In my company there is a coding rule that says, after freeing any memory, reset the variable to NULL. For example ...
void some_func ()
{
int *nPtr;
nPtr = malloc (100);
free (nPtr);
nPtr = NULL;
return;
}
I feel that, in cases like the code shown above, setting to NULL does not have any meaning. Or am I missing something?
If there is no meaning in such cases, I am going to take it up with the "quality team" to remove this coding rule. Please advice.
Setting unused pointers to NULL is a defensive style, protecting against dangling pointer bugs. If a dangling pointer is accessed after it is freed, you may read or overwrite random memory. If a null pointer is accessed, you get an immediate crash on most systems, telling you right away what the error is.
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. Even for local variables, it may be a good approach if the function continues after the memory is released.
To complete the style, you should also initialize pointers to NULL before they get assigned a true pointer value.
Most of the responses have focused on preventing a double free, but setting the pointer to NULL has another benefit. Once you free a pointer, that memory is available to be reallocated by another call to malloc. 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). If you had set the pointer to NULL after free, any attempt to read/write through that pointer later would result in a segfault, which is generally preferable to random memory corruption.
For both reasons, it can be a good idea to set the pointer to NULL after free(). It's not always necessary, though. For example, if the pointer variable goes out of scope immediately after free(), there's not much reason to set it to NULL.
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. 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).
Allegedly, setting a pointer to NULL after free is supposed to prevent the dreaded "double free" problem when the same pointer value is passed to free more than once. In reality though, in 9 cases out of 10 the real "double free" problem occurs when different pointer objects holding the same pointer value are used as arguments for free. Needless to say, setting a pointer to NULL after free achieves absolutely nothing to prevent the problem in such cases.
Of course, it is possible to run into "double free" problem when using the same pointer object as an argument to free. However, in reality situations like that normally indicate a problem with the general logical structure of the code, not a mere accidental "double free". 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 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. It simply won't work in general case, because the problem with the code structure will always find another way to manifest itself.
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. But as a general "good practice" rule (as in "always set your pointer to NULL after free") it is, once again, a well-known and pretty useless fake, often followed by some for purely religious, voodoo-like reasons.
This is considered good practice to avoid overwriting memory. In the above function, it is unnecessary, but oftentimes when it is done it can find application errors.
Try something like this instead:
#if DEBUG_VERSION
void myfree(void **ptr)
{
free(*ptr);
*ptr = NULL;
}
#else
#define myfree(p) do { void ** p_tmp = (p); free(*(p_tmp)); *(p_tmp) = NULL; } while (0)
#endif
The DEBUG_VERSION lets you profile frees in debugging code, but both are functionally the same.
Edit: Added do ... while as suggested below, thanks.
If you reach pointer that has been free()d, it might break or not. That memory might be reallocated to another part of your program and then you get memory corruption,
If you set the pointer to NULL, then if you access it, the program always crashes with a segfault. No more ,,sometimes it works'', no more ,,crashes in unpredictible way''. It's way easier to debug.
Setting the pointer to the free'd memory means that any attempt to access that memory through the pointer will immediately crash, instead of causing undefined behavior. It makes it much easier to determine where things went wrong.
I can see your argument: since nPtr is going out of scope right after nPtr = NULL, there doesn't seem to be a reason to set it to NULL. 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. It's not immediately apparent whether or not that pointer will be used again by code that shouldn't be using it.
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. It doesn't hurt to set pointers to NULL after every free, but it has the potential of pointing out big problems.
the most common bug in c is the double free. Basically you do something like that
free(foobar);
/* lot of code */
free(foobar);
and it end up pretty bad, the OS try to free some already freed memory and generally it segfault. So the good practice is to set to NULL, so you can make test and check if you really need to free this memory
if(foobar != null){
free(foobar);
}
also to be noted that free(NULL) won't do anything so you don't have to write the if statement. I am not really an OS guru but I am pretty even now most OSes would crash on double free.
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.
The idea behind this, is to stop accidental reuse of the freed pointer.
Recently I come across the same question after I was looking for the answer. I reached this conclusion:
It is best practice, and one must follow this to make it portable on all (embedded) systems.
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. This may not be the case for some library implemented for the platform.
so always go for
free(ptr);
ptr = NULL;
This (can) actually be important. Although you free the memory, a later part of the program could allocate something new that happens to land in the space. Your old pointer would now point to a valid chunk of memory. It is then possible that someone would use the pointer, resulting in invalid program state.
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. Random pointers pointing to random memory is hard to debug. It's obviously not necessary but then that's why it's in a best practices document.
From the ANSI C standard:
void free(void *ptr);
The free function causes the space
pointed to by ptr to be deallocated,
that is, made available for further
allocation. If ptr is a null pointer,
no action occurs. Otherwise, if the
argument does not match a pointer
earlier returned by the calloc ,
malloc , or realloc function, or if
the space has been deallocated by a
call to free or realloc , the behavior
is undefined.
