Some uses of the null pointer are: a) To initialize a pointer variable when that pointer variable isn’t assigned any valid memory address yet. Does having several kids really mean I don't pay any federal taxes? [], you can use the ?? e.g. What is a smart pointer and when should I use one? And finally to anyone reading this and getting confused by the idea that memset to 0 does not create NULL pointers this is because the value of the NULL pointer is not set by the C/C++ standard. If the gravitational force were inversely proportional to distance (rather than distance squared), will celestial bodies fall into each other? @Coder Completely wrong. having to leave to developer the Then their secret internal pointer … I accessed some already free'd memory at some point (the free was some lines too early). If the way you access the pointer means that a large enough offset is applied to it before dereferencing, then it may reach addressable memory: ((MyHugeStruct *)0)->fieldNearTheEnd. memory management as a whole. This (can) actually be important. For example if the pointer is a member of a class and I've deleted what it points to then the "contract" if you like of the class is that that member will point to something valid at any time so it must be set to null for that reason. Is becoming an Amazon seller profitable? In general, I would advise you to set it to NULL when you think it is a good idea, and not bother when you think it isn't worth it. edit close. Edit: Added do ... while as suggested below, thanks. The two goals, keep only good or eliminate clearly bad, would seem to be diametrically opposed, but there is usually a third group that's neither one nor the other, some of both. I don't remember ever causing a double-free that would be fixed by setting the pointer to NULL after freeing, but I've caused plenty that would not. tst.c 8 Warning 438: Last value assigned to variable 'nPtr' (defined at line 5) not used. It's much more likely that setting pointers to NULL will, I always assign dead pointers to NULL since their addressed memory is no longer valid. What is the appropriate length of an antenna for a handheld on 2 meters? no action occurs. the error is. *test = NULL; Where did you get that? What software should I buy to have a macOS VM on my Linux machine? In c++ it's important to always be thinking who owns this data when you allocate some memory (unless you are using smart pointers but even then some thought is required). values = None >>> Sometimes you have instances of incompatible types. Do I have to pay a web hosting company for an SSL certificate? Wilhelm, the point is that with a null pointer dereference you get a determinate crash and the actual location of the problem. For example PC-lint/FlexeLint from Gimpel Software says The preprocessor macro NULL is defined as an implementation-defined null pointer constant, which in C99 can be portably expressed as the integer value 0 converted to the type void* (pointer to void). It is assumed that message_type is an integer and message is a pointer to an array of characters that were allocated dynamically. I wouldn't do so in your example as I think there is no benefit to doing so and it adds "clutter" which in my experience is just as responsible for bugs and bad code as this kind of thing. That memory might be reallocated to another part of your program and then you get memory corruption. I am not site design / logo © 2021 Stack Exchange Inc; user contributions licensed under cc by-sa. I personally think that in any none-trivial codebase getting an error for dereferencing null is as vague as getting an error for dereferencing an address you don't own. In such cases setting the pointer to NULL and considering the problem "fixed" is nothing more than an attempt to sweep the problem under the carpet. A null pointer is a value that any pointer can take to represent that it is pointing to "nowhere", while a void pointer is a type of pointer that can point to somewhere without a specific type. ?-Operator verwenden, um einen alternativen Ausdruck zum Auswerten für den Fall bereitzustellen, dass das Ergebnis des NULL-bedingten Vorgangs null ist:In expressions with the null-conditional operators ?. Why doesn't free() zero out the memory prior to releasing it? Why does this script running su never seem to terminate if I change user inside the script? If you NULL out the pointer, then any attempt to use it is going to dereference 0x0 and crash right there, which is easy to debug. However, if message_type == value_1 evaluates to true and message_type == value_2 also evaluates to true, then messageis freed … A pointer declaration is any simple declaration whose declaratorhas the form There are no pointers to references and there are no pointers to bit fields. Appeal process for being designated a "Terrorist Group" (Canada). Stack Overflow for Teams is a private, secure spot for you and On the other hand, defensively NULLing a pointer that has been freed can be quite useful when someone thoughtlessly adds a new block of code beneath the free(), when the design of the original module isn't correct, and in the case of it-compiles-but-doesn't-do-what-I-want bugs. After that pCreatureN = NULL; is ok. site design / logo © 2021 Stack Exchange Inc; user contributions licensed under cc by-sa. For help clarifying this question so that it can be reopened, Stack Overflow works best with JavaScript enabled, Where developers & technologists share private knowledge with coworkers, Programming & related technical career opportunities, Recruit tech talent & build your employer brand, Reach developers & technologists worldwide. ?