C library functions that make changes to arrays or objects take at least two arguments: a pointer to the array or object and an integer indicating the number of elements or bytes to be manipulated. For the purposes of this rule, the element count of a pointer is the size of the object to which it points, expressed by the number of elements that are valid to access. Supplying arguments to such a function might cause the function to form a pointer that does not point into or just past the end of the object, resulting in undefined behavior.

Annex J of the C Standard [ISO/IEC 9899:2011] states that it is undefined behavior if the "pointer passed to a library function array parameter does not have a value such that all address computations and object accesses are valid." .

In the following code,

the element count of the pointer p is sizeof(arr) / sizeof(arr[0]) , that is, 5 . The element count of the pointer p2 is sizeof(arr) , that is,  20 , on implementations where sizeof(int) == 4 . The element count of the pointer p3 is  12  on implementations where sizeof(int) == 4 , because p3 points two elements past the start of the array arr .  The element count of p4 is treated as though it were unsigned char * instead of void * , so it is the same as p2 .

Pointer + Integer

The following standard library functions take a pointer argument and a size argument, with the constraint that the pointer must point to a valid memory object of at least the number of elements indicated by the size argument.

¹ Takes two pointers and an integer, but the integer specifies the element count only of the output buffer, not of the input buffer.
² Takes two pointers and an integer, but the integer specifies the element count only of the input buffer, not of the output buffer.
³ Takes two pointers and two integers; each integer corresponds to the element count of one of the pointers.
⁴ Takes a pointer and two size-related integers; the first size-related integer parameter specifies the number of bytes available in the buffer; the second size-related integer parameter specifies the number of bytes to write within the buffer.

For calls that take a pointer and an integer size, the given size should not be greater than the element count of the pointer.

 Noncompliant Code Example (Element Count)

In this noncompliant code example, the incorrect element count is used in a call to wmemcpy() . The sizeof operator returns the size expressed in bytes, but wmemcpy() uses an element count based on wchar_t * .

Compliant Solution (Element Count)

When using functions that operate on pointed-to regions, programmers must always express the integer size in terms of the element count expected by the function. For example, memcpy() expects the element count expressed in terms of void * , but wmemcpy() expects the element count expressed in terms of wchar_t * .  Instead of the sizeof operator, functions that return the number of elements in the string are called, which matches the expected element count for the copy functions. In the case of this compliant solution, where the argument is an array A of type T , the expression sizeof(A) / sizeof(T) , or equivalently sizeof(A) / sizeof(*A) , can be used to compute the number of elements in the array.

Noncompliant Code Example (Pointer + Integer)

This noncompliant code example assigns a value greater than the number of bytes of available memory to n , which is then passed to memset() :

Compliant Solution (Pointer + Integer)

This compliant solution ensures that the value of n is not greater than the number of bytes of the dynamic memory pointed to by the pointer p :

Noncompliant Code Example (Pointer + Integer)

In this noncompliant code example, the element count of the array a is ARR_SIZE elements. Because memset() expects a byte count, the size of the array is scaled incorrectly by sizeof(int) instead of sizeof(long) , which can form an invalid pointer on architectures where sizeof(int) != sizeof(long) .

Compliant Solution (Pointer + Integer)

In this compliant solution, the element count required by memset()  is properly calculated without resorting to scaling:

Two Pointers + One Integer

The following standard library functions take two pointer arguments and a size argument, with the constraint that both pointers must point to valid memory objects of at least the number of elements indicated by the size argument. 

For calls that take two pointers and an integer size, the given size should not be greater than the element count of either pointer.

Noncompliant Code Example (Two Pointers + One Integer)

In this noncompliant code example, the value of n is incorrectly computed, allowing a read past the end of the object referenced by q :

Compliant Solution (Two Pointers + One Integer)

This compliant solution ensures that n is equal to the size of the character array:

One Pointer + Two Integers

The following standard library functions take a pointer argument and two size arguments, with the constraint that the pointer must point to a valid memory object containing at least as many bytes as the product of the two size arguments.

For calls that take a pointer and two integers, one integer represents the number of bytes required for an individual object, and a second integer represents the number of elements in the array. The resulting product of the two integers should not be greater than the element count of the pointer were it expressed as an unsigned char *.  

Noncompliant Code Example (One Pointer + Two Integers)

This noncompliant code example allocates a variable number of objects of type struct obj . The function checks that num_objs is small enough to prevent wrapping, in compliance with INT30-C. Ensure that unsigned integer operations do not wrap. The size of  struct obj  is assumed to be 16 bytes to account for padding to achieve the assumed alignment of long long . However, the padding typically depends on the target architecture, so this object size may be incorrect, resulting in an incorrect element count.

Compliant Solution (One Pointer + Two Integers)

This compliant solution uses the sizeof operator to correctly provide the object size and num_objs to provide the element count:

Noncompliant Code Example (One Pointer + Two Integers)

In this noncompliant code example, the function  f()  calls  fread()  to read  nitems  of type  wchar_t , each  size  bytes in size, into an array of  BUFFER_SIZE  elements,  wbuf . However, the expression used to compute the value of  nitems  fails to account for the fact that, unlike the size of  char , the size of  wchar_t  may be greater than 1. Consequently,  fread()  could attempt to form pointers past the end of  wbuf  and use them to assign values to nonexistent elements of the array. Such an attempt is undefined behavior. (See undefined behavior 109 .)  A likely consequence of this undefined behavior is a buffer overflow. For a discussion of this programming error in the Common Weakness Enumeration database, see CWE-121, "Stack-based Buffer Overflow," and CWE-805, "Buffer Access with Incorrect Length Value."

Compliant Solution (One Pointer + Two Integers)

This compliant solution correctly computes the maximum number of items for  fread()  to read from the file:

Noncompliant Code Example (Heartbleed)

CERT vulnerability 720951 describes a vulnerability in OpenSSL versions 1.0.1 through 1.0.1f, popularly known as "Heartbleed." This vulnerability allows an attacker to steal information that under normal conditions would be protected by Secure Socket Layer/Transport Layer Security (SSL/TLS) encryption.

Despite the seriousness of the vulnerability, Heartbleed is the result of a common programming error and an apparent lack of awareness of secure coding principles. Following is the vulnerable code:

The p pointer, along with payload and p1 , contains data from a packet. The code allocates a buffer sufficient to contain payload bytes, with some overhead, then copies payload bytes starting at p1 into this buffer and sends it to the client. Notably absent from this code are any checks that the payload integer variable extracted from the heartbeat packet corresponds to the size of the packet data. Because the client can specify an arbitrary value of payload , an attacker can cause the server to read and return the contents of memory beyond the end of the packet data, which violates INT04-C. Enforce limits on integer values originating from tainted sources. The resulting call to memcpy() can then copy the contents of memory past the end of the packet data and the packet itself, potentially exposing sensitive data to the attacker. This call to memcpy() violates ARR38-C. Guarantee that library functions do not form invalid pointers. A version of ARR38-C also appears in ISO/IEC TS 17961:2013, "Forming invalid pointers by library functions [libptr]." This rule would require a conforming analyzer to diagnose the Heartbleed vulnerability.