"the undefined behavior" is almost always a program crash. So as to avoid this it is safe to reset the pointer to NULL. free() itself cannot do this as it is passed only a pointer, not a pointer to a pointer. You can also write a safer version of free() that NULLs the pointer:
void safe_free(void** ptr)
{
free(*ptr);
*ptr = NULL;
}
I find this to be little help as in my experience when people access a freed memory allocation it's almost always because they have another pointer to it somewhere. And then it conflicts with another personal coding standard which is "Avoid useless clutter", so I don't do it as I think it rarely helps and makes the code slightly less readable.
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. 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. A small distinction but I think an important one.
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). 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.
A common pattern is to set all the member pointers to NULL in the constructor and have the destructor call delete on any pointers to data that your design says that class owns. 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.
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. 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.
It is always advisable to declare a pointer variable with NULL such as,
int *ptr = NULL;
Let's say, ptr is pointing to 0x1000 memory address.
After using free(ptr), it's always advisable to nullify the pointer variable by declaring again to NULL.
e.g.:
free(ptr);
ptr = NULL;
If not re-declared to NULL, the pointer variable still keeps on pointing to the same address (0x1000), this pointer variable is called a dangling pointer.
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. 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)).
e.g.
*ptr = 20; //Points to 0x1000
free(ptr);
int *q = (int *)malloc(sizeof(int) * 2); //Points to 0x1000
*ptr = 30; //Since ptr and q are pointing to the same address, so the value of the address to which q is pointing would also change.
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.
2) The pointer is a member variable of a class that has fairly complex behavior and you don't want to accidentally reuse the pointer to deleted memory in other functions.
In your scenario, it doesn't make a whole lot of sense, but if the function were to get longer, it might matter.
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.
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. Focus instead on writing short functions and well designed classes.
This might be more an argument to initialize all pointers to NULL, but something like this can be a very sneaky bug:
void other_func() {
int *p; // forgot to initialize
// some unrelated mallocs and stuff
// ...
if (p) {
*p = 1; // hm...
}
}
void caller() {
some_func();
other_func();
}
p ends up in the same place on the stack as the former nPtr, so it might still contain a seemingly valid pointer. Assigning to *p might overwrite all kinds of unrelated things and lead to ugly bugs. Especially if the compiler initializes local variables with zero in debug mode but doesn't once optimizations are turned on. 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. In this way , you can avoid 1) using a freed pointed 2)free it towice
There are two reasons:
Avoid crashes when double-freeing
Written by RageZ in a duplicate question.
The most common bug in c is the double
free. Basically you do something like
that
free(foobar);
/* lot of code */
free(foobar);
and it end up pretty bad, the OS try
to free some already freed memory and
generally it segfault. So the good
practice is to set to NULL, so you
can make test and check if you really
need to free this memory
if(foobar != NULL){
free(foobar);
}
also to be noted that free(NULL)
won't do anything so you don't have to
write the if statement. I am not
really an OS guru but I am pretty even
now most OSes would crash on double
free.
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 developer the
memory management as a whole.
Avoid using already freed pointers
Written by Martin v. Löwis in a another answer.
Setting unused pointers to NULL is a
defensive style, protecting against
dangling pointer bugs. If a dangling
pointer is accessed after it is freed,
you may read or overwrite random
memory. If a null pointer is accessed,
you get an immediate crash on most
systems, telling you right away what
the error is.
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. Even
for local variables, it may be a good
approach if the function continues
after the memory is released.
To complete the style, you should also
initialize pointers to NULL before
they get assigned a true pointer
value.
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. Something like:
if(ptr)
ptr->CallSomeMethod();
Explicitly marking the pointer as NULL after freeing it allows for this kind of usage in C/C++.
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.
Double-free leads to undefined behaviour - usually heap corruption or immediately crashing the program. Calling free() for a NULL pointer does nothing and is therefore guaranteed to be safe.
So the best practice unless you now for sure that the pointer leaves scope immediately or very soon after free() is to set that pointer to NULL so that even if free() is called again it is now called for a NULL pointer and undefined behaviour is evaded.
The idea is that if you try to dereference the no-longer-valid pointer after freeing it, you want to fail hard (segfault) rather than silently and mysteriously.
But... be careful. Not all systems cause a segfault if you dereference NULL. On (at least some versions of) AIX, *(int *)0 == 0, and Solaris has optional compatibility with this AIX "feature."
To the original question:
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. 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.
