- (NSString *)fooFromPath:(NSString *)path error:(NSError **)anError {

    const char *fileRep = [path fileSystemRepresentation];
    int fd = open(fileRep, O_RDWR|O_NONBLOCK, 0);

    if (fd == -1) {

        if (anError != NULL) {
            NSString *description = nil;
            NSDictionary *uDict = nil;
            int errCode;

            if (errno == ENOENT) {
                description = NSLocalizedString(@"No file or directory at requested location", @"");
                errCode = MyCustomNoFileError;
            } else if (errno == EIO) {
                // Continue for each possible POSIX error...
            }

            // Make underlying error.
            NSError *underlyingError = [[[NSError alloc] initWithDomain:NSPOSIXErrorDomain
                code:errno userInfo:nil] autorelease];
            // Make and return custom domain error.
            NSArray *objArray = [NSArray arrayWithObjects:description, underlyingError, path, nil];
            NSArray *keyArray = [NSArray arrayWithObjects:NSLocalizedDescriptionKey,
                NSUnderlyingErrorKey, NSFilePathErrorKey, nil];
            NSDictionary *eDict = [NSDictionary dictionaryWithObjects:objArray
                forKeys:keyArray];

            *anError = [[[NSError alloc] initWithDomain:MyCustomErrorDomain
                    code:errCode userInfo:eDict] autorelease];
        }
        return nil;
    }
    // ...