I mean to ask if it follows some specific algorithm and actually are not junk.
In other words, how exactly the "garbage" values be present? Considering not invoking UB, if a garbage value is read, what is the source of that value?
The standard does not mention the term "garbage", it mentions "indeterministic / indeterminate values". The value can be anything.Note
From the user point of view, if we are unable to get a fix on a certain value (for any variable), then the "expectation" is not matched anytime and the value (if) we get is not of any use, thus terming them as "garbage" is common.
The most relatable and common observation / implementation is, for an automatic variable left uninitialized, only the storage is allocated, the content of that storage is not touched. So, probably it still contains the last stored value which was put there. Now, that value, probably being a valid one in other (previous) scenario, in present case, does not make any sense, so it is "garbage" in current scenario.
TL;DR The "garbage" value is not generated, most of the cases, it's just the last stored value in that memory location.
Note:
Related quoting from C11, chapter §6.7.9
If an object that has automatic storage duration is not initialized explicitly, its value is
indeterminate. [....]
§§ Additional Read:
This is very closely related to the topic, so adding it as a footnote.
In case, there exist a variable, which holds indeterministic value and
the data type can have trap representation
the address of the variable is not taken
then, trying to read the value actually causes undefined behavior. Be aware.
When C allocates memory on the stack and heap, it does not modify what's stored at the memory addresses, it simply designates the space for your variable. An un-initialized variable will contain whatever was in memory at that location before you declared the variable. Some times the values will be previous memory from your program that was used and cleared, and some times they will be values from the OS, stack canaries, etc. There is no way to predict what will be there.
The garbage values exist for variables on the stack (also known as automatic variable) if not initialized. They are pushed, popped, initialized with previous variable residing on the address in the stack, previous function call, etc.
Every specific memory addresses (either stack or heap) will be having some data. Before assigning that memory to your variable, it might be used by another variable (may be by OS or other programs). So, it might contain last values assigned by those last allocated variables. And those values are now useless to us. Thats why they are garbage for us
Related
I heard several times that if you do not initialise a variable then garbage value is stored in it.
Say
int i;
printf("%d",i);
The above code prints any garbage value, but I want to know that what is the need for storing garbage value if uninitialized?
The value of an uninitialized value is not simply unknown or garbage, but indeterminate and evaluating that variable may invoke undefined behavior or implementation-defined behavior.
One possible scenario (which is probably the scenario you are seeing) is that the variable, when evaluated, will return the value that was previously present in that memory address. Therefore, it's not like garbage is explicitly written to that variable.
It's worth noting that languages (or even C implementations) that do not exhibit the behavior you're seeing, do so by explicitly writing zeroes (or other initial values) to that area, before allowing you to use it.
It is not storing garbage, it prints whatever happens to be there in memory at that address when it is running. This is in the name of efficiency. You don't pay for what you didn't ask for.
EDIT
To answer why there is something in memory. All sort of program runs and need to share memory. When memory is allocated to your process, it is not reset, again for performance reason. Since the variable we are observing is declared on the stack, it could even be your program that put the value there in a previous function call.
C only does what you tell it to. The standard defines reading an uninitialized variable as undefined behavior.
This question elaborates: (Why) is using an uninitialized variable undefined behavior?
The accepted answer has a very good explanation.
EDIT:
A funny sidenote though, if you declare the variable static it is guaranteed to be initialized to zero per the standard. Can't find a quote right now, working on it..
EDIT2:
I left my C reference at work and CBA to download one. This answer elaborates on the initial values of variables, whether they be local/auto, global, static or indeterminate: https://stackoverflow.com/a/1597491/700170
The other answers point out (correctly) that what's being printed is whatever's already in memory in the memory location that happens to have been assigned to i.
They don't, however, clarify why there are any values stored in these locations in the first place, which is perhaps what you're really asking.
There are two reasons for this: first, upon startup, we can't be sure exactly how the memory circuits will initialize themselves. So they could be set to any arbitrary value. The second (and, in general, more likely reason, unless you just restarted your computer) is that before you started your program, that memory location had been used by another program, which stored something there--something that wasn't garbage at the time, since it was stored intentionally. From the perspective of your program, however, it is garbage, since your program has no way of knowing why that particular value was stored there.
