Programming Perl, 4th Edition

fcntl

fcntl FILEHANDLE, FUNCTION, SCALAR

This function calls your operating system’s file control functions, as documented in the fcntl(2) manpage. Before you call fcntl, you’ll probably first have to say:

use Fcntl;

to load the correct constant definitions.

SCALAR will be read or written (or both) depending on the FUNCTION. A pointer to the string value of SCALAR will be passed as the third argument of the actual fcntl call. (If SCALAR has no string value but does have a numeric value, that value will be passed directly rather than passing a pointer to the string value.) See the Fcntl module for a description of the more common permissible values for FUNCTION.

The fcntl function will raise an exception if used on a system that doesn’t implement fcntl(2). On systems that do implement it, you can do such things as modify the close-on-exec flags (if you don’t want to play with the $^F ($SYSTEM_FD_MAX) variable), modify the nonblocking I/O flags, emulate the lockf(3) function, and arrange to ...