Beginning with Inno Setup 4.0.8, the uninstaller will return a non-zero exit code if the user cancels or a fatal error is encountered. Programs checking the exit code to detect failure should not check for a specific non-zero value; any non-zero exit code indicates that the uninstaller was not run to completion.

Note that at the moment you get an exit code back from the uninstaller, some code related to uninstallation might still be running. Because Windows doesn't allow programs to delete their own EXEs, the uninstaller creates and spawns a copy of itself in the TEMP directory. This "clone" performs the actual uninstallation, and at the end, terminates the original uninstaller EXE (at which point you get an exit code back), deletes it, then displays the "uninstall complete" message box (if it hasn't been suppressed with /SILENT or /VERYSILENT).