=operators can be useful in the following scenarios: 1. C++ [closed], Sequencing your DNA with a USB dongle and open source code, Podcast 310: Fix-Server, and other useful command line utilities, Opt-in alpha test for a new Stacks editor, Visual design changes to the review queues. Trojan "Win32/Tnega!MSR" found by Windows Defender - aliases used by other antiviruses? For example when freeing a struct that holds resources, pointers to allocated memory, file handles, etc., as I free the contained memory pointers and close the contained files, I NULL respective members. But in the real world my code will be modified and/or mantained by someone who probably doesn't know me and doesn't have the time and/or skills to do things properly (because the deadline is always yesterday). For local variables, it may be a little bit pointless if it is "obvious" that the pointer isn't accessed anymore after being freed, so this style is more appropriate for member data and global variables. Should one really set pointers to `NULL` after freeing them? Applying the indirection operator to a null pointer causes an implementation-defined behavior. When I had to write a stack for an university exercise some weeks ago, I had a problem, I investigated a few hours. Random pointers pointing to random memory is hard to debug. It is one of those fake truths that belong to the "sounds right" category but in reality achieve absolutely nothing useful (and sometimes leads to negative consequences). value. In this way , you can avoid 1) using a freed pointed 2)free it towice. In c++ you should not set the pointer to null, since the Object will still exists in memory, but without any reference to it-> memory leak. To complete the style, you should also initialize pointers to NULL before they get assigned a true pointer value. After using free(ptr), it's always advisable to nullify the pointer variable by declaring again to NULL. after the memory is released. In c++ you should not set the pointer to null, since the Object will still exists in memory, but without any reference to it-> memory leak. accessed anymore after being freed, so Dataset Association of Association vs Hierarchical data, why does adding one character to my mysql password lock me out. So as to avoid this it is safe to reset the pointer to NULL. I reached this conclusion: It is best practice, and one must follow this to make it portable on all (embedded) systems. If in case, you use the same pointer (ptr) and update the value at the address pointed by the same pointer (ptr), then the program will end up writing a value to the place where q is pointing (since p and q are pointing to the same address (0x1000)). earlier returned by the calloc , 3. Unlike reference types, pointer types are not tracked by the default garbage collection mechanism. really an OS guru but I am pretty even member data and global variables. 2. What all C operations does valgrind treat as 'malloc' and 'free'? nullptr in C++. One common mistake is to assume that a newly created pointer has a NULL value. Why does starship flip vertical at the last moment instead of earlier. That macro is a perfect place for the comma operator: free(, The macro shouldn't be in bare brackets, it should be in a. As you have a quality assurance team in place, let me add a minor point about QA. Then I do a ton of looping and use tons of classes. Should I be worried that I don't have ideas of questions to ask during seminars? A similar operation occurs when message_type equals value_2. 'test' being an array, it's a constant pointer. ctypes provides a cast() function which can be used in the same way. There is no scenario in which writing a double free is justified. Creating a string. The "module" that allocates a resource (memory, file, ...) is responsible for freeing it and has to provide a function for doing so that keeps care for the pointers, too. free() is a library function, which varies as one changes the platform, so you should not expect that after passing pointer to this function and after freeing memory, this pointer will be set to NULL. Raw, unsafe pointers, *const T, and *mut T. See also the std::ptr module.. Clearly in this case you have to set the pointer to NULL when you delete something to indicate that you don't own any data before. is undefined. Finally, if your code is specifically designed to rely on the pointer value being NULL or not NULL, it is perfectly fine to set the pointer value to NULL after free. pointers are used to store address of variable. Certainely not from a good C-book. Good answer. It makes it much easier to determine where things went wrong. Get C string equivalent. But in the context of the original question, keep in mind it would be hard to know how many invalid pointer problems were prevented, or at least caught so early as to not have made it into the bug database. If a dangling pointer is accessed after it is freed, you may read or overwrite random memory. There are ways to selectively suppress messages, so you can still satisfy both QA requirements, should your team decide so. If the value of the pointer is ever used in a true/false sense, it's not only a valid practice, it's a best practice. Especially if the compiler initializes local variables with zero in debug mode but doesn't once optimizations are turned on. Making statements based on opinion; back them up with references or personal experience. systems, telling you right away what ?