Is the openssl / bn.h library part of the ANSI C standard?

Standard libraries do not provide support for arithmetic operations with values greater than 64 bits.

Operations with large integers can be done using a GNU library called gmplib .

Here is an example where the two arguments (large integers) passed to main() are multiplied and their result displayed on the screen:

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

#define MAX_BUF_LEN    (128)

int main( int argc, char * argv[] )
{
    char res[ MAX_BUF_LEN ];

    mpz_t a, b, c;

    mpz_init(a);
    mpz_init(b);
    mpz_init(c);

    mpz_set_str( a, argv[1], 10 );
    mpz_set_str( b, argv[2], 10 );

    mpz_mul( c, a, b );

    mpz_get_str( res, 10, c );

    printf("%s\n", res );

    mpz_clear(a);
    mpz_clear(b);
    mpz_clear(c);

    return 0;
}

compiling:

$ gcc -lgmp -Wall bigintmul.c -o bigintmul

Testing:

$ ./bigintmul 1234567890123456789012345678901234567890 9876543219876543219876543219876543210

Output:

12193263124676116335924401657049230176967230591384377380136092059011263526900

Checking: