is specified and the file does not exist.

Returns

• FileDescriptor