Hi,
I'm currently learning libgmp and to that end I'm writing a small program which find prime factors. My program calls a function which fills an array with a varying amount of mpz_t integers, prime factors of a given number, which I need to return. I'm planning on setting the last element to NULL, so I know how many mpz_t integers the function found.
My problem is I'm getting double free errors with my array of pointers to mpz_t integers. I've written up some sample code illustrating my problem:
#include <stdlib.h>
#include <stdio.h>
#include <gmp.h>
int main(void)
{
mpz_t *p = malloc(5*sizeof(mpz_t*));
mpz_init_set_ui(p[0], 2UL);
mpz_init_set_ui(p[1], 5UL);
gmp_printf("%Zd %Zd\n", p[0], p[1]);
mpz_clear(p[0]);
mpz_clear(p[1]);
free(p);
return 0;
}
2 and 5 are printed to stdout, so allocation seems to be fine. But I'm getting the double free error below:
2 5
*** glibc detected *** ./lol: double free or corruption (out): 0x08e20020 ***
======= Backtrace: =========
/lib/libc.so.6(+0x6b6c1)[0xb77126c1]
/lib/libc.so.6(+0x6cf18)[0xb7713f18]
/lib/libc.so.6(cfree+0x6d)[0xb7716f8d]
/usr/lib/libgmp.so.3(__gmp_default_free+0x1d)[0xb77f53fd]
/usr/lib/libgmp.so.3(__gmpz_clear+0x2c)[0xb77ff08c]
./lol[0x80485e3]
/lib/libc.so.6(__libc_start_main+0xe6)[0xb76bdb86]
./lol[0x80484e1]
I'm still getting totally used to pointers, and gcc gives no errors, however I'm fairly sure this is wrong and I should be doing something like
mpz_init_set_ui(*p[0], 2UL);
instead of:
mpz_init_set_ui(p[0], 2UL);
But that gives me a compiler error
test.c:8: error: incompatible type for argument 1 of ‘__gmpz_init_set_ui’
/usr/include/gmp.h:925: note: expected ‘mpz_ptr’ but argument is of type ‘__mpz_struct’
Anyway, my questions are:
- I'm sure I should be dereferencing the pointer in the mpz_init_set_ui() call, why is that wrong?
- Is there a better way of doing this? Should I use a linked list?(I've not learned linked lists yet, I figure an array is best for this but if I'm really making things way more difficult, tell me) 3.Would it be better to create a struct with a pointer to my array and another variable with the amount of elements in my array and return a pointer to that instead?
The platform is linux 32-bit just in case that's relevant.
Here is the code I have just now, which I want to modify, I declare the array of mpz_t on the stack. But I want to make main() a function:
#include <stdio.h>
#include <stdlib.h>
#include "prime.h"
#define MAXFACTORS 100
int main(void)
{
mpz_t numToFactor, factor;
mpz_t result;/* used to pass return values from getPrimeFactor() */
mpz_t primeFactors[MAXFACTORS];
mpz_init_set_str(numToFactor, "18 446 744 073 709 551 615 436 457 568", 10);
mpz_init(factor);
mpz_init(result);
int pFLen = 0;
mpz_init(primeFactors[pFLen]);
getPrimeFactor(numToFactor, result);
mpz_set(factor, result);
while(mpz_cmp_ui(factor, 0UL))
{
mpz_set(primeFactors[pFLen], factor);
pFLen++;
if(pFLen == MAXFACTORS)
{
puts("Ran out of space to store prime factors, quitting...");
}
mpz_init(primeFactors[pFLen]);
mpz_divexact(factor, numToFactor, factor);
mpz_set(numToFactor, factor);
getPrimeFactor(factor, result);
mpz_set(factor, result);
}
mpz_set(primeFactors[pFLen], numToFactor);
pFLen++;
int i;
for(i = 0; i < pFLen; i++)
{
gmp_printf("%Zd ", primeFactors[i]);
}
puts("");
mpz_clear(numToFactor);
mpz_clear(factor);
return 0;
}
Thanks in advance people,