How to Find Memory Leak in a C++ Code/Project

How to find memory leak in a C++ code/project?

Instructions

Things You'll Need

• Proficiency in C++
• C++ compiler
• Debugger and other investigative software tools

1

Understand the operator basics. The C++ operator new allocates heap memory. The delete operator frees heap memory. For every new, you should use a delete so that you free the same memory you allocated:

char* str = new char [30]; // Allocate 30 bytes to house a string.

delete [] str; // Clear those 30 bytes and make str point nowhere.

2

Reallocate memory only if you've deleted. In the code below, str acquires a new address with the second allocation. The first address is lost irretrievably, and so are the 30 bytes that it pointed to. Now they're impossible to free, and you have a memory leak:

char* str = new char [30]; // Give str a memory address.

// delete [] str; // Remove the first comment marking in this line to correct.

str = new char [60]; /* Give str another memory address with
                                                    the first one gone forever.*/

delete [] str; // This deletes the 60 bytes, not just the first 30.

3

Watch those pointer assignments. Every dynamic variable (allocated memory on the heap) needs to be associated with a pointer. When a dynamic variable becomes disassociated from its pointer(s), it becomes impossible to erase. Again, this results in a memory leak:

char* str1 = new char [30];

char* str2 = new char [40];

strcpy(str1, "Memory leak");

str2 = str1; // Bad! Now the 40 bytes are impossible to free.

delete [] str2; // This deletes the 30 bytes.

delete [] str1; // Possible access violation. What a disaster!

4

Be careful with local pointers. A pointer you declare in a function is allocated on the stack, but the dynamic variable it points to is allocated on the heap. If you don't delete it, it will persist after the program exits from the function:

void Leak(int x){

char* p = new char [x];

// delete [] p; // Remove the first comment marking to correct.

}

5

Pay attention to the square braces after "delete." Use delete by itself to free a single object. Use delete [] with square brackets to free a heap array. Don't do something like this:

char* one = new char;

delete [] one; // Wrong

char* many = new char [30];

delete many; // Wrong!

6

If the leak yet allowed - I'm usually seeking it with deleaker (check it here: http://deleaker.com).

Detecting memory leaks in C programs?

As suggested, there already exist excellent tools like Valgrind to do this.

Further:

I would like to do this without any external utilities for practice

This is interesting and I am sure would be fulfilling,

You can use macro trick to detect such memory usage and leak errors, in fact write your own neat leak detector. You should be able to do this as long as you have a single allocation and deallocation function in your project.

#define malloc(X) my_malloc( X, __FILE__, __LINE__, __FUNCTION__)

void* my_malloc(size_t size, const char *file, int line, const char *func)
{

    void *p = malloc(size);
    printf ("Allocated = %s, %i, %s, %p[%li]\n", file, line, func, p, size);

    /*Link List functionality goes in here*/

    return p;
}

You maintain a Linked List of addresses being allocated with the file and line number from where there allocated. You update the link list with entries in your malloc.

Similar to above you can write an implementation for free, wherein you check the address entries being asked to be freed against your linked list. If there is no matching entry its a usage error and you can flag it so.

At the end of your program you print or write the contents of your linked list to an logfile. If there are no leaks your linked list should have no entries but if there are some leaks then the logfile gives you exact location of where the memory was allocated.

Note that in using this macro trick, you lose the type checking which functions offer but it's a neat little trick I use a lot of times.

Hope this helps and All the Best :)

Finding memory leaks in C code on Windows

Application Verifier will do this quite well, http://msdn.microsoft.com/en-us/library/ms220948.aspx

C++ memory leak, how to detect

You never free the _a and _b allocated in SSE_4_Matrix.

You also allocate a lot dynamically just to throw it away a bit later. For example the _a and _b could be arrays of 16 floats in stack.

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