What happens if you free a pointer twice. When no longer needed, return it to the heap using free.

What happens if you free a pointer twice Unless Setting a pointer to null doesn't free the memory. The program runs perfectly. Once upon a very long time ago, there were implementations of 'free()' that crashed when given a null pointer to free. sandeepmorya6274 sandeepmorya6274 09. so a pointer is an address, think of it as an index into a big array with all the memory available to your program. If you already have that pointer, great. If you want safety there, use smart pointers, which free the memory for you and give errors when you try to access them when they don't hold something. 07. The quiz seems confused anyway, since "deleting a pointer" is likely not what it means. If you really need to increment by 2 values at a time, then consider having a vector of std::pair or a vector of a struct with 2 elements. How this affect when debugging? 0 Answers So if you have some variable fp declared to be a FILE* (i. The System do not know if you are going to assign the ptr with a new valid address right after the free call, or you just will abandon it unused. 2 also says that 'free' on a NULL pointer does no action. If you free oldPointer and the address changed, UB (crash likely). 3 rd scenario: If we free the same pointer second time without reallocating memory to that pointer, then what happens? As per ANSI/ISO C standard, this is undefined behaviour. Managed One would think that negating it twice would, in this case, return false, but since calling a member function on a null pointer is undefined behavior, the compiler is free to assume it never happens and optimize the function to always return true, or in x86 assembly terms, store 1 (true) in the register used for the return value (eax). Then, we can use it and free it again. Almost always this masks a logical bug because there is no reason to call free() a second time. When you delete the pointer for the first time , you are relinquishing your ownership and the memory will be going back to the free pool getting ready to be given out again. But I know of no real examples. ,NULL,it may corrupt the heap store and cause a trap. If we free the same pointer two or more time, then the behavior is undefined. This (a) forces you to spend a very long time trying to debug it, since the symptoms are nowhere near the cause and additionally (b) creates a serious security vulnerability in the software, which your employer gets sued over, loses, goes bust, and your What happens if you free a pointer twice? If you free a pointer, use it to allocate memory again, and free it again, of course it’s safe. Answer: Option A . For the first (non-array) form, expression must be a pointer to an object type or a class type contextually implicitly convertible to such pointer, and its value must be either null or pointer to a non-array object created by a What happens if you set the pointer to NULL before freeing the memory? If you try to free a null pointer, nothing will happen. BUT, the two file descriptors are indepedent in that they point to two different open file descriptions(an open file description is an entry in the system-wide table of open files). Ask questions, find answers and collaborate at work with Stack Overflow for Teams. When a program If you start by using the Standard containers - e. Teams. m, technically it means that the iterator's operator-> returns a raw pointer to the contained object. Since you might get the test backwards, and since most testing methodologies force you to explicitly test every branch point, you should not put in the redundant if test. Nobody's responded to this post yet. If there are other pointers to the same memory, they can continue to be used to reference and eventually free the memory. It just means that str_c now points to the newly allicated memory space. If it's garbage (let's say a huge number, you know what might A pointer takes you to a place in memory that you can change; the data off in memory that you point to is not copied or anything when you pass a parameter by value to a function. Actually this is also useful (C11): The value of a pointer becomes indeterminate when the object it points to (or just past) reaches the end of its lifetime. When you free a pointer you are not even allowed >>What happens if a pointer is deleted twice Depends. In general the allocator usually maintains a list of free memory in some way and freeing memory just adds it to that list In fact, in this case, when you call open() twice on the same file, you get two different file descriptors pointing to the same file (same physical file). To give a more useful answer, that is, knowing what typically could happen when you delete a pointer twice might help you recognize the situation if you see those symptoms. The free function causes the space pointed to by ptr to be deallocated, that is, made available for further allocation. Description. When this happens, if you aren't careful, you might free the same pointer