= können in folgenden Szenarios nützlich sein:The ?? "obvious" that the pointer isn't We get the output as 0, which indicates that the pointer does not intend to point to any accessible memory location i.e. After that pCreatureN = … Die Operatoren ?? It's common practice to set newly created or newly freed pointers to NULL to make sure you don't use this unhelpful address by accident. Join Stack Overflow to learn, share knowledge, and build your career. That's also a main reason why all Basically you do something like that, and it end up pretty bad, the OS try to free some already freed memory and generally it segfault. How can my town be public knowledge while still keeping outsiders out? Asking for help, clarification, or responding to other answers. Be aware that a void pointer-to-pointer has its problems: @Chris, no, the best approach is code structure. Reviewing the bug database is good to do for a variety of reasons. why Grothendieck said that capacity to be alone and what is the actual meaning of this statement in term of researcher? However, the program is more likely to crash with a segfault. also to be noted that free(NULL) A small distinction but I think an important one. Allocating memory and freeing them. It can't be modified. To add to what other have said, one good method of pointer usage is to always check whether it is a valid pointer or not. Pointers are used to store the address of variable. won't do anything so you don't have to languages with garbage collection Not the reasoning about "double free" (which is completely bogus), but. What is the earliest mention of space travel? practice is to set to NULL, so you Setting a pointer to NULL after free is a dubious practice that is often popularized as a "good programming" rule on a patently false premise. What happens if a prosecutor refuses to file charges? Calling free() for a NULL pointer does nothing and is therefore guaranteed to be safe. However, in the case of a struct member or somewhere else where the pointer is not immediately going out of scope, it makes more sense. are they dynamically allocated and must they be destroyed? I am not really an OS guru but I am pretty even now most OSes would crash on double free. Recently I come across the same question after I was looking for the answer. It is then possible that someone would use the pointer, resulting in invalid program state. Appeal process for being designated a "Terrorist Group" (Canada). For both reasons, it can be a good idea to set the pointer to NULL after free(). On Windows, at least, debug builds will set the memory to 0xdddddddd so when you use a pointer to deleted memory you know right away. It's difficult to tell what is being asked here. Is it safe to sell them? There should be similar mechanisms on all platforms. Before you make a case with the quality department, try looking through the bug data base to see how often, if ever, invalid pointer values caused problems that had to be written down. Is it safe to sell them? I don't understand why you would "initialize pointers to NULL before they get assigned a true pointer value"? This point in particular is very useful in C. In C, we can use function pointers to avoid code redundancy. Settings a pointer to NULL is to protect agains so-called double-free - a situation when free() is called more than once for the same address without reallocating the block at that address. And this process tends to lead to pointers generally being a member of some class and generally you want a class to be in a valid state at all times, and the easiest way to do that is to set the member variable to NULL to indicate it points to nothing now. Consider a method that returns a pointer to a local variable through an in, out, or ref parameter or as the function result. It all depends on the use case. In C++, we can assign a pointer a null value by initializing or assigning it the literal 0: 1. Something like: Explicitly marking the pointer as NULL after freeing it allows for this kind of usage in C/C++. In this noncompliant code example, the type of a message is used to determine how to process the message itself. If you reach pointer that has been free()d, it might break or not. NULL means nothing at all when used with integers because 0 is a valid integer value. Mathematica cannot seem to properly handle times in the 1 hour before clocks go back. Unless the logic of your code actually explicitly handles that value for the pointer (i.e. How should I go about setting the pointer back to NULL after that monster dies? If you define another pointer variable (let's say, q) and dynamically allocate address to the new pointer, there is a chance of taking the same address (0x1000) by new pointer variable. int* pInt = NULL; chevron_right. (Like freeing too much. A pointer can be null. So the debug builds don't show any signs of the bug while release builds blow up randomly... Set the pointer that has just been freed to NULL is not mandatory but a good practice. It's way easier to debug. allocation. The pointers to these monster objects get passed through about 6 or 7 classes.. one of which I need to set to NULL when its HP hits 0. I'm working on a roguelike game and I have it set up so that at MOST there will be 3 monsters on a level. Definitely. A bad access may or may not crash, and corrupt data or behavior in unexpected ways in unexpected places. For any specific resource, you then have exactly one place where it is allocated and one place where it is released, both close together. 4. float *ptr { 0 }; // ptr is now a null pointer. rev 2021.2.5.38499. The Bar structure defined above accepts POINTER(c_int) pointers or c_int arrays for its values field, but not instances of other types: … for local variables, it may be a good If you write code that frees the same pointer twice, your program has a logical error in it. Some automated QA tools for C will flag assignments to freed pointers as "useless assignment to ptr". Thus a null-terminated string contains the characters that compris However, if you do not set the pointer to NULL, and by mistake try to de-reference the pointer, or change the value of that address; YOU CAN STILL DO IT. Another problem with NULL is that people sometimes mistakenly believe that it is different from 0 and/or not an integer. Pointers to functions C++ allows operations with pointers to functions. a pointer has its own memory address and size on the stack. they get assigned a true pointer Normally NULL is used to initialize pointers to a known address so that the pointer is never confused with some valid address. Even An object has always a value. To check for a null pointer you can use an if statement as follows − if(ptr) // succeeds if p is not null if(!ptr) // succeeds if p is null Thus, if all unused pointers are given the null value and you avoid the use of a null pointer, you can avoid the accidental misuse of an uninitialized pointer. filter_none. initialize pointers to NULL before What are the differences between a pointer variable and a reference variable in C++? But... be careful. Essentially I need the monster to be removed until that pointer is regenerated as another monster. Is it unethical to accidentally benefit from online material in a take-home exam? When you use NULL for initializing pointers then before using the pointer anywhere in the program, first check that it is non-zero or non-NULL as shown in the above code fragment. the space has been deallocated by a your coworkers to find and share information. A pointer holding a null value is called a null pointer. Many times, uninitialized … Typically, mentions of "pointers" without elaboration do not include pointers to (non-static) members. und ? Bug history provides better evidence for adding coding rules. set pointer to NULL after free the heap memory. So to summarise, yes i often set the pointer to NULL after deleting something, but it's as part of a larger design and thoughts on who owns the data rather than due to blindly following a coding standard rule. If you still have the original pointer around you might end up with a bug where you attempt to use the pointer after free and corrupt some other variable, and then your program enters an unknown state and all kinds of bad things can happen (crash if you're lucky, data corruption if you're unlucky). How can I determine if a variable is 'undefined' or 'null'? For example a simple qsort() function can be used to sort arrays in ascending order or descending or by any other order in case of array of structures. It's likely the rule is stated without making a distinction between these two cases, because it's much more difficult to automatically enforce the rule, let alone for the developers to follow it. And that's even before you deal with hardware that doesn't segfault on 0 access at all. It doesn't hurt to set pointers to NULL after every free, but it has the potential of pointing out big problems. Setting unused pointers to NULL is a @AnT "dubious" is a bit much. c and c++ languages do not have any way to tell us that an integer is uninitialized, or has no value at all. Instead you should delete the Object: delete pCreatureN; This will basically remove the object from memory and makes the pointer being a null pointer. In Ausdrücken mit den NULL-bedingten Operatoren „?.