Can anyone share concept/program in C to add two 64-bit numbers on a 32-bit machine

Just use long long or __int64 data types. One of those are supported by most, if not all, modern compilers.

> Can anyone share concept
I assume you know how to add say 16 and 18 together to get 34

I mean, once you've figured out the concept of 'carry', whether you're storing each element of your overall number in a character, or byte, or long really doesn't make a bean of difference.

commented: Good to see you posting again :) +9

for(i  =  0; i < 2; i++)
{
rslt1 = ( a[i] & 0x7fffffffu ) + ( b[i] & 0x7fffffffu ) + carry;//first add 31lsb bits
carry= rslt1 >> 31;// 32 th bit is carry
rslt2 = (a[i] >> 31) + (b[i] >> 31) + carry; //add this carry with 32th bit's of operands
carry = rslt2 >> 1; //msb of answer is the carry from 32 bit addition
rslt1 = rslt1 & 0x7fffffffu;
rslt2 = rslt1 + (rslt2 << 31);//lsb of that answer is the 32th bit of original answer
result[i] = rslt2;//result stored in to an array
}

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commented: cool post +1

Why all that crazy code??

__int64 a = 123;
__int64 b = 234;
__int64 c = a+b;