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This function is available as OS\open() in Facebook's www repository.

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.


  • 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.