Check for Integer Overflow with Boolean

Question

This little project is based on this discussion about the best way to detect integer overflow before an operation is performed. What I want to do is have a program demonstrate the effectivity of utilizing the integer check. It should produce an integer overflow unchecked for some numbers, whereas it should quit before performing the operation if the check (-c) flag is used. The -m is for multiplication.

The program runs fine without the boolean part, but I need some help with the boolean part that conducts the highestOneBitPosition check. I am getting compilation errors after adding the true/false logic. I am not sure if I am calling and using the highestOneBitPosition function properly. Thanks!

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
/*boolean */ 
#define true 1
#define false 0
typedef int bool;

void ShowUsage ()
{
    printf (
    "Integer Overflow Check before performing an arithmetic.\n"
    "=======================================================\n"
    "Usage:\n"
    "Integer Operant (-a, -s, -m, -d) Checked/Unchecked (-u, -c)\n"
    "Example: ./overflowcheck 2 -a 2 -u\n"
    "\n"
);
}

size_t highestOneBitPosition(uint32_t a) {
size_t bits=0;
while (a!=0) {
    ++bits;
    a>>=1;
};
return bits;
}

int main(int argc, char *argv[]) {
    if      (argc != 5) {ShowUsage (); return (0);}
    else if (strcmp(argv[2],"-m") == 0 && strcmp(argv[4],"-u") == 0)
            {printf("%s * %s = %d -- Not checked for integer overflow.\n",argv[1],argv[3], atoi(argv[1])*atoi(argv[3]));return 0;}
/*Works fine so far */
    else if (strcmp(argv[2],"-m") == 0 && strcmp(argv[4],"-c") == 0)
    {
    bool multiplication_is_safe(uint32_t a, uint32_t b) {
    a = atoi( argv[1] );
    b = atoi( argv[3] );
    size_t a_bits=highestOneBitPosition(a), b_bits=highestOneBitPosition(b);
    return (a_bits+b_bits<=32);}
        if (multiplication_is_safe==true) 
        {printf("%s * %s = %d -- Checked for integer overflow.\n",argv[1],argv[3], atoi(argv[1])*atoi(argv[3]));return 0;}
        if (multiplication_is_safe==false)
        {printf("Operation not safe, integer overflow likely.\n");}    
    }

    ShowUsage ();
    return (0);}

compilation:

gcc integer_overflow2.c -o integer_overflow
integer_overflow2.c:40:61: error: function definition is not allowed here
        bool multiplication_is_safe(uint32_t a, uint32_t b) {
                                                            ^
integer_overflow2.c:45:17: error: use of undeclared identifier
      'multiplication_is_safe'
            if (multiplication_is_safe==true) 
                ^
integer_overflow2.c:47:17: error: use of undeclared identifier
      'multiplication_is_safe'
            if (multiplication_is_safe==false)
                ^

Answer 1

[to long for a comment]

Nested functions are not supported in C.

Properly indented C sources might look like this:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
/*boolean */
#define true 1
#define false 0
typedef int bool;

void ShowUsage()
{
  printf("Integer Overflow Check before performing an arithmetic.\n"
      "=======================================================\n"
      "Usage:\n"
      "Integer Operant (-a, -s, -m, -d) Checked/Unchecked (-u, -c)\n"
      "Example: ./overflowcheck 2 -a 2 -u\n"
      "\n");
}

size_t highestOneBitPosition(uint32_t a)
{
  size_t bits = 0;
  while (a != 0)
  {
    ++bits;
    a >>= 1;
  };
  return bits;
}

bool multiplication_is_safe(uint32_t a, uint32_t b)
{
  a = atoi(argv[1]);
  b = atoi(argv[3]);
  size_t a_bits = highestOneBitPosition(a), b_bits = highestOneBitPosition(b);
  return (a_bits + b_bits <= 32);
}

int main(int argc, char *argv[])
{
  if (argc != 5)
  {
    ShowUsage();
    return (0);
  }
  else if (strcmp(argv[2], "-m") == 0 && strcmp(argv[4], "-u") == 0)
  {
    printf("%s * %s = %d -- Not checked for integer overflow.\n", argv[1],
        argv[3], atoi(argv[1]) * atoi(argv[3]));
    return 0;
  }

  /*Works fine so far */
  else if (strcmp(argv[2], "-m") == 0 && strcmp(argv[4], "-c") == 0)
  {
    if (multiplication_is_safe == true)
    {
      printf("%s * %s = %d -- Checked for integer overflow.\n", argv[1],
          argv[3], atoi(argv[1]) * atoi(argv[3]));
      return 0;
    }

    if (multiplication_is_safe == false)
    {
      printf("Operation not safe, integer overflow likely.\n");
    }
  }

  ShowUsage();
  return (0);
}

There however still is a bug, which you might like to find and fix yourself. Look closely what the compiler warns you about. To enable all warnings use -Wall -Wextra -pedantic for gcc.