Comp 210 Lab 12: C Pitfalls

Table of Contents

Cotchas (C Gotchas)

int a = 2, b = 3;
float x = 2.0, y = 3.0;

    expr1,expr2;

char buff[BIG_ENOUGH_CONSTANT];
... // buff gets some message to be printed
printf(buff);

buff[i] = '%';
buff[i+1] = 'd';

printf("%s", buff);

int f( int n );   // this is a declaration of f()

int g( int m ) {  // this is both the declaration and definition of g()
  return f(m-3);  // It's now legal to mention f().
  }

int f() {         // f() was previously declared; now its defined
  if (n < 0)
    return n;
  else
   return g(n);
  }

  int x = 10;
  while (x >= 0) {
    int x = x - 1;     // this declares a different x!
    cout << x << " ";
  }

  if (condition)
    action

int main() {
  //...
  if (x == y)
    y = y + x;
    printf( "I just increased the value of y.\n" );
  printf( "Continuing onward...\n" );
  //...
  }

  int i = 1;
  while (i < 100);
    i = 2*i;
  printf( "The first power of two bigger than 100 is %d.\n", i );

  for ( int i = 0;  i < 100; i = 2*i )
    ;  // Look at that null statement there!
  printf( "blahblahblah\n" );

char x;
if (condition1)
    if (condition1a)
        x = 'a';
  else             // where to indent
   x = '?';        //   this?

  int n;
  printf( "Enter a value for n: " );
  scanf( "%d", &n );

/*
 * Copy this program, which can be found in
 * ~comp210/Labs/lab12/wrongo.c,
 * and compile it to see how well the compiler likes it.
 * The file has a number of (hopefully) fairly obvious mistakes,
 * as well as some hideous indentation.
 * Modify it, so that it shines.
 */

#include <stddef.h>     // get declaration of NULL
#include <stdio.h>      // get declaration of printf
int main() {
  double x;
    printf( "Enter a positive x, and I'll calculate how many doublings\n" );
    printf( "of 1 it takes until x is met or exceeded: " );
    scanf( "%lf", &x );
  
int n = 1,doublings=0;
  for ( bool notDoneYet = x > 0; notDoneYet = true; notDoneYet == n >= x );
    {
      n == n * 2;
      doublings++;
    }

printf( "ceil(log_2(%g)) = %d.\n", x, doublings );
return 0;
  }

Pointers and Seg Faults and Bus Errors, Oh My!

When I see a segmentation fault, my first suspicions are that my code is

  char str1[200] = "hi";         // This is fine.
  char *str2;                    // str2 has no value yet; okay but be cautious.
  strcpy( str2, "Katasrophe" );  // WRONG we're copying into str2 which
                                 //   could be pointing at anywhere in memory.
                                 //   "str2 = str1" would have saved us,
                                 //   by having str2 and str1 both point
                                 //   at a (valid) block of 200*sizeof(char)

  #include <stddef.h>            // Get declaration of NULL.
  #include <stdio.h>             // Get declaration of printf.

  struct intCons {             // Declare a type "struct intCons"
    int car;                   //   with a car and a cdr.
    struct intCons *cdr;
    }
    
  intCons* l2 = new intCons;
  l2->car = 5;
  l2->cdr = NULL;
  intCons* l1;

  printf( "%d", l2->car );        // Fine, prints 5.
  l1 = l2->cdr;                   // Fine, l1 gets NULL.
  if (l2->cdr->cdr == NULL)       // WRONG: l2->cdr is NULL, so 'the thing
                                  // pointed to by l2->cdr' makes no sense.
    printf( "l2 has only one item.\n" );
                                  // Compare with also-incorrect Scheme:
                                  //   (if (null? (cdr (cdr l2))) ...)

(Advanced:) Here is one way a bus error can occur, for those who care: On some machines, when the women in the controller ask for the contents of a memory bucket/byte, they actually get several (perhaps 4) buckets, a "word". All good and fine, except that when requesting a memory location, the request must be a multiple of 4 ("on a word boundary"). That is, not every memory-location (bucket) can be fetched individually (it's not "byte-addressable"). A bus error occurs if a bucket is requested that isn't a multiple of 4.

Now usually the C compiler takes care of this for you (after all, this word size, 4, varies from machine to machine, being 1 in the trivial case). However, if you're accessing through a garbage pointer, then it may not be pointing at a word boundary, and the memory doesn't know what values to ship to the women via the data bus.

Common C Error Messages