WIN30-C. Properly pair allocation and deallocation functions

Windows provides several APIs for allocating memory.  While some of these functions have converged over time, it is still important to always properly pair allocations and deallocations.  The following table shows the proper pairings.

Noncompliant Code Example

In this example, the FormatMessage() function allocates a buffer and stores it in the buf parameter.  From the documentation of FORMAT_MESSAGE_ALLOCATE_BUFFER [MSDN]:

The function allocates a buffer large enough to hold the formatted message, and places a pointer to the allocated buffer at the address specified by lpBuffer.  The lpBuffer parameter is a pointer to anL PTSTR; you must cast the pointer to an LPTSTR (for example, (LPTSTR)&lpBuffer). The nSize parameter specifies the minimum number of TCHARs to allocate for an output message buffer. The caller should use the LocalFree function to free the buffer when it is no longer needed.

Instead of freeing the memory using LocalFree(), this code example uses GlobalFree() erroneously.

LPTSTR buf;
DWORD n = FormatMessage(FORMAT_MESSAGE_ALLOCATE_BUFFER |
                        FORMAT_MESSAGE_FROM_SYSTEM |
                        FORMAT_MESSAGE_IGNORE_INSERTS, 0, GetLastError(),
                        LANG_USER_DEFAULT, (LPTSTR)&buf, 1024, 0);
if (n != 0) {
  /* Format and display the error to the user */

  GlobalFree(buf);
}

Compliant Solution

The compliant solution uses the proper deallocation function as described by the documentation.

LPTSTR buf;
DWORD n = FormatMessage(FORMAT_MESSAGE_ALLOCATE_BUFFER |
                        FORMAT_MESSAGE_FROM_SYSTEM |
                        FORMAT_MESSAGE_IGNORE_INSERTS, 0, GetLastError(),
                        LANG_USER_DEFAULT, (LPTSTR)&buf, 1024, 0);
if (n != 0) {
  /* Format and display the error to the user */

  LocalFree(buf);
}

Risk Assessment

Mixing allocation and deallocation functions can lead to memory corruption issues, or result in accessing out-of-bounds memory.

Automated Detection

Related Vulnerabilities

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

Related Guidelines