multiple times when cleaning up the objects. Calling malloc isn't something special: it simply gives you a pointer where you can start storing data. char * ptr; ptr = malloc(10); free(ptr); free(ptr); Page -- However, calling free on a pointer does not reset it to NULL, so if you do a "auth " followed by two "reset"s (without another "auth " in between), then you will get a double-free. It can only be used to deallocate the heap memory previously allocated using malloc(), calloc() and realloc() You will probably ask why ptr has the same value? Well, the answer is simple: velocity. ) When you manually delete a pointer, there is absolutely no indication anywhere that the pointer has been deleted. just treat it as a 1D array of values, and keep a separate width value. The following example is silly, but safe: What happens if we free a pointer twice? If you free a pointer, use it to allocate memory again, and free it again, of course it’s safe. You might be wondering – if I free the same pointer twice, what‘s the worst that could happen? Well, let me tell you – double frees lead to chaos! While immediate crashes are rare, double frees corrupt the internal heap metadata used to track allocated blocks. It is gone, and there is a new value in x. Only delete what you new. A fairly common one is when multiple different objects all have pointers to one another and start getting cleaned up by calls to free. Double free errors occur when free() is called more than once with the same memory address as an argument. Although I can't test your code right now and it looks like there could occur quite a few problems it might actually work in your supplied example. What happens when a pointer is deleted twice? 1) a. This Question Belongs to C Plus Plus Programming >> Object Oriented Programming In C Plus First of all, free() is not a direct call to the OS. Normally it also tries to meld together adjacent blocks in the address space. You should not be manually deleting pointers at all, unless again you have some specific reason to do so. What does freeing the same memory twice mean? 2) Freeing same memory twice refers to a condition like this : Suppose you have char *cptr; Now here what happens is the memory pointed to by cptr is free for use. You get undefined behaviour if you try to delete an object through a pointer more than once. The allocator has to keep track of the pices it gave out and adding checks to it would probably What happens if you attempt to free the same block of memory twice in C? a) It leads to undefined behavior b) The program terminates c) Memory is released, but the pointer remains valid d) Memory is leaked In C programming, how do you insert quote characters (‘ and “) into the output screen? 0 Answers A* a = new A(); A* b = a; a = NULL; After this, b still points to the object allocated in the first line. What happens if you call free on a pointer twice? If you set free d pointers to NULL your program will But the free() method is not compulsory to use. Suppose that you have int *a = malloc (sizeof (int)) and a has 0xdeadbeef and you execute free (a) then after execution a still contains 0xdeadbeef but after the free call this memory address is no more reserved for you. General Knowledge Multiple choice GK Questions MCQ on current affairs quiz. You are deleting a pointer that didn't come from new, so you have undefined behavior (anything can happen). This can happen for example if you call free on ptr twice and value of ptr wasn't changed since first call to free. As an aside, C Standard $7. The free function cannot determine if what you've passed it is a valid pointer, it will dereference it regardless and then the nasal demons are on the loose. Header is behind the data pointed by data. The real problem is here: ptr = new Something(); otherPtr = ptr; delete ptr; delete otherPtr; This can happen if you have several pointers to the same object and it is quite dangerous. A final operator-> is applied to that and you end up with a reference to the contained object. In this case, freeing the pointer twice is OK, but only because you've been lucky. Undefined behaviour. open() what happens if I open twice the same file? 0. If you free oldPointer and it was the same address as newPointer, this is the same as freeing newPointer, of course. You first allocate storage for the array, then you allocate storage for each array element to place strings in. : Free'd pointer may or may not point to NULL afterwards - it's simply undefined behaviour to access free'd 5 likes, 0 comments - embeddedsystemstutorials on February 1, 2025: "Tricky C Interview Questions on Pointer & Memory Management 1️⃣ What happens if you free a pointer twice? 2️⃣ What is a dangling pointer? How do you avoid it? 3️⃣ What is a wild pointer? How is it different from a dangling pointer? 