ROC Curve for SVM

Hi , I want to plot ROC curve for my svm results with perfcurve, and i dont know what to put exactly for labels,scores,posclass parameters, here is my code :

model=svmtrain(lab_train,train,'-t 2 -d 2 -c 7 -g 0.5');
[labeltest,valtest,prectest]= svmpredict(lab_test,test,model);

% labels are the test data labels
% scores returned by a classifier for some test data
% posclass is is the positive class label 
[X,Y] = perfcurve(labels,scores,posclass);

thank you