The second example is illegal syntax, maybe you meant this?
pp = (int**) malloc (m*sizeof(int))

The first example is ok in C language but not in C++, which requires the return value of malloc() to be typecast to the appropriate type of variable. Other than the typecast they both are identical.

And BTW you shouldn't expect people to respond so quickly to a question.

This is all I have to say about the topic title.
http://catb.org/~esr/faqs/smart-questions.html#urgent