A question about memory and garbage collection

int main(int argc, char *argv[])

{

pari_init(1000000,0);

n = gp_read_str(argv[1]);

i = gen_0;

{

GEN t = prng();

}

while (mpcmp(i,n)<0);

pari_close();

return 0;

}

Here prng() is a function which returns a GEN. Basically it takes a value x, does some computations (using a few auxiliary variables), and returns an updated value. It may do this 10^12 times. Clearly I need to have some sort of memory management: once a value x is updated and used to create the next value, it is needed no longer. So I need some way of removing the previous value of x so that the stack doesn't overflow.

Here's an example of the sort of prng() I might use (I'm not actually using this one, but you get the idea):

GEN prng(void)

{

y = Fp_pow(x,7,p);

x = y;

return(x);

}

(where p is a GEN set to a large prime). How do I keep the memory and stack at a reasonable size? I've read the manual, and experimented with cgiv, and with avma, gerepile etc, but so far none of my attempts has worked.

I'm sorry if this is a stupid question, but I am quite new to PARI, and not a very experienced C programmer: most of my work up to now has been in high level languages (Maple, Sage, Maxima, etc).

Thanks,

Alasdair

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