Finit loop doesn't finishes or show segfault

I used GNU BC ( basic calculator ) for exponentiation 999,999 to 999,999 and it answered in 10 minutes but when I want to find 999,999 root of a big file ( 1.1 GB) it shows segfault after 5 hours and GDB core shows mpz_clear(y) is the problem and when I remove it , It takes 20 hours and nothing happens and loop doesn't finish ; Here's the code :

#include <gmp.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(void) {
    FILE *fr, *fw;
    char filename[200];
    int size = 0;
    unsigned long int rt = 2;
    unsigned char ch[1] = {112};

    printf("Enter filename :\n");
    scanf("%s", &filename);
    fr = fopen(filename, "rb+");

    if (fr == NULL) {
        fprintf(stderr, "Error during opening of file %s\n", filename);
        perror("");
        return 1;
    }

    fseek(fr, 0L, SEEK_END);
    size = ftell(fr);
    fseek(fr, 0L, SEEK_SET);

    unsigned char* buf = NULL;
    buf = malloc((size_t)size);

    fread(buf, size, 1, fr);
    if (ferror(fr)) {
        fprintf(stderr, "Error during reading from file %s\n", filename);
        perror("");
        fclose(fr);
        return 1;
    }
    fclose(fr);
    fr = NULL;

    mpz_t x;
    mpz_init(x);
    mpz_t y;
    mpz_init(y);

    mpz_import(x, size, 1, (size_t)1u, 0, 0, buf);

    do {
        mpz_root(y, x, rt);
        if ((mpz_cmp_ui(y, 999999)) < 0)
            break;
        else
            mpz_clear(y);
        rt++;
    } while (rt <= 99999);

    int rootnum = 0;

    while (rt != 0) {
        int num;
        num = rt / 10;
        ++rootnum;
    }

    memset(buf, 0u, size);

    mpz_export(buf, NULL, 1, (size_t)1u, 0, 0, y);

    printf("Enter copied filename :\n");
    scanf("%s", &filename);

    fw = fopen(filename, "wb");
    if (fw == NULL) {
        fprintf(stderr, "Error during opening of file %s\n", filename);
        perror("");
        return 1;
    }

    fwrite(buf, mpz_sizeinbase(y, 10), 1, fw);
    if (ferror(fw)) {
        fprintf(stderr, "Error during writing to file %s\n", filename);
        perror("");
        fclose(fw);
        return 1;
    }

    fclose(fw);

    fw = fopen(filename, "ab+");
    fwrite(ch, 1, 1, fw);

    fclose(fw);

    fw = fopen(filename, "ab+");
    fwrite(&rt, rootnum, 1, fw);

    fclose(fw);

    free(buf);
    mpz_clear(x);
    mpz_clear(y);

    return 0;
}

Can any one tell me why the loop doesn't finish and how to fix it ?

Thanks I will be grateful

Edit : The code has been changed but segfault still exist and when I use GDB for debugging and open core file and enter the where command GDB shows mpz_clear(y) and if I remove it ( of course with else option ) the loop never ends ; The new code :

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>
#include <gmp.h>

int main(void)
{

FILE * fr, * fw;
char filename[201];
unsigned long long int size = 0;
unsigned long int rt = 2;
unsigned char ch[1] = {112};

printf("Enter filename : ");
fgets(filename, 200, stdin);
filename[strlen (filename) - 1] = '\0';
fr = fopen(filename, "rb+");

if (fr == NULL)
{
    fprintf(stderr, "Error during opening of file %s\n", filename);
    perror("");
    return 1;
}

fseek(fr, 0L, SEEK_END);
size = ftell(fr);
fseek(fr, 0L, SEEK_SET);

if (size == -1)
{

fprintf(stderr, "Size doesn't match\n'");
perror("");
return 1;

}

if ( size > SIZE_MAX)
{

fprintf(stderr, "The size of file is greater than acceptable\n");
perror("");
return 1;

}

unsigned char * buf = NULL;
buf = malloc((size_t)size);
if(buf == NULL)
{

    fprintf(stderr, "Buffer failure\n");
    perror("");
    return 1;

}

int r;
r = fread(buf, size, 1, fr);
printf("%d\n", r);
if (ferror(fr))
{
    fprintf(stderr, "Error during reading from file %s\n", filename);
    perror("");
    fclose(fr);
    return 1;
}
fclose(fr);
fr = NULL;

mpz_t x;
mpz_init(x);
mpz_t y;
mpz_init(y);

mpz_import(x, size, 1, (size_t)1u, 0, 0, buf);

do
{
    mpz_root (y, x, rt);
    if ((mpz_cmp_ui(y, 999999))<0)
    break;
    else
    {
    mpz_clear(y);
    mpz_init(y);
    rt++;
}while(rt<=99999);

int rootnum = 0;
unsigned long int num = rt;

do
{

    num = num/10;
    ++rootnum;

}while(num != 0);

memset (buf, 0u, size);

mpz_export(buf, NULL, 1, (size_t)1u, 0, 0, y);

printf("\nEnter copied filename : ");
fgets(filename, 200, stdin);
filename[strlen (filename) - 1] = '\0';

fw = fopen(filename, "wb");
if (fw == NULL) {
    fprintf(stderr, "Error during opening of file %s\n", filename);
    perror("");
    return 1;
}

fwrite(buf, mpz_sizeinbase(y, 10), 1, fw);
if (ferror(fw)) {
    fprintf(stderr, "Error during writing to file %s\n", filename);
    perror("");
    fclose(fw);
    return 1;
}

fclose(fw);

fw = fopen(filename, "ab+");
if(fw == NULL)
printf("File can't be written\n'");
if(ferror(fw))
{

    fprintf(stderr, "Error during writing to file %s\n", filename);
    perror("");
    fclose(fw);
    return 1;
    
}
fwrite(ch, 1, 1, fw);

fclose(fw);

fw = fopen(filename, "ab+");
if(fw == NULL)
printf("File can't be written\n'");
if(ferror(fw))
{

fprintf(stderr, "Error during writing to file %s\n", filename);
    perror("");
    fclose(fw);
    return 1;
    
}
int w;
w = fwrite(&rt, rootnum, 1, fw);
printf("%d\n", w);

fclose(fw);

free(buf);
mpz_clear(x);
mpz_clear(y);

return 0;

}