“ und „? But by convention, if a pointer contains the null (zero) value, it is assumed to point to nothing. It's obviously not necessary but then that's why it's in a best practices document. A proper way to deal with the problem in such cases is to review and rethink the structure of the code in order to avoid the situation when the same pointer is passed to free more than once. In pre-standard code, NULL was/is sometimes defined to something unsuitable and therefore had/has to be avoided. Is calling a character a "lunatic" or "crazy" ableist when it is in reference to their erratic behavior? The DEBUG_VERSION lets you profile frees in debugging code, but both are functionally the same. What's the rationale for null terminated strings? C# supports pointers in a limited extent. this style is more appropriate for In the UK, can a landlord/agent add new tenants to a joint tenancy agreement without the consent of the current tenants? That's also a main reason why all languages with garbage collection (Java, dotnet) was so proud of not having this problem and also not having to leave to developers the memory management as a whole. little bit pointless if it is Working with raw pointers in Rust is uncommon, typically limited to a few patterns. Can we free dynamically allocation memory using realloc? In any system, there is an unobtainable goal of making it easiest to the right thing, and the irreducible cost of inaccurate measurements. IF your goal is to eliminate the not good, then the ambiguities may stay with the good. This rule is useful when you're trying to avoid the following scenarios: 1) You have a really long function with complicated logic and memory management and you don't want to accidentally reuse the pointer to deleted memory later in the function. Or maybe I have the wrong idea? If you could access a pointer after freeing it through that same pointer, it's even more likely that you'd access a pointer after freeing the object it points to through some other pointer. On (at least some versions of) AIX, *(int *)0 == 0, and Solaris has optional compatibility with this AIX "feature. How should I prevent a player from instantly recognizing a magical impostor without making them feel cheated? How can I temporarily repair a lengthwise crack in an ABS drain pipe? write the if statement. Also, please tag this question with the appropriate language tag (e.g. When you delete a pointer, you should set it to NULL, right? generally it segfault. C. In C, two null pointers of any type are guaranteed to compare equal. Written by RageZ in a duplicate question. If the goal is do keep only good practices, then some ambiguous ones get tossed out with the actually not good. If you set the pointer to NULL, then if you access it, the program always crashes with a segfault. @katze_sonne Even a stopped clock is right twice a day. Because, when you free the address, you allow that address in the heap to be allocated to some other application. This code do: if (x) myfree(&x); else do_foo(); becomes if (x) { free(*(&x)); *(&x) = null; } void 0; else do_foo(); The else is an error. And people, being naturally risk averse, do irrational things like checking a pointer for NULL value before calling free with it… The most common bug in c is the double can make test and check if you really need to free this memory. You may argue that setting it to NULL may actually mask logic errors later on, or in the case where you assume it is valid, you still crash on NULL, so it doesn't matter. The macro version has a subtle bug if you use it after an if statement without brackets. They shouldn't be destroyed. I just need them to be blank and ready to point to another object. Setting the pointer to NULL directly after freeing the contents is a complete waste of time, provided the code meets all requirements, is fully debugged and will never be modified again. What's with the (void) 0? If message_type equals value_1, the message is processed accordingly. However - I won't set the variable to null if the pointer isn't supposed to be used again, but often the higher level design gives me a reason to set it to null anyway. What rules have been written for 5e D&D on Failing Forward? play_arrow. We know that a string is a sequence of characters which we save in an array. You might as well use that method to protect in the same pointer case too. float *ptr2; // ptr2 is uninitialized. If a hidden double free breaks your program, it's an error. Are the sticks of RAM in my desktop computer volatile? Avoid this mistake: … Otherwise, if the It depends on whether the pointers "own" the monsters, i.e. NULL pointer in C. A null pointer is a pointer which points nothing. c++ delete pointer issue, can still access data. @DrPizza: I've just found an argument why one should set it to. The idea behind this, is to stop accidental reuse of the freed pointer. No more ,,sometimes it works'', no more ,,crashes in unpredictible way''. your coworkers to find and share information. and ? Are the sticks of RAM in my desktop computer volatile? Why is SAT so important in theoretical computer science? Long story short: You do not want to accidentally (by mistake) access the address that you have freed. @Paul: In the specific case, the declaration could read. To complete the style, you should also link brightness_4 code. Although you free the memory, a later part of the program could allocate something new that happens to land in the space. The measurement problem is that whenever you attempt to divide good from less good, the more complex the case, the more likely you get an ambiguous measurement. ), There are few things in the world that give away the lack of professionalism on part of the C code author. Needless to say, setting a pointer to NULL after free achieves absolutely nothing to prevent the problem in such cases. you may read or overwrite random For local variables, it may be a