Lesson 34: Runtime Errors and Segfaults

int a[5];
a[10] = 99;  // only a[0]..a[4] exist; writing a[10] = instant crash

// Common mistake: wrong loop bound
for (int i = 0; i <= 5; i++)  // accesses a[5] when i=5 -> NG
  a[i] = i;

int *p = NULL;
*p = 10;  // writing to the address NULL points to -> Segfault

// Fix: NULL-check before use
if (p != NULL) { *p = 10; }

int x;
scanf("%d", x);   // NG: uses the "value" of x as an address -> crash
scanf("%d", &x);  // OK: passes the "address" of x

int *p;     // uninitialized -> contains garbage address
*p = 42;    // writing to a garbage address -> Segfault

// Fix: always initialize, or allocate with malloc before use

int fact(int n) {
  return n * fact(n - 1); // no base case -> recurses forever -> stack overflow
}
// Fix: add if (n <= 1) return 1;

char name[5];
strcpy(name, "HelloWorld"); // only 5 bytes but 10+NUL -> overflow

// Fix: strncpy(name, "Hello", sizeof(name)-1);

int *p = malloc(sizeof(int) * 100);
// ... use p ...
// forgot free(p); -> memory leak

free(p);
*p = 10;   // writing to freed memory -> undefined behavior
free(p);   // double free -> possible crash

// Fix: set p = NULL; after free

int a = 7, b = 2;
double avg = a / b;     // -> 3.000000 (not 3.5!)
double avg = (double)a / b; // -> 3.500000 (OK)

if (x & 1 == 0)   // parsed as x & (1==0) - not what you meant!
if ((x & 1) == 0) // OK: explicit parens

switch (n) {
  case 1: printf("one\n");  // no break -> case 2 and case 3 also run
  case 2: printf("two\n");
  case 3: printf("three\n");
}

if (x > 0);  // ← this ; ends the if statement
{
  printf("positive\n"); // ← always runs!
}