4️⃣ What is memory leakage? How can you prevent it in C? 5️⃣ ನೀವು ಪಾಯಿಂಟರ್ ಅನ್ನು ಮುಕ್ತಗೊಳಿಸಿದರೆ, ಮೆಮೊರಿಯನ್ನು ಮತ್ತೆ The pointer only stores the value returned by malloc and this function does not know anything about this pointer. It depends on the method receiver and on the type of the variable. Dereferencing Shared Pointers using * and -> Accessing the Raw Pointer using get() Transferring Ownership using std::move() Swapping Ownership using swap() Resetting Ownership using reset() Sharing Ownership. If you're talking about IEE754 floats, 12. If you free a pointer, the memory you freed might be reallocated. However, C programmers in a hurry (that’s all of us, right?) will talk about “a freed pointer” to If you free a non-null pointer twice, memory is freed twice, and that is a problem. 2020 Computer Science Secondary School If delete is applied to one of the pointers then the object's memory is returned to the free store. Example 3: you declare an array of pointers to characters. You won't be able to declare a new variable aod again while you are in the scope of the first aod. Can you use a pointer after freeing it? You can safely use delete on a null pointer - it will yield a no-op. They both have the same idental affect on the original variable The reason you don't see the first example is because it's wrong. Afterwards you remove the front page and the book's back and hand it back to the librarian. You are not calling new[] twice on the same pointer. To be precise and accurate, the pointed-to memory, not the pointer itself, has been freed. What exactly happens when you free memory depends on the implementation of the allocator. when freeing the memory you dont tell free how much memory you want to free, the runtime knows this so some housekeeping is needed. A very likely scenario is that it will stick the same memory block twice in the list of free memory blocks - which means the first and second malloc with a matching size, it will return the same pointer. Double frees can happen in all sorts of cases. Or you have two (or more) different pointers pointing to same memory: if you call free on one, you are not allowed to call free on other pointers now too. If you free a pointer, the memory you freed might be In many ways, the most insidious thing that can happen if code double-frees a pointer is that between the two calls to free, the address gets returned by a later call to malloc(). – Oliver Charlesworth. Short answer: if you're using the database/sql package, your deferred Rows. Explain what happens when a pointer is deleted twice. Thus, the memory that p points to is being freed twice. See reasoning and explanation below. Write a client and server program in C language using UDP, where client program interact with the Server as given below: i) The client begins by sending a request to send a string of 8 characters or series of 7 numbers, the server sends back a characters or numbers as per the request of the client. So the real problem is not delete on a null pointer. When you de-reference it, you'll simply get the char representation of the first part (but see below to understand what this really means, it's not as clear as you may think) of the float. You can still "use" the object via the b pointer, and you can delete the object via delete b;. You can cast a float* to a char* just fine, it's the using of such a beast that may be problematic. 2 in IEEE754 float is (abcd are the octets): S Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company The free() function in C is used to free or deallocate the dynamically allocated memory and helps in reducing memory wastage. In your case it means it returns a raw pointer to the unique_ptr. You can assign pointer as many times as you need with the value returned by the malloc function if you free previously stored value, otherwise you will have a memory leak. I would definitely avoid it if at all possible. D. The C free() function cannot be used to free the statically allocated memory (e. One way to think of it is that the pointer has the "same" value before and after the call, but that value is valid before the call and indeterminate after the call. The implementation of malloc/free in K&R for example, allocates a header and then the data. When no longer needed, return it to the heap using free. So, although it is undefined behaviour, as arkoenig mentioned, there are a number of things that would typically happen if you delete a pointer a second time, but it is, of course dependent on the when allocating say 5 bytes you get back from malloc an address pointing to these 5 bytes, hence the name pointer. you can learn and practice Online free. Ad. Anyway, the rule is simple: do not use oldPointer after it's be realloc'ed. And what happen exactly when you free a pointer from the heap memory. Click here 👆 to get an answer to your question ️ What happens when a pointer is deleted twice. new[] simply returns a pointer to the allocated memory. Then, we can allocate memory for same pointer variable. You can delete a; here - it will have no effect (since a is NULL), but it is safe and will not lead to undefined behavior. It is safe. You cannot use oldPointer after that realloc. Avoid crashing when double-freeing pointers. , a pointer to whatever type