malloc: *** error for object 0x1ecc0eb0: pointer being freed was not allocated

extern "C" {
const char* getWeiboAppInstallUrl() {
NSString *str = [[NSString alloc] initWithCString:"Sheldon" encoding:NSStringEncodingConversionAllowLossy];
return [str UTF8String];
}
}

extern "C" {
const char* getWeiboAppInstallUrl() {
const char * a = "Sheldon";
return a;
}
}

extern "C" {
char * getWeiboAppInstallUrl() {
NSString * str = [[NSString alloc]initWithString:@"Sheldon"];
char * x = (char *)malloc(strlen([str UTF8String]) + 1);
strcpy(x, [str UTF8String]);
return x;
}
}