uint64_t a = xxx, b = yyy;

x = a * b;
if (a != 0 && x / a != b) {
    // overflow handling
}

uint64_t hi(uint64_t x) {
    return x >> 32;
}

uint64_t lo(uint64_t x) {
    return ((1L << 32) - 1) & x;
}

void multiply(uint64_t a, uint64_t b) {
    // actually uint32_t would do, but the casting is annoying
    uint64_t s0, s1, s2, s3;

    uint64_t x = lo(a) * lo(b);
    s0 = lo(x);

    x = hi(a) * lo(b) + hi(x);
    s1 = lo(x);
    s2 = hi(x);

    x = s1 + lo(a) * hi(b);
    s1 = lo(x);

    x = s2 + hi(a) * hi(b) + hi(x);
    s2 = lo(x);
    s3 = hi(x);

    uint64_t result = s1 << 32 | s0;
    uint64_t carry = s3 << 32 | s2;
}

x = s2 + hi(a) * hi(b) + hi(x)

x <= (B - 1) + (B - 1)(B - 1) + (B - 1)
              <= B*B - 1
               < B*B

if (b > 0 && a > 18446744073709551615 / b) {
     // overflow handling
}; else {
    c = a * b;
}

18446744073709551615 == (1<<64)-1

// split input numbers into 32-bit digits
uint64_t a0 = a & ((1LL<<32)-1);
uint64_t a1 = a >> 32;
uint64_t b0 = b & ((1LL<<32)-1);
uint64_t b1 = b >> 32;


// The following 3 lines of code is to calculate the carry of d1
// (d1 - 32-bit second digit of result, and it can be calculated as d1=d11+d12),
// but to avoid overflow.
// Actually rewriting the following 2 lines:
// uint64_t d1 = (a0 * b0 >> 32) + a1 * b0 + a0 * b1;
// uint64_t c1 = d1 >> 32;
uint64_t d11 = a1 * b0 + (a0 * b0 >> 32);
uint64_t d12 = a0 * b1;
uint64_t c1 = (d11 > 18446744073709551615 - d12) ? 1 : 0;

uint64_t d2 = a1 * b1 + c1;
uint64_t carry = d2; // needed carry stored here

/* Multiply with overflow checking, emulating clang's builtin function
 *
 *     __builtin_umull_overflow
 *
 * This code benchmarks five possible schemes for doing so.
 */

#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <limits.h>

#ifndef BOOL
    #define BOOL int
#endif

// Option 1, check for overflow a wider type
//    - Often fastest and the least code, especially on modern compilers
//    - When long is a 64-bit int, requires compiler support for 128-bits
//      ints (requires GCC >= 3.0 or Clang)

#if LONG_BIT > 32
    typedef __uint128_t long_overflow_t ;
#else
    typedef uint64_t long_overflow_t;
#endif

BOOL
umull_overflow1(unsigned long lhs, unsigned long rhs, unsigned long* result)
{
        long_overflow_t prod = (long_overflow_t)lhs * (long_overflow_t)rhs;
        *result = (unsigned long) prod;
        return (prod >> LONG_BIT) != 0;
}

// Option 2, perform long multiplication using a smaller type
//    - Sometimes the fastest (e.g., when mulitply on longs is a library
//      call).
//    - Performs at most three multiplies, and sometimes only performs one.
//    - Highly portable code; works no matter how many bits unsigned long is

BOOL
umull_overflow2(unsigned long lhs, unsigned long rhs, unsigned long* result)
{
        const unsigned long HALFSIZE_MAX = (1ul << LONG_BIT/2) - 1ul;
        unsigned long lhs_high = lhs >> LONG_BIT/2;
        unsigned long lhs_low  = lhs & HALFSIZE_MAX;
        unsigned long rhs_high = rhs >> LONG_BIT/2;
        unsigned long rhs_low  = rhs & HALFSIZE_MAX;

        unsigned long bot_bits = lhs_low * rhs_low;
        if (!(lhs_high || rhs_high)) {
            *result = bot_bits;
            return 0;
        }
        BOOL overflowed = lhs_high && rhs_high;
        unsigned long mid_bits1 = lhs_low * rhs_high;
        unsigned long mid_bits2 = lhs_high * rhs_low;