FILE happens to be), then you are free to reassign it if you find that convenient. Still, there are some important reasons to free() after using malloc(): FILE * It's a pointer to a FILE structure, when you call fclose() it will destroy/free FILE structure but will not change the value of FILE* pointer means still it has the address of that FILE structure which is now not exits. Asking OS for memory is quite slow, so there is layer in between called an allocator that asks the OS for big chunks of memory and gives you a piece of it whenever you call malloc, which brings average cost of each allocation down. what is trap. This is also not an issue. >>value or pass by reference Nothing -- almost. A* a = new A(); A* b = a; delete a; After the Troubleshooting Javax. . Calling free() twice on the same value can lead to memory leak. B. Even if you fail to call free() (and thus leak memory in the heap), the operating system will reclaim all the memory of the process (including those pages for code, stack, and, as relevant here, heap) when the program is In the above snippet, on call to line 3, twice destructors of Test gets called, and allocates memory for array of size 1, So, who call the reset? Is it make_unique does it? Also, can I get detail implementation of make_unique and make_shared. In any case it is a good practice assign ptr with a NULL pointer right after the free call: free(ptr); ptr = NULL; Ad. If ptr is a NULL pointer, no operation is performed. The free block list is just a circular list of memory chunks which have of course some admin data in the beginning. This only applies to implementations that pre-date the C89 (C90) standard that have not been modified to deal with the problem since. If you delete it for the second time, that memory might be used by any other program now and when they try to access it it will crash. Shared Pointers. If set to null, it wont have much affect if deleted twice. If you really want the convenience of mat[y][x], you can improve it by doing a single call to malloc() that allocates man free The free() function deallocates the memory allocation pointed to by ptr. ” A pointer is simply a variable that holds an address, the same way an int is a variable that holds an integer value (within a range). g. 20. free(ptr);/*C*/ or delete(ptr);//C++. The possible solutions are: It helps to understand correctly what you are actually doing. If that happens, you might get that pointer back. Setting a pointer to null after freeing the memory makes sure that you don't accidentally try to free the same memory twice. std::make_shared() Using Shared Pointers. 2 nd scenario: We can free a pointer. What happens if a pointer is deleted twice in a program as shown in the following C++ statements? int *ptr = new int; delete ptr; delete ptr; A. This is why no further chaining of operator-> occurs. This means that pretty much anything can happen from 'appearing to work' to What happens if you free memory twice? Calling free () twice on the same value can lead to memory leak. SSLHandshakeException: Received Fatal Alert - Bad_Certificate You need to free() the same pointer as you received from malloc(). – Moreover you should be careful in such cases to not delete the memory twice (e. So when you try to the header, you will access garbage and anything can happen. Both versions of your program destroy the same object twice because you basically create two separate shared_ptrs from the same raw pointer. If you free a pointer, the memory you freed might be 3 rd scenario: If we free the same pointer second time without reallocating memory to that pointer, then what happens? As per ANSI/ISO C standard, this is undefined behaviour. 2. It is a solid memory corruption because regarding the code, the variable is still usable but the memory pointed to that variable can be free. You aren't closing the pointer, you're closing the object that the pointer points to. If you free a block twice then you might have the problem that someone did a new malloc, got the memory you just freed, overwrites it and the second free reads invalid pointers from it. Memory is not allocated “to a pointer. It can abort the program : We provide fully solved Online General Knowledge Quiz preparation. For an iterator it, the expression it->m is equivalent to (*i). To avoid Running without crashing doesn't mean your program is correct. Example 2: you request storage from the heap using malloc. Unique Pointers. Once the function is complete the parameter p come to end of existence. What happens if you free a pointer twice? If you free a pointer, use it to allocate memory again, and free it again, of course it’s safe. The second level of management is within each process, for example within the heap when you call malloc() and free(). Close() methods will properly close both of your Rows instances because Rows. Similarly, when Passing p to func() by value, which will copy the pointer and creates the local copy to func() which frees the memory. What happens at that point is undefined by the C spec, so