CERT C++: MEM36-C

Do not modify the alignment of objects by calling realloc()

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Description

Rule Definition

Do not modify the alignment of objects by calling realloc().¹

Polyspace Implementation

The rule checker checks for Alignment changed after memory reallocation.

Examples

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Alignment changed after memory reallocation

Issue

Alignment changed after memory reallocation occurs when you use realloc() to modify the size of objects with strict memory alignment requirements.

Risk

The pointer returned by realloc() can be suitably assigned to objects with less strict alignment requirements. A misaligned memory allocation can lead to buffer underflow or overflow, an illegally dereferenced pointer, or access to arbitrary memory locations. In processors that support misaligned memory, the allocation impacts the performance of the system.

Fix

To reallocate memory:

Resize the memory block.
- In Windows^®, use _aligned_realloc() with the alignment argument used in _aligned_malloc() to allocate the original memory block.
- In UNIX/Linux, use the same function with the same alignment argument used to allocate the original memory block.
Copy the original content to the new memory block.
Free the original memory block.

Note

This fix has implementation-defined behavior. The implementation might not support the requested memory alignment and can have additional constraints for the size of the new memory.

Example - Memory Reallocated Without Preserving the Original Alignment

#include <stdio.h>
#include <stdlib.h>

#define SIZE1024 1024

void func(void)
{
    size_t resize = SIZE1024;
    size_t alignment = 1 << 12; /* 4096 bytes alignment */
    int *ptr = NULL;
    int *ptr1;

	/* Allocate memory with 4096 bytes alignment */
	
    if (posix_memalign((void **)&ptr, alignment, sizeof(int)) != 0) 
    {
        /* Handle error */      
	  }
	  
	/*Reallocate memory without using the original alignment. 
	ptr1 may not be 4096 bytes aligned. */
		
    ptr1 = (int *)realloc(ptr, sizeof(int) * resize); //Noncompliant
	
    if (ptr1 == NULL)
    {
        /* Handle error */
    }

    /* Processing using ptr1 */

    /* Free before exit */
    free(ptr1);
}

In this example, the allocated memory is 4096-bytes aligned. realloc() then resizes the allocated memory. The new pointer ptr1 might not be 4096-bytes aligned.

Correction — Specify the Alignment for the Reallocated Memory

When you reallocate the memory, use posix_memalign() and pass the alignment argument that you used to allocate the original memory.

#include <stdio.h>
#include <stdlib.h>

#define SIZE1024 1024

void func(void)
{
    size_t resize = SIZE1024;
    size_t alignment = 1 << 12; /* 4096 bytes alignment */
    int *ptr = NULL;

	/* Allocate memory with 4096 bytes alignment */
    if (posix_memalign((void **)&ptr, alignment, sizeof(int)) != 0) 
    {
        /* Handle error */
    }
	
	/* Reallocate memory using the original alignment. */
    if (posix_memalign((void **)&ptr, alignment, sizeof(int) * resize) != 0) 
    {
        /* Handle error */
        free(ptr);
        ptr = NULL;
    }

    /* Processing using ptr */

    /* Free before exit */
    free(ptr);
}

Check Information

Group: 06. Memory Management (MEM)

Version History

Introduced in R2019a

¹ This software has been created by MathWorks incorporating portions of: the “SEI CERT-C Website,” © 2017 Carnegie Mellon University, the SEI CERT-C++ Web site © 2017 Carnegie Mellon University, ”SEI CERT C Coding Standard – Rules for Developing safe, Reliable and Secure systems – 2016 Edition,” © 2016 Carnegie Mellon University, and “SEI CERT C++ Coding Standard – Rules for Developing safe, Reliable and Secure systems in C++ – 2016 Edition” © 2016 Carnegie Mellon University, with special permission from its Software Engineering Institute.

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