        *result = bot_bits + ((mid_bits1+mid_bits2) << LONG_BIT/2);
        return overflowed || *result < bot_bits
            || (mid_bits1 >> LONG_BIT/2) != 0
            || (mid_bits2 >> LONG_BIT/2) != 0;
}

// Option 3, perform long multiplication using a smaller type (this code is
// very similar to option 2, but calculates overflow using a different but
// equivalent method).
//    - Sometimes the fastest (e.g., when mulitply on longs is a library
//      call; clang likes this code).
//    - Performs at most three multiplies, and sometimes only performs one.
//    - Highly portable code; works no matter how many bits unsigned long is

BOOL
umull_overflow3(unsigned long lhs, unsigned long rhs, unsigned long* result)
{
        const unsigned long HALFSIZE_MAX = (1ul << LONG_BIT/2) - 1ul;
        unsigned long lhs_high = lhs >> LONG_BIT/2;
        unsigned long lhs_low  = lhs & HALFSIZE_MAX;
        unsigned long rhs_high = rhs >> LONG_BIT/2;
        unsigned long rhs_low  = rhs & HALFSIZE_MAX;

        unsigned long lowbits = lhs_low * rhs_low;
        if (!(lhs_high || rhs_high)) {
            *result = lowbits;
            return 0;
        }
        BOOL overflowed = lhs_high && rhs_high;
        unsigned long midbits1 = lhs_low * rhs_high;
        unsigned long midbits2 = lhs_high * rhs_low;
        unsigned long midbits  = midbits1 + midbits2;
        overflowed = overflowed || midbits < midbits1 || midbits > HALFSIZE_MAX;
        unsigned long product = lowbits + (midbits << LONG_BIT/2);
        overflowed = overflowed || product < lowbits;

        *result = product;
        return overflowed;
}

// Option 4, checks for overflow using division
//    - Checks for overflow using division
//    - Division is slow, especially if it is a library call

BOOL
umull_overflow4(unsigned long lhs, unsigned long rhs, unsigned long* result)
{
        *result = lhs * rhs;
        return rhs > 0 && (SIZE_MAX / rhs) < lhs;
}

// Option 5, checks for overflow using division
//    - Checks for overflow using division
//    - Avoids division when the numbers are "small enough" to trivially
//      rule out overflow
//    - Division is slow, especially if it is a library call

BOOL
umull_overflow5(unsigned long lhs, unsigned long rhs, unsigned long* result)
{
        const unsigned long MUL_NO_OVERFLOW = (1ul << LONG_BIT/2) - 1ul;
        *result = lhs * rhs;
        return (lhs >= MUL_NO_OVERFLOW || rhs >= MUL_NO_OVERFLOW) &&
            rhs > 0 && SIZE_MAX / rhs < lhs;
}

#ifndef umull_overflow
    #define umull_overflow2
#endif

/*
 * This benchmark code performs a multiply at all bit sizes,
 * essentially assuming that sizes are logarithmically distributed.
 */

int main()
{
        unsigned long i, j, k;
        int count = 0;
        unsigned long mult;
        unsigned long total = 0;

        for (k = 0; k < 0x40000000 / LONG_BIT / LONG_BIT; ++k)
                for (i = 0; i != LONG_MAX; i = i*2+1)
                        for (j = 0; j != LONG_MAX; j = j*2+1) {
                                count += umull_overflow(i+k, j+k, &mult);
                                total += mult;
                        }
        printf("%d overflows (total %lu)n", count, total);
}

