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#include <assert.h>
#include <limits.h>
#include <stdio.h>
 
int foo(int a) {
  assert(a + 100 > a);
  printf("%d %d\n", a + 100, a);
  return a;
}

int main(void) {
  foo(100);
  foo(INT_MAX);
  return 0;
}

This code checks for signed integer overflow by testing whether a + 100 > a. This test cannot evaluate to false unless an integer overflow occurs. However, because a conforming implementation is not required to generate code for undefined behavior, and signed integer overflow is undefined behavior, this code may be compiled out. For example, GCC version 4.1.1 optimizes out the assertion for all optimization levels, and version 4.2.3 optimizes out the assertion for programs compiled with -O2-level optimization and higher.

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#include <assert.h>
#include <limits.h>
#include <stdio.h>

int foo(int a) {
  assert(a < (INT_MAX - 100));
  printf("%d %d\n", a + 100, a);
  return a;
}

int main(void) {
  foo(100);
  foo(INT_MAX);
  return 0;
}

Risk Assessment

Although it is rare that the entire application can be strictly conforming, the goal should be that almost all the code is allowed for a strictly conforming program (which among other things means that it avoids undefined behavior), with the implementation-dependent parts confined to modules that the programmer knows are needed to adapt to the platform when it changes.

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