CODEWITHSUNDEEP
cgnubignum
Marauder
Marauder

Reputation: 1

How to convert abitrary raw data into integer using C and GMP_Bignum

I am writing a program with GNU Bignum and what i want to do is simply read a binary file, and use the raw data as a Bignum integer, But whenever i read this file even though it is about 2MB long and try to print the number it gives me a very small number like 67 or 300, i have tried it with different files and it all behaves the same. the source code below might give you guys an idea of what i am talking about.

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


int main()
{
  mpz_t data_in;
  mpz_t data_out;
  FILE *in_file;
  FILE * out_file;
  unsigned long file_length;
  void* data;

  //initialize data
  mpz_init(data_in);
  mpz_init(data_out);

  in_file = fopen("main.c","rb");
  out_file = fopen("out.txt","wb");

  //get file length.
  fseek(in_file,0,SEEK_END);
  file_length = ftell(in_file);
  fseek(in_file,0,SEEK_SET);

  //allocate memory.
  data = malloc(file_length);

  //read file into memory.
  fread(data,file_length,1,in_file);
   //check to see if the first byte is zero
  /*if(data[1]== 0)
    {
      printf(" first byte zero\n");
    }
  else
    {
      printf("first byte OK\nFile length %lu\n Data read %d\n",file_length,strlen(data));
    }
  */

  //import data as integer.
  mpz_import(data_in,1,file_length,sizeof(data[0]),0,0,data);

  //output number in the screen
  gmp_printf(" Data is %Zd\n",data_in);
  mpz_out_str(NULL,10,data_in);


  fclose(in_file);
  fclose(out_file);


  return 0;

}

What could be wrong here ?

Upvotes: 0

Views: 975

Answers (1)

mu is too short
mu is too short

Reputation: 434675

I think you have the second and third arguments to mpz_import reversed, I think you want this (with ugly comments for clarity):

mpz_import(
        data_in,                /* mpz_t rop      */
        file_length,            /* size_t count   */
        1,                      /* int order      */
        sizeof(data[0]),        /* int size       */
        0,                      /* int endian     */
        0,                      /* size_t nails   */
        data                    /* const void *op */
);

The result of reversing the second and third arguments would be mpz_import reading less data than you want and hence your small values.

Upvotes: 1

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