errors.md

Potential Errors

Pointers and Memory

Declaring Multiple Pointers

An asterisk is needed before each variable name when declaring multiple pointers.

int *p, q, r;   // incorrectly declares a pointer and two ints
int *i, *j, *k; // correctly declares three pointers

Dangling Pointers

Local variables in C are not initialized; hence, newly declared pointers are not initialized to null. Dereferencing and altering uninitialized pointers will result in unexpected behavior.

/* Bad things happen here. */
int *p;
*p = 5;

Stale Pointers

If two pointers are pointing to the same memory address and the data stored is freed or reallocated (using, for example, realloc) using one pointer, the other pointer will not redirect itself to the correct memory address.

/* Creates a integer array and points p and q to it. */
int *p, *q;
p = malloc(sizeof(int) * 10);
q = p;

/* Reallocates the integer array using p. q may no longer point 
to valid memory. */
p = realloc(sizeof(int) * 20);

Returned Pointers

Functions should not return pointers to local variables, because the stack frame is erased and the memory is deallocated upon reaching return. While the pointer will point to the correct location, this unallocated memory may be overwritten in the future, causing the values to change unexpectedly.

/** A function that returns a pointer to the local variable y.*/
int *foo() {
    int y = 3;
    return &y;
}

/** The main function. */
main() {
    int *p;
    p = foo();          // calls foo, pointing p to y
    printf("%d", *p);   // correctly prints 3
    printf("%d", *p);   // prints garbage left by printf
}

Null Pointers

The functions malloc and realloc will return NULL if the program is unable to find suitable space to allocate memory; attempting the modify the null pointer will result in an error.

int *p = malloc(sizeof(int) * 99999); // likely impossible
*p = 1;                               // error