I always use p for pointers, but not p_. Mainly it's because of aesthetics when scanning code. The -> operator and the _ always break things up too much for my brain. If I'm in a well protected piece of code where I know I don't need to do null check, I may write something like this:

    pFoo->pBar->pBaz=blah;

with the underscore, I find that much harder to parse (probably just my dyslexia, but maybe it's useful for you too). This is much harder for me to read.

    p_foo->p_bar->p_baz=blah;

Another thing that I've learned over the years is any time I need to do a malloc, make it typed-ish and have an accompanying free. Sometimes you may need to use a different library or a different allocator and then you have conflicts on who creates something and who deletes it. So I always have paired

    foo* new_foo();
    void delete_foo(foo* pFoo);

    bar* new_bar();
    void delete_bar(bar* pBar);

    baz* new_baz();
    void delete_baz(baz* pBaz);

Those functions may simply be wrappers around a malloc/free, or they may be more complex, but I like to start with having them exist in case I need to change the implementation later. When you're managing your memory, try to make it hard to make mistakes.

I do mostly agree with the other commentors though, just dig in an have fun and learn what works for you.