it depends on your particular implementations of malloc and free , but in practice is usually exploitable for memory If you free a pointer, use it to allocate memory again, and free it again, of course it's safe. when we delete a pointer that was pointing to an object,the memory allocated in the free store is released ,but if i happen to delete the pointer twice which now points to nothing,i. Incrementing, altering or changing it is undefined behaviour, that is usually a segmentation fault. Only newPointer exists. Example. As soon as a pointer is passed to free() , the object it pointed to reaches the end of its lifetime. a now points to nothing. In the same way, you can assign it to any number of pointers but as soon as it's free'd via one pointer, it's done. Free will put the memory block in its own free block list. So using a realistic factor that an impotent call to free() is 6x the check you win unless more than 5/6 of pointers are non-NULL. func() then sets it's own instance of the pointer p to NULL but which is useless. Short: Calling free() a second time, by accident, doesn't crash when it's set to NULL. When a program calls free () twice with the same argument, the program’s What happens if you free a pointer twice? If you free a pointer, use it to allocate memory again, and free it again, of course it's safe. I believe C++ is more prone to double freeing where there could be a method and a destructor which is executed implicitly, and both free the same memory. Add your If you free a pointer, use it to allocate memory again, and free it again, of course it’s safe. Not to be confused with SNMP The only thing that will remove the pointer var from the stack would be if it went out of scope, just like any other variable. NVD Categorization. Any attempt to refer to the pointed-to object has undefined behavior (i. Think of the pointer you receive as a book from a library. But if you don't then the program will probably crash. For example, the header may contain the length of data allocated. S. If you're really, really unlucky, the allocator reused that pointer and you freed a different allocation. To access an element at (x,y) use mat[y * width + x]. When you must track or otherwise handle elements using pointers, prefer "smart pointers" which can enforce an ownership/lifetime model and avoid leaks: in particular, the C++11 Standard library What is double free exploit? A double-free vulnerability occurs when, as the name says, a variable is free()’d twice. This happens when the code goes outside the curly brace pair where the var is declared. Of course in many applications the pointer is rarely non-NULL. There are lots of other cases as well, though. In calling function you still have pointer p holding an address, but the block is now on the free list During my tests, I have found that it is possible to use pointer after free(). From cppreference on delete:. Unless every address that ever gets returned from malloc is permanently and forevermore associated What happens if you free a pointer twice? Answer: If you free a pointer, use it to allocate memory again, and free it again, of course it's safe. If free() is not used in a program the memory allocated using malloc() will be de-allocated after completion of the execution of the program (included program execution time is relatively small and the program ends normally). So in practice you were right in Dec 2017. Most likely the program will crash, sometimes strange things might happen. C. In fact, all four answers basically say the same thing. This leads to a slow death by Memory Corruption – unpredictable consequences, security holes and data As to exactly what happens when you free twice, it is very much dependent on the implementation of free(). If ptr is a null pointer, no action occurs. More crucially, you must pay attention to the "can" in the multiple choice answers, because there is no guarantee that anything will happen when you double-delete an object. first via p1 then via p2) - otherwise you will trigger undefined behavior. In this scenario, the erroneous call to free() would be passing an address that matches that of a live object. so void foo (int *ip) { *ip = 42; } chn change the value POINTED TO even though this function does not use a reference parameter. It's safer to let the application crash and be able It could happen that you are releasing memory after using it and as part of cleanup for example, and end up calling free on the same pointer twice. To avoid leaking memory, you have to free that memory again at some point. Nothing about the pointer has changed. Try Teams for free Explore Teams. There is a practice in coding to keep some code blocks in comment symbols than delete it when debugging. you can implement It's true that free()'s argument is passed by value (like all C function arguments), and so free can't actually modify the pointer. Getting the Owner Count using use_count() Pitfall: Not FAQ: Does delete p delete the pointer p, or the pointed-to-data *p? FAQ: Is it safe to delete the