ioctl

ioctl FILEHANDLE,FUNCTION,SCALAR

Implements the ioctl(2) function. You'll probably first have to say

require "sys/ioctl.ph";  # probably in
                         # $Config{archlib}/sys/ioctl.ph

to get the correct function definitions. If sys/ioctl.ph doesn't exist or doesn't have the correct definitions you'll have to roll your own, based on your C header files such as <sys/ioctl.h>. (There is a Perl script called h2ph that comes with the Perl kit that may help you in this, but it's nontrivial.) SCALAR will be read and/or written depending on the FUNCTION; a C pointer to the string value of SCALAR will be passed as the third argument of the actual ioctl call. (If SCALAR has no string value but does have a numeric value, that value will be passed rather than a pointer to the string value. To guarantee this to be true, add a 0 to the scalar before using it.) The pack and unpack functions may be needed to manipulate the values of structures used by ioctl.

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

if OS returns:      then 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:

$retval = ioctl(...) || -1;
printf "System returned %d\n", $retval;

The special string "0 but true" is exempt from -w complaints about improper numeric conversions.

Portability issues: ioctl in perlport.