Question

Does any standard specifies what should be the output?

For example this code:

#include <stdio.h>
#include <math.h>

int main(int argc, char** argv) {
  float a = INFINITY;
  float b = -INFINITY;
  float c = NAN;

  printf("float %f %f %f\n", a, b, c); 
  printf("int %d %d %d\n", (int) a, (int) b, (int) c); 
  printf("uint %u %u %u\n", (unsigned int) a, (unsigned int) b, (unsigned int) c); 
  printf("lint %ld %ld %ld\n", (long int) a, (long int) b, (long int) b); 
  printf("luint %lu %lu %lu\n", (unsigned long int) a, (unsigned long int) b, (unsigned long int) c); 

  return 0;
}

Compiled on gcc version 4.2.1 (Apple Inc. build 5664) Target: i686-apple-darwin10

Outputs:

$ gcc test.c && ./a.out 
float inf -inf nan
int -2147483648 -2147483648 -2147483648
uint 0 0 0
lint -9223372036854775808 -9223372036854775808 -9223372036854775808
luint 0 9223372036854775808 9223372036854775808

Which is quite weird. (int)+inf < 0 !?!

Answer 1

As Paul said, it’s undefined:

From §6.3.1.4:

6.3.1.4 Real floating and integer

When a finite value of real floating type is
converted to an integer type other
than _Bool, the fractional part is
discarded (i.e., the value is
truncated toward zero). If the value
of the integral part cannot be
represented by the integer type, the
behavior is undefined.50)

Infinity isn’t finite, and the integral part can’t be represented in an integral type, so it’s undefined.