FA> Does passing a pointer to a function effect performance, as opposed to
 FA> having the contents of said pointer global or local ?

The only differences is the overhead associated with passing any parameter.
A parameter is a relative (to the stack) address, whereas a global is not.
Same issue, and it really doesn't much matter about the data type.

For the sake of 99.9999% of applications, the 'overhead' of using a
relative address is irrelevant.

 FA> Also, does re-entrancy mean, that one can have multiple instances of a
 FA> function/program ?

Reentrancy means that a function can be called more than once, even at the
same time or recursively, and each invocation will not affect the other. It
means that the function can be REENTERED.

In the C library, for example, there are some functions which are not
re-entrant. Like strtok(), since it stores its own static 'state'
information that is modified each time it is called. So you can't for
example, expect that this state information will not change whenever it is
called. In a multithreaded environment (such as OS2), where you have
pseudo-simultaneous execution of different threads, if each thread called
the same non-reentrant function at the same time, they would tend to
interfere with each other. I should add that for strtok() and similar C
library functions, most OS/2 multithreaded libraries actually work around
this by having separate static state information for each thread, and other
functions that are not strictly reentrant such as printf() use semaphores
to ensure that only one thread can execute it at the same time.

Reentrancy is an issue also when you want a function to be reusable, and
you don't want to have to worry about when and where you call it. Reentrant
functions tend to use and work with parameters and auto variables only, and
avoid using (non-const) static data at all.

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