+------------------+----------+----------+----------+----------+----------+
|                  | Option 1 | Option 2 | Option 3 | Option 4 | Option 5 |
|                  |  BigInt  | LngMult1 | LngMult2 |   Div    |  OptDiv  |
+------------------+----------+----------+----------+----------+----------+
| Clang 3.5 i386   |    1.610 |    3.217 |    3.129 |    4.405 |    4.398 |
| GCC 4.9.0 i386   |    1.488 |    3.469 |    5.853 |    4.704 |    4.712 |
| GCC 4.2.1 i386   |    2.842 |    4.022 |    3.629 |    4.160 |    4.696 |
| GCC 4.2.1 PPC32  |    8.227 |    7.756 |    7.242 |   20.632 |   20.481 |
| GCC 3.3   PPC32  |    5.684 |    9.804 |   11.525 |   21.734 |   22.517 |
+------------------+----------+----------+----------+----------+----------+
| Clang 3.5 x86_64 |    1.584 |    2.472 |    2.449 |    9.246 |    7.280 |
| GCC 4.9 x86_64   |    1.414 |    2.623 |    4.327 |    9.047 |    7.538 |
| GCC 4.2.1 x86_64 |    2.143 |    2.618 |    2.750 |    9.510 |    7.389 |
| GCC 4.2.1 PPC64  |   13.178 |    8.994 |    8.567 |   37.504 |   29.851 |
+------------------+----------+----------+----------+----------+----------+

x = a * b;
    if (a != 0 && x / a != b) {
        // overflow handling
    }

#include <stdint.h>

uint64_t mul(uint64_t a, uint64_t b) {
  uint32_t ah = a >> 32;
  uint32_t al = a;  // truncates: now a = al + 2**32 * ah
  uint32_t bh = b >> 32;
  uint32_t bl = b;  // truncates: now b = bl + 2**32 * bh
  // a * b = 2**64 * ah * bh + 2**32 * (ah * bl + bh * al) + al * bl
  uint64_t partial = (uint64_t) al * (uint64_t) bl;
  uint64_t mid1    = (uint64_t) ah * (uint64_t) bl;
  uint64_t mid2    = (uint64_t) al * (uint64_t) bh;
  uint64_t carry   = (uint64_t) ah * (uint64_t) bh;
  // add high parts of mid1 and mid2 to carry
  // add low parts of mid1 and mid2 to partial, carrying
  //    any carry bits into carry...
}

struct INT32struct {INT16 high, low;};
typedef union
{
  struct INT32struct s;
  INT32 ll;
} INT32union;

INT16 mulFunction(INT16 a, INT16 b)
{
  INT32union result.ll = a * b; //32Bits result
  if(result.s.high > 0)
      Overflow();
  return (result.s.low)
}

INT32 mulFunction(INT32 a, INT32 b)
{

  INT32union s_a.ll = abs(a);
  INT32union s_b.ll = abs(b); //32Bits result
  INT32union result;
  if(s_a.s.hi > 0 && s_b.s.hi > 0)
  {
      Overflow();
  }
  else if (s_a.s.hi > 0)
  {
      INT32union res1.ll = s_a.s.hi * s_b.s.lo;
      INT32union res2.ll = s_a.s.lo * s_b.s.lo;
      if (res1.hi == 0)
      {
          result.s.lo = res1.s.lo + res2.s.hi;
          if (result.s.hi == 0)
          {
            result.s.ll = result.s.lo << 16 + res2.s.lo;
            if ((a.s.hi >> 15) ^ (b.s.hi >> 15) == 1)
            {
                result.s.ll = -result.s.ll;
            }
            return result.s.ll
          }else
          {
             Overflow();
          }
      }else
      {
          Overflow();
      }
  }else if (s_b.s.hi > 0)
{

