---
name: fileno
status: documented
runtime: pp
source: src/runtime/pp/fileio.rs
---
```{index} single: fileno; Perl built-in (pp runtime)
```

# fileno

## Synopsis

```perl
$fd = fileno(STDIN);     # 0
$fd = fileno(STDOUT);    # 1
$fd = fileno($fh);       # fd number or undef
```

## Description

Returns the underlying OS file descriptor for a Perl filehandle.

Returns the numeric file descriptor associated with the given
filehandle, or `undef` if the handle is not connected to a real OS
file descriptor (e.g. in-memory handles, closed handles).

Accepts bareword filehandles (`STDIN`), lexical filehandles (`$fh`),
and IO::Handle / IO::File objects (blessed hashrefs with `__fh_id`).

The standard streams always return their well-known descriptors:
STDIN = 0, STDOUT = 1, STDERR = 2.

## See also

open, close, IO::Handle