Question

Can anyone give an example of how a string can be returned from a call using Win32::API() function? I need to return a string and print using $val. Please give an example if the same can be handled using pointer as return type.

use Win32::API;  
my $res = new Win32::API('abc.dll','MyFun','_argument type list_','_Return type list_')or die $^E;  
my $val= $res->Call();   
print ($val);

Answer 1

The documentation for Win32::API‘s Call() method suggests that you must pass Call() a scalar which will be used as a buffer to store the returned value; Call() itself will return whether the call succeeded or not.

Example:

my $return_buffer = " " x 80;
if ($res->Call(80, $return_buffer)) {
    print "OK, the API call returned '$return_buffer'\n";
} else {
    print "The API call failed for some reason.\n";
}

EDIT: quoting from the docs for completeness:

The two parameters needed here are the length of the buffer that will hold the returned temporary path, and a pointer to the buffer itself. For numerical parameters, you can use either a constant expression or a variable, while for pointers you must use a variable name (no Perl references, just a plain variable name). Also note that memory must be allocated before calling the function, just like in C. For example, to pass a buffer of 80 characters to GetTempPath(), it must be initialized before with:

$lpBuffer = " " x 80;

This allocates a string of 80 characters. If you don’t do so, you’ll probably get Runtime exception errors, and generally nothing will work. The call should therefore include:

$lpBuffer = " " x 80;
$GetTempPath->Call(80, $lpBuffer);

And the result will be stored in the $lpBuffer variable. Note that you don’t need to pass a reference to the variable (eg. you don’t need \$lpBuffer), even if its value will be set by the function.