Why does Perl's sysopen report success but $! has an error?

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).

Answer 2

Many similar special variables in Perl don't change their value until they have to. You always should check for the condition that would actually set the variable. Two of the most common examples are resetting error variables when there is an error or resetting regex capture variables on a successful match.

 unless( sysopen( ... ) ) {
      die "Error was $!";
      }

 if( m/(...)(...)/ ) {
      print "Found $1 and $2\n";
      }

This is certainly one of Perl's warts since it relies on side effects of distant global variables to denote what happened locally. It's even a bit worse in some cases because something else can change the special variable before you get a chance to look at it so you always should inspect it immediately before running any other statements.