dl: add dl.open_opt/2, dl.sym_opt/2 and dl.dlerror/0 utility functions

vlib/dl/dl.v

@@ -24,3 +24,25 @@ pub fn get_shared_library_extension() string {
 pub fn get_libname(libname string) string {
 	return '$libname$dl.dl_ext'
 }
+
+// open_opt - loads the dynamic shared object.
+// Unlike open, open_opt return an option.
+pub fn open_opt(filename string, flags int) ?voidptr {
+	shared_object_handle := open(filename, flags)
+	if shared_object_handle == 0 {
+		e := dlerror()
+		return error(e)
+	}
+	return shared_object_handle
+}
+
+// sym_opt returns the address of a symbol in a given shared object, if found.
+// Unlike sym, sym_opt returns an option.
+pub fn sym_opt(shared_object_handle voidptr, symbol string) ?voidptr {
+	sym_handle := sym(shared_object_handle, symbol)
+	if sym_handle == 0 {
+		e := dlerror()
+		return error(e)
+	}
+	return sym_handle
+}

vlib/dl/dl_nix.c.v

@@ -13,6 +13,8 @@ fn C.dlsym(handle voidptr, symbol charptr) voidptr
 
 fn C.dlclose(handle voidptr) int
 
+fn C.dlerror() charptr
+
 // open loads the dynamic shared object.
 pub fn open(filename string, flags int) voidptr {
 	return C.dlopen(charptr(filename.str), flags)
@@ -27,3 +29,11 @@ pub fn close(handle voidptr) bool {
 pub fn sym(handle voidptr, symbol string) voidptr {
 	return C.dlsym(handle, charptr(symbol.str))
 }
+
+// dlerror provides a text error diagnostic message for functions in `dl`
+// it returns a human-readable string, describing the most recent error 
+// that occurred from a call to one of the `dl` functions, since the last
+// call to dlerror()
+pub fn dlerror() string {
+	return unsafe { cstring_to_vstring(byteptr(C.dlerror())) }
+}

vlib/dl/dl_windows.c.v

@@ -26,3 +26,14 @@ pub fn close(handle voidptr) bool {
 pub fn sym(handle voidptr, symbol string) voidptr {
 	return C.GetProcAddress(handle, symbol.str)
 }
+
+// dlerror provides a text error diagnostic message for functions in `dl`
+// it returns a human-readable string, describing the most recent error 
+// that occurred from a call to one of the `dl` functions, since the last
+// call to dlerror()
+pub fn dlerror() string {
+	// https://docs.microsoft.com/en-us/windows/win32/api/errhandlingapi/nf-errhandlingapi-getlasterror
+	// Unlike dlerror(), GetLastError returns just an error code, that is function specific.
+	cerr := int(C.GetLastError())
+	return 'error code $cerr'
+}