ARR00-C. Understand how arrays work

The incorrect use of arrays has traditionally been a source of exploitable vulnerabilities. Elements referenced within an array using the subscript operator [] are not checked unless the programmer provides adequate bounds checking. As a result, the expression array [pos] = value can be used by an attacker to transfer control to arbitrary code.

An attacker who can control the values of both pos and value in the expression array [pos] = value can perform an arbitrary write (which is when the attacker overwrites other storage locations with different content). The consequences range from changing a variable used to determine what permissions the program grants to executing arbitrary code with the permissions of the vulnerable process. Arrays are also a common source of buffer overflows when iterators exceed the bounds of the array.

An array is a series of objects, all of which are the same size and type. Each object in an array is called an array element. The entire array is stored contiguously in memory (that is, there are no gaps between elements). Arrays are commonly used to represent a sequence of elements where random access is important but there is little or no need to insert new elements into the sequence (which can be an expensive operation with arrays).

Arrays containing a constant number of elements can be declared as follows:

enum { ARRAY_SIZE = 12 };
int array[ARRAY_SIZE];

These statements allocate storage for an array of 12 integers referenced by array. Arrays are indexed from 0..n-1 (where n represents an array bound). Arrays can also be declared as follows:

int array[];

This array is called an incomplete type because the size is unknown. If an array of unknown size is initialized, its size is determined by the largest indexed element with an explicit initializer. At the end of its initializer list, the array no longer has incomplete type.

int array[] = { 1, 2 };

Although these declarations work fine when the size of the array is known at compile time, it is not possible to declare an array in this fashion when the size can be determined only at runtime. The C Standard adds support for variable length arrays or arrays whose size is determined at runtime. Before the introduction of variable length arrays in C99, however, these "arrays" were typically implemented as pointers to their respective element types allocated using malloc(), as shown in this example:

int *dis = (int *)malloc(ARRAY_SIZE * sizeof(int));

Always check that malloc() returns a non-null pointer, as per ERR33-C. Detect and handle standard library errors.

It is important to retain any pointer value returned by malloc() so that the referenced memory may eventually be deallocated. One possible way to preserve such a value is to use a constant pointer:

int * const dat = (int * const)malloc(
  ARRAY_SIZE * sizeof(int)
);
/* ... */
free(dat);

Below we consider some techniques for array initialization.  Both dis and dat arrays can be initialized as follows:

for (i = 0; i < ARRAY_SIZE; i++) {
   dis[i] = 42; /* Assigns 42 to each element of dis */ 
   dat[i] = 42; /* Assigns 42 to each element of dat */
}

The dis array can also be initialized as follows:

for (i = 0; i < ARRAY_SIZE; i++) {
   *dis = 42;
   dis++;
}
dis -= ARRAY_SIZE;

This technique, however, will not work for dat.  The dat identifier cannot be incremented (produces a fatal compilation error), as it was declared with type int * const.  This problem can be circumvented by copying dat into a separate pointer:

int *p = dat;
for (i = 0; i < ARRAY_SIZE; i++)  {
  *p = 42; /* Assigns 42 to each element */
  p++;
}

The variable p is declared as a pointer to an integer, initialized with the value stored in dat, and then incremented in the loop. This technique can be used to initialize both arrays, and is a better style of programming than incrementing the original pointer to the array (e.g., dis++, in the above example), as it avoids having to reset the pointer back to the start of the array after the loop completes. 

Obviously, there is a relationship between array subscripts [] and pointers. The expression dis[i] is equivalent to *(dis+i) for all integral values of i. In other words, if dis is an array object (equivalently, a pointer to the initial element of an array object) and i is an integer, dis[i] designates the i^th element of dis. In fact, because *(dis+i) can be expressed as *(i+dis), the expression dis[i] can be represented as i[dis], although doing so is not encouraged. Because array indices are zero-based, the first element is designated as dis[0], or equivalently as *(dis+0) or simply *dis.

Risk Assessment

Arrays are a common source of vulnerabilities in C language programs because they are frequently used but not always fully understood.

Automated Detection

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

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