Why does Perl think "0 but true" is a number? [duplicate]

Question

Can someone explain what exactly the string "0 but true" means in Perl? As far as I understand, it equals zero in an integer comparison, but evaluates to true when used as a boolean. Is this correct? Is this a normal behavior of the language or is this a special string treated as a special case in the interpreter?

Answer 1

It's normal behaviour of the language. Quoting the perlsyn manpage:

The number 0, the strings '0' and "", the empty list (), and undef are all false in a boolean context. All other values are true. Negation of a true value by ! or not returns a special false value. When evaluated as a string it is treated as "", but as a number, it is treated as 0.

Because of this, there needs to be a way to return 0 from a system call that expects to return 0 as a (successful) return value, and leave a way to signal a failure case by actually returning a false value. "0 but true" serves that purpose.

Answer 2

Because it's hardcoded in the Perl core to treat it as a number. This is a hack to make Perl's conventions and ioctl's conventions play together; from perldoc -f 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.