same pointer twice? (this FAQ) FAQ: Can I free() pointers allocated with new? Can I delete pointers allocated with malloc()? FAQ: Benefits of new over malloc()? FAQ: Can I use realloc() on pointers allocated via new? FAQ: Checking for NULL after p There seem to be two arguments why one should set a pointer to NULL after freeing them. net. If we subsequently delete the second pointer then the free store may be corrupted. That's why care is needed when copying pointers around (because if you free one, it may still be inadvertently used). When you set the pointer to NULL after free() you can call free() on it again and no operation will be performed. e. P. And why we should set pointer to null after freeing it to free it again. If there is enough space you don't need to free or malloc again. Something like you have rented a flat with malloc used for some I would think in practice the worst that can plausibly happen is that some time later, the memory allocator misbehaves. Query() returns a pointer (and so rows is a pointer). But do watch out for one thing (which has nothing to do with declarations): when you open a file, you are using some operating system resources, which you will continue to use until you close the file (or your Never free memory twice. However, if a Causing malloc twice shouldn't cause any problems. same things happem with any pointer getting with malloc. If you delete a pointer and set it to NULL, it it possibly cannot have an adverse effect. Given the memory capacity of computers today, unless you're creating massive numbers of variables, it's not a problem. , you’re no longer allowed to dereference the pointer). ssl. What happen when a pointer is deleted twice Mcq? A pointer if not nullified and deleted twice, leads to a trap. (what will happen when a pointer is deleted twice Kindly detail about trap srinivas 08-22-2014 10:02 PM c++0 good question monika 07-27-2014 03:23 PM « Previous; Next » Write your comments * * * * Enter the code shown above: (Note: If you cannot read the numbers in the above image, reload the page to generate a new one. I have the following code: typedef struct{ int module_id; int adc_id; struct config_line * pnext; } A smart compiler could inline the check and only call when free() was needed. Of course it would be pretty unmaintainable for a large increment since you need a check for each, but it is another option. In giving the pointer an address to an existing location, you don't want to use the type specifier to the left of it again, since that variable already exists (that's declaring a variable). NOTE To be precise and accurate, the pointed-to memory, not the pointer itself, has been freed. A pointer if not nullified and deleted twice, leads to a trap. You get home and read it. Given some int x = 3;, when the program executes x = 8;, nothing is done with the 3 that is in x before the assignment except to overwrite it. It's also not required for the pointer to set itself to NULL after you delete it. In many ways, the most insidious thing that can happen if code double-frees a pointer is that between the two calls to free, the address gets returned by a later call to malloc(). int a malloc(10); free(a); still a will not be NULL If your matrix isn't "ragged", i. all rows have the same length, you might want to consider: Accessing it manually, i. So one of those tries to delete an invalid pointer which results in undefined behavior. std::string, std::vector, std::set - they'll manage and release memory for you automatically if you store elements "by value". If that's the only pointer to that memory, the memory cannot be referenced again. In this Double free errors occur when free () is called more than once with the same memory address as an argument. This is why most modern C++ texts will tell you that, unless you have some specific reason otherwise, do not use naked pointers. Semantically incorrect. , local variables) or memory allocated on the stack. Close() has pointer receiver and because DB. I know that this answer is a little bit late, but I just read this answer and wrote some code to verify the following:. Just hold on to the allocated chunk. Commented Feb 15, 2014 at 11:41. When a program calls free () twice with the same argument, the Deallocating a memory area with free does not make the contents of the pointer NULL. C - Opening differents files . Syntactically incorrect. You are calling new[] twice, and storing both results into the same pointer. CWE-415: Double Free: The product calls free() twice on the same memory address, potentially leading to modification of unexpected memory locations. 3. If you set the value of the pointer to 0 after the first delete then nothing will happen. solve General Knowledge Multiple choice GK Quiz online, Study and Tips general knowledge mcq. When you hit undefined behavior the result is undefined. So, if we free the same pointer which is freed already, the program will stop its execution. jewgfa ofomwb klz ozsus ktnlnh aqrzm ywxq tdjoggvl acxw gmbd ywifu gdfio fzak kmekgt gedpk