In any system, there is an unobtainable goal of making it easiest to the right thing, and the irreducible cost of inaccurate measurements. In C we're offered a set of very sharp, very strong tools, which can create many things in the hands of a skilled worker, and inflict all sorts of metaphoric injuries when handled improperly. Some are hard to understand or use correctly. And people, being naturally risk averse, do irrational things like checking a pointer for NULL value before calling free with it…
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. If the goal is do keep only good practices, then some ambiguous ones get tossed out with the actually not good. IF your goal is to eliminate the not good, then the ambiguities may stay with the good. 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.
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. If you want to make real difference, identify the most common problem in your production code and propose three ways to prevent it
As you have a quality assurance team in place, let me add a minor point about QA. Some automated QA tools for C will flag assignments to freed pointers as "useless assignment to ptr". For example PC-lint/FlexeLint from Gimpel Software says
tst.c 8 Warning 438: Last value assigned to variable 'nPtr' (defined at line 5) not used
There are ways to selectively suppress messages, so you can still satisfy both QA requirements, should your team decide so.
Long story short: You do not want to accidentally (by mistake) access the address that you have freed. Because, when you free the address, you allow that address in the heap to be allocated to some other application.
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. BUT NOT SOMETHING THAT YOU WOULD LOGICALLY WANT TO DO.
Why can I still access the memory location that I have freed? Because: You may have free the memory, but the pointer variable still had information about the heap memory address. So, as a defensive strategy, please set it to NULL.
I am reading a book that has the following code:
int list_ins_next(List *list, ListElmt *element, const void *data) {
ListElmt *new_element;
/* Allocate storage for the element.*/
if ((new_element = (ListElmt *)malloc(sizeof(ListElmt))) == NULL)
return -1;
/* Insert the element into the list.*/
new_element->data = (void *)data;
/* and so on */
}
As we see, the function gets const void *data as one of the arguments.
Why is it re-cast in the line:
new_element->data = (void *)data; ?
Additional information:
/* Define a structure for linked list elements. */
typedef struct ListElmt_ {
void *data;
struct ListElmt_ *next;
} ListElmt;
/* Define a structure for linked lists.*/
typedef struct List_ {
ListElmt *head;
ListElmt *tail;
} List;
Without the cast, and with sufficient compiler warning levels enabled, your compilation would fail with something like:
warning: assignment discards 'const' qualifier from pointer target type
What this means is that the following assignment is breaking some rules:
new_element->data = data; // <-- danger!
What's wrong?
Well, the message tells you that a 'const qualifier' is being discarded as a result of the assignment. Let's unpack what that implies.
The variable data has the type const void*, which means the memory it points at should never be modified as a result of following that pointer. In other words, the function list_ins_next is not allowed to do anything that will result in that memory being changed.
So now, the list node that the pointer is being stored in does not make such promises. It has a data member of type void* which means anyone following the pointer stored in the list is allowed to modify the memory it points to.
Some memory really is not allowed to be modified. String literals are an example. If you have the string "hello" in your program, that's actually stored in memory somewhere as part of the code. And you can have a pointer: const char* hello = "hello"; ... but you really should not drop the const and start messing with that data.
Do you see the problem? If this were allowed to pass, then the function list_ins_next might be unknowingly breaking the rules. Even though it's not allowed to modify memory pointed to by data, it is storing that pointer in a structure not bound by such restrictions. Later on, someone using that structure could happily go and use data as if it were non-const, without realizing it's supposed to be const.
Analogy time (naughty)
Imagine you went on holiday, and had someone look after your house. Before you left, you made them promise not to touch your top-shelf whisky. You return a week later, the whisky is gone. You ask them why they broke their promise, and they insist that they really didn't touch it, but they did invite a friend over and gave them full permission to drink it all.
Obviously, this is untenable behavior and something we want to avoid. Being in this situation could provoke a violent, irrational response that could damage friendships, property, and bodies. In computer land, we call that Undefined Behavior.
Anyway, let's put aside my anxieties about stewardship of expensive single malt, and return to talking about code.
Good behavior
The issue here is that the compiler wants to stop us from accidentally making mistakes, wherever it can. It rightly warns us: hey, this is bad, you might be using this wrong.
Now, back to why the cast is there. It is a little hard to say for sure without more context. What I will say is that the cast is making a promise:
Dear C compiler, by this cast I solemnly swear that I'm not going to break the rules, or maybe I might break the rules but I really really REALLY know what I'm doing and all of the ramifications, so please let's just pretend that this const pointer is not const.
Thanks <3 - A. C. Programmer.
This is the best case scenario. If the way that this list is used will really not result in modifications to the data, OR if the data didn't really need to be const (but for some reason the insert function made it const) then all is well. It's now the programmer's responsibility to keep their word.
Analogy time (nice)
You can actually trust your friend with your whisky. While you're away, they looked through the collection, but only to catalogue it. They had a friend around, and they spent time researching where it all came from and finding out information about distilleries. You return from holiday and it's all right where you left it, safe and secure.
