What does it mean double free detected in tcache 2 while using mpz?

Question

I use this program to store a mpz value but when I add a 0 ( 400000000000000000000000000000000000000 instead of 40000000000000000000000000000000000000 -> 38 0s instead of 37) I get

free(): double free detected in tcache 2

Aborted (core dumped)

#include <iostream>
#include <gmpxx.h>
#include <vector>

using namespace std;

int main(const int argc, const char * const argv[])
{
    char *str = (char*)malloc(sizeof(char)*1024);
    mpz_class l;
    l = 40000000000000000000000000000000000000_mpz;
    mpz_set_str(l.get_mpz_t(), str, 10);
    cout << endl << str;
    return 0;
}

Is there a possibility to store large numbers?

Thank you

Furlough

How should I declare str? Only after I read the arguments I can find out how long the "message is". Is there a way to declare only a pointer? – Furlough

This is a common problem when working with C-style strings. If you don't know how big a message could be, you're usually best off assuming the worst case and allocating for the biggest possible message. Since this is now a statically-sized allocation, consider using an automatic variable. For example, consider using char str[1024]; in place of char *str= (char*)malloc(sizeof(char)*1024);. Fewer memory management headaches in the future. – Billfish

Answer 1

Your code has undefined behaviour because you are trying to assign l from an uninitialised array str.

I'm guessing you got your functions confused and meant to write the opposite

mpz_get_str(str, 10, l.get_mpz_t());

That code assigns l to str.

Use the following code to work out how big str needs to be

size_t size = mpz_sizeinbase(l.get_mpz_t(), 10) + 2;

Liss

Here's an explanation for the + 2 part: "The right amount of allocation is normally two more than the value returned by mpz_sizeinbase, one extra for a minus sign and one for the null-terminator." – Mitra