Question

My sysopen fails:

sysopen(DEV, "/dev/ttyS0", O_NONBLOCK|O_RDONLY)

returns 1, which is a success! Then, why does $! have the error “Illegal seek” in it (where it is undef before the call)

before sysopen $!:
after sysopen $!: Illegal seek

EDIT:
Here is the full script: (using the actual O_NONBLOCK|O_RDONLY value 2048)

 #!/usr/bin/perl -w
 use Device::SerialPort;
 my $ob;

 $ob = new Device::SerialPort("/dev/ttyS0");

 print $!, "\n";
 $! = 0;

 my $ret = sysopen(DEV, "/dev/ttyS0", 2048);

 print $!, "\n";
 $! = 0;

 print "ret from sysopen: ", $ret, "\n";
 #my $dev = <DEV>;

which prints out:
./filehandle.pl
Illegal seek
Illegal seek
ret from sysopen: 1

Answer 1

That’s how the C errno variable works. To quote from man errno:

Its value is significant only when
the return value
of the call indicated an error (i.e., -1 from most system
calls; -1 or NULL from most library functions); a function
that succeeds is allowed to change errno.

$! is just Perl’s interface to errno, and has the same behavior:

This means that the value of $! is
meaningful only immediately after a
failure

You must check the return value of sysopen to determine whether it failed. If it failed, $! will tell you why it failed, but you can’t use $! to determine whether it failed (unless the particular function you’re using is documented to set $! to 0 on success. Most don’t, including sysopen).