hard real time operation is time dependent and is completed at a fixed duration whereas in soft real time os operation is guaranted but it is not time dependent.

sizeof(0) returns the size of integer whose value is 0. Hence malloc would allocate sizeof(int) bytes and return a valid pointer.

++*ip will increment the value to which ip points.
ex: *ip=3;
++*ip;
printf("%d",*ip);

the output will be 4
after this operation ip will point towards the same memory location. It doesn't increments the memory location in this operation.

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#define cat(x,y) x##y concatenates x to y. But cat(cat(1,2),3) does not expand but gives preprocessor warning. Why?

in this case the cat(x,y) is the macro which is defined by using the preprocessor directive , this will be substituted only at the place where it is called in this example it happens like this

cat(1,2)##3 which will once again become 1##2##3
here if we use ## in between we can join or concatenat only two variables that why it gives a preprocessor warning

The public data members of the base class will becoome the private members of thederived class and hence are visible but private data members of base class are not accessible to derived class .