Next to it there's a notebook containing detailed information about what you have and how much money you probably wasted on booze. There's also a note asking if you'll consider organizing a whisky tasting session one day.
Bad behavior
However, this sadly is not the most common reason for casting away const.
What we see very frequently is addition of a cast by someone who just wanted to get around a compiler error without fully understanding it. The compiler said "whoah buddy, hold on" and the programmer said "please shut up so I can run my code".
And, you know... I get the feeling that the author of this book might possibly fall into the latter category, but I won't outright claim this because I haven't seen the full context of the code example to comment on why it was written this way.
At face value, either the list should be storing const data or the insert function should be accepting non-const data. End of story.
Summary
We commonly say "casts hide bugs". Keep this in mind. When you cast, do it with intent. Do it because you know it's valid, not just because the language will let you do it. C will happily let you hold a nailgun to your foot and pull the trigger.
Anyways, I hope this lengthy explanation has helped somewhat, even if I can't peer inside the head of whoever wrote that code.
And that really touches on another topic -- if you cast away a const, you should probably at the very least somewhere have a comment in the code explaining why. Because one day, someone is gonna read it and ask why. And that person might even be the same person who wrote the code years prior.
I'm making a data structures and algorithms library in C for learning purposes (so this doesn't necessarily have to be bullet-proof), and I'm wondering how void functions should handle errors on preconditions. If I have a function for destroying a list as follows:
void List_destroy(List* list) {
/*
...
free()'ing pointers in the list. Nothing to return.
...
*/
}
Which has a precondition that list != NULL, otherwise the function will blow up in the caller's face with a segfault.
So as far as I can tell I have a few options: one, I throw in an assert() statement to check the precondition, but that means the function would still blow up in the caller's face (which, as far as I have been told, is a big no-no when it comes to libraries), but at least I could provide an error message; or two, I check the precondition, and if it fails I jump to an error block and just return;, silently chugging along, but then the caller doesn't know the List* was NULL.
Neither of these options seem particularly appealing. Moreover, implementing a return value for a simple destroy() function seems like it should be unnecessary.
EDIT: Thank you everyone. I settled on implementing (in all my basic list functions, actually) consistent behavior for NULL List* pointers being passed to the functions. All the functions jump to an error block and exit(1) as well as report an error message to stderr along the lines of "Cannot destroy NULL list." (or push, or pop, or whatever). I reasoned that there's really no sensible reason why a caller should be passing NULL List* pointers anyway, and if they didn't know they were then by all means I should probably let them know.
Destructors (in the abstract sense, not the C++ sense) should indeed never fail, no matter what. Consistent with this, free is specified to return without doing anything if passed a null pointer. Therefore, I would consider it reasonable for your List_destroy to do the same.
However, a prompt crash would also be reasonable, because in general the expectation is that C library functions crash when handed invalid pointers. If you take this option, you should crash by going ahead and dereferencing the pointer and letting the kernel fire a SIGSEGV, not by assert, because assert has a different crash signature.
Absolutely do not change the function signature so that it can potentially return a failure code. That is the mistake made by the authors of close() for which we are still paying 40 years later.
Generally, you have several options if a constraint of one of your functions is violated:
Do nothing, successfully
Return some value indicating failure (or set something pointed-to by an argument to some error code)
Crash randomly (i.e. introduce undefined behaviour)
Crash reliably (i.e. use assert or call abort or exit or the like)
Where (but this is my personal opinion) this is a good rule of thumb:
the first option is the right choice if you think it's OK to not obey the constraints (i.e. they aren't real constraints), a good example for this is free.
the second option is the right choice, if the caller can't know in advance if the call will succeed; a good example is fopen.
the third and fourth option are a good choice if the former two don't apply. A good example is memcpy. I prefer the use of assert (one of the fourth options) because it enables both: Crashing reliably if someone is unwilling to read your documentation and introduce undefined behaviour for people who do read it (they will prevent that by obeying your constraints), depending on whether they compile with NDEBUG defined or not. Dereferencing a pointer argument can serve as an assert, because it will make your program crash (which is the right thing, people not reading your documentation should crash as early as possible) if these people pass an invalid pointer.
So, in your case, I would make it similar to free and would succeed without doing anything.
HTH
If you wish not to return any value from function, then it is good idea to have one more argument for errCode.
void List_destroy(List* list, int* ErrCode) {
*ErrCode = ...
}
Edit:
Changed & to * as question is tagged for C.
I would say that simply returning in case the list is NULL would make sense at this would indicate that list is empty(not an error condition). If list is an invalid pointer, you cant detect that and let kernel handle it for you by giving a seg fault and let programmer fix it.