   //Same code changing a with b

}else
{
    return (s_a.lo * s_b.lo);
}
}

func hex128 (_ hi: UInt64, _ lo: UInt64) -> String
{
    var s: String = String(format: "%08X", hi >> 32)
                  + String(format: "%08X", hi & 0xFFFFFFFF)
                  + String(format: "%08X", lo >> 32)
                  + String(format: "%08X", lo & 0xFFFFFFFF)
    return (s)
}

func mul64to128 (_ multiplier: UInt64, _ multiplicand : UInt64)
             -> (result_hi: UInt64, result_lo: UInt64)
{
    let x: UInt64 = multiplier
    let x_lo: UInt64 = (x & 0xffffffff)
    let x_hi: UInt64 = x >> 32

    let y: UInt64 = multiplicand
    let y_lo: UInt64 = (y & 0xffffffff)
    let y_hi: UInt64 = y >> 32

    let mul_lo: UInt64 = (x_lo * y_lo)
    let mul_hi: UInt64 = (x_hi * y_lo) + (mul_lo >> 32)
    let mul_carry: UInt64 = (x_lo * y_hi) + (mul_hi & 0xffffffff)
    let result_hi: UInt64 = (x_hi * y_hi) + (mul_hi >> 32) + (mul_carry >> 32)
    let result_lo: UInt64 = (mul_carry << 32) + (mul_lo & 0xffffffff)

    return (result_hi, result_lo)
}

var c: UInt64 = 0
var d: UInt64 = 0

(c, d) = mul64to128(0x1234567890123456, 0x9876543210987654)
// 0AD77D742CE3C72E45FD10D81D28D038 is the result of the above example
print(hex128(c, d))

(c, d) = mul64to128(0xFFFFFFFFFFFFFFFF, 0xFFFFFFFFFFFFFFFF)
// FFFFFFFFFFFFFFFE0000000000000001 is the result of the above example
print(hex128(c, d))

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

int might_be_mul_oflow(unsigned long a, unsigned long b)
{
  if (!a || !b)
    return 0;

  a = a | (a >> 1) | (a >> 2) | (a >> 4) | (a >> 8) | (a >> 16) | (a >> 32);
  b = b | (b >> 1) | (b >> 2) | (b >> 4) | (b >> 8) | (b >> 16) | (b >> 32);

  for (;;) {
    unsigned long na = a << 1;
    if (na <= a)
      break;
    a = na;
  }

  return (a & b) ? 1 : 0;
}

int main(int argc, char **argv)
{
  unsigned long a, b;
  char *endptr;

  if (argc < 3) {
    printf("supply two unsigned long integers in C formn");
    return EXIT_FAILURE;
  }

  a = strtoul(argv[1], &endptr, 0);

  if (*endptr != 0) {
    printf("%s is garbagen", argv[1]);
    return EXIT_FAILURE;
  }

  b = strtoul(argv[2], &endptr, 0);

  if (*endptr != 0) {
    printf("%s is garbagen", argv[2]);
    return EXIT_FAILURE;
  }

  if (might_be_mul_oflow(a, b))
    printf("might be multiplication overflown");

  {
    unsigned long c = a * b;
    printf("%lu * %lu = %lun", a, b, c);
    if (a != 0 && c / a != b)
      printf("confirmed multiplication overflown");
  }

  return 0;
}

might_be_mul_oflow

int might_be_mul_oflow(unsigned long a, unsigned long b)
{
  if (!a || !b)
    return 0;

  {
    unsigned long arng = ULONG_MAX >> 1;
    unsigned long brng = 1;

    while (arng != 0) {
      if (a <= arng && b <= brng)
        return 0;
      arng >>= 1;
      brng <<= 1;
      brng |= 1;
    }

    return 1;
  }
}

int64_t safemult(int64_t a, int64_t b) {
  double dx;

  dx = (double)a * (double)b;

  if ( fabs(dx) < (double)9007199254740992 )
    return (int64_t)dx;

  if ( (double)INT64_MAX < fabs(dx) )
    return INT64_MAX;

  return a*b;
}