cocos2d-x3.0rc0 CCRANDOM_MINUS1_1 CCRANDOM_0_1 alway get the same result

cocos2d-x C++
#1

xcode5.1 lastest sdk, maybe sdk have some bug

srand(time(NULL)); //srand(time(0)); srandom(time(0));
float rand_num = CCRANDOM_MINUS1_1();

alway get the same result(same time the result is equal):
rand = -0.049565

rand = -0.049487

rand = -0.049362

rand = -0.049221

rand = -0.049158

rand = -0.049080

–other result
rand = -0.417533
rand = -0.870324
rand = -0.870324