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MSC37-C-EX1: According to the C Standard, 5.1.2.2.3, paragraph 1 [ISO/IEC 9899:2011], "Reaching the } that terminates the main function returns a value of 0." As a result, it is permissible for control to reach the end of the main() function without executing a return statement.

MSC37-C-EX2: It is permissible for a control path to not return a value if that code path is never taken and a function marked _Noreturn is called as part of that code path. For example:

#include <stdio.h>
#include <stdlib.h>
 
_Noreturn void unreachable(const char *msg) {
  printf("Unreachable code reached: %s\n", msg);
  exit(1);
}

enum E {
  One,
  Two,
  Three
};
 
int f(enum E e) {
  switch (e) {
  case One: return 1;
  case Two: return 2;
  case Three: return 3;
  }
  unreachable("Can never get here");
}

Risk Assessment

Using the return value from a non-void function where control reaches the end of the function without evaluating a return statement can lead to buffer overflow vulnerabilities as well as other unexpected program behaviors.

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