C ++ overflows prematurely

Question

I had a strange problem with C ++, where a long data type overflowed long before it appeared. What I am doing (with success so far) is for integers to behave like float, so that the range [-32767,32767] maps to [-1.0,1.0]. Where it stumbles, there are larger arguments representing float greater than 1.0:

inline long times(long a, long b) { printf("a=%ld b=%ld ",a,b); a *= b; printf("a*b=%ld ",a); a /= 32767l; printf("a*b/32767=%ld\n",a); return a; } int main(void) { printf("%ld\n",times(98301l,32767l)); } 

What I get as output:

 a=98301 b=32767 a*b=-1073938429 a*b/32767=-32775 -32775 

Thus, the times (98301,32767) are similar to 3.0 * 1.0. This code works fine when arguments at times less than 32767 (1.0), but none of the intermediate steps with the arguments above should overflow 64 bits in length.

Any ideas?

Answer 1

long is not necessarily 64 bits. try "long long".

Answer 2

The long type is not necessarily 64 bits. If you use a 32-bit architecture (at least on MS Visual C ++), the long type is 32 bits. Check this out with sizeof (long) . There is also a long long data type that can help.

Answer 3

You probably have 32 bit long. Use long long instead.

98301 * 32767 = 3221028867, while 32-bit overflows at 2147483648

Answer 4

The C standard ensures that long will have at least 32 bits (which is actually the case on most 32-bit platforms).

If you need 64 bits, use long long . It is guaranteed to hold at least 64 bits.