Reference: fcntl
Author: Ellen Siever, Stephen Spainhour & Nathan Patwardhan
Date: 1999-07-02T01:34:49Z

#

| Chapter 5 Function Reference |

## fcntl

    fcntl filehandle, function, arg

Calls the file control function (with the function-specific arg) to use on the file or device opened with filehandle. `fcntl` calls Unix's `fcntl` function (not available on Win32 platforms). If the function is not implemented, the program exits with a fatal error. `fcntl` sets file descriptors for a filehandle. This built-in command is usable when you use the Fcntl module in the standard distribution:

>     use Fcntl;

This module imports the correct function definitions. See the description of the Fcntl module in Chapter 8, Standard Modules.

The return value of `fcntl` (and `ioctl`) is as follows:

System call returns | Perl returns
--- | ---
`-1` | Undefined value
`0` | String "`0 but true`"
Anything else | That number

Thus Perl returns true on success and false on failure, yet you can still easily determine the actual value returned by the operating system.

Reference: exp |  | Reference: fileno