EDIT: As I mentioned in a comment on another answer, even if the value stored in memory under some uninitialized variable is actually 0, that's not the same thing as "not having a value stored." The value stored is 0, which is to say, the physical hardware that represents one bit of memory is faithfully storing the value 0. As long as a circuit is active (i.e. turned on), the memory cells must store something; for an explanation of why this is, look into flip-flop gates. (There's a decent overview here, assuming you already understand a little bit about NAND gates: http://computer.howstuffworks.com/boolean4.htm)
It's happens only in case of local varibales. As memory for local variables are allocated on stack and while allocating the memory the runtime system does not clear the memory before allocating it to the variable unlike in case of allocating memory in heap for global and static variables. Hence the default value of local varibles beomes the content of its memory on stack while that of constant and static variables is 0.
I know that if a pointer is declared in C (and not initialized), it will be pointing to a "random" memory address which could contain anything.
How is where it actually points to determined though? Presumably it's not truly random, since this would be inefficient and illogical.
If this pointer is defined outside of all functions (or is static), it will be initialized to NULL before main() gets control.
If this pointer is created in the heap via malloc(sizeof(sometype*)), it will contain whatever happens to be at its location. It can be the data from the previously allocated and freed buffer of memory. Or it can be some old control information that malloc() and free() use to manage the lists of the free and allocated blocks. Or it can be garbage if the OS (if any) does not clear program's memory or if its system calls for memory allocation return uninitialized memory and so those can contain code/data from previously run programs or just some garbage that your RAM chips had when the system was powered on.
If this pointer is local to a function (and is not static), it will contain whatever has been at its place on the stack. Or if a CPU register is allocated to this pointer instead of a memory cell, it will contain whatever value the preceding instructions have left in this register.
So, it won't be totally random, but you rarely have full control here.
Uninitialized is undefined. Generally speaking, when the pointer is allocated the memory space is not cleared, so whatever the memory contained is now a pointer. It is random, but it is also efficient in the sense that the memory location is not being changed in the operation.
http://en.wikipedia.org/wiki/Uninitialized_variable
Languages such as C use stack space for variables, and the collection
of variables allocated for a subroutine is known as a stack frame.
While the computer will set aside the appropriate amount of space for
the stack frame, it usually does so simply by adjusting the value of
the stack pointer, and does not set the memory itself to any new state
(typically out of efficiency concerns). Therefore, whatever contents
of that memory at the time will appear as initial values of the
variables which occupy those addresses.
Although I would imagine this is implementation specific.
Furthermore you should probably always initialize your pointers, see the answer provided at How do you check for an invalid pointer? and the link given on the first answer: -
http://www.lysator.liu.se/c/c-faq/c-1.html
As far as the C standard is concerned, an uninitialized pointer doesn't point anywhere. It is illegal to dereference it. Thus it is impossible in principle to observe its target, and thus the target simply doesn't exist for all intents and purposes.
If you want a trite analogy, asking for the value of an uninitialized pointer is like asking for the value of the smallest positive real number, or the value of the last digit of π.
(The corolloary is that only Chuck Norris can dereference an uninitialized pointer.)
It is implementation specific / undefined behavior. It's possible the pointer was automatically initialized to NULL... or it is just whatever value is in memory at the time.
A pointer is an address in memory that points to another address in memory. The fact that it's not initialized does not mean that the pointer itself doesn't have an address, it only means that the address of the thing it points to is not known. So, either the compiler initialized it to NULL by default, or the address is whatever was in memory in the pointer's variable space at the time.
In large part, it is like an unerased whiteboard when you enter class. If you draw a box on part of the board but do not erase what is in the box, then what is in the box?
It is whatever was left there previously.
Similarly, if you allocate space for a pointer but do not erase the space, then what is in the space?
Data may be left over from earlier parts of your program or from special code that runs before any normal part of your program (such as the main function) starts running. (The special code may load and link certain libraries, set up a stack, and otherwise prepare the environment needed by a C program.) On embedded systems, rather than typical multi-user systems, data might be left over from previous processes. Quite likely, the previous use of the space was for something other than a pointer, so the value in it does not make sense when interpreted as a pointer. Its value as a pointer might point somewhere but not be meaningful in any way.
However, you cannot rely on this. The C standard does not define the behavior when you use an uninitialized object with automatic storage duration. In many C implementations, an uninitialized pointer will simply contain left over data. But, in some C implementations, the system may detect that you are using an uninitialized object and cause your program to crash. (Other behaviors are possible too.)
I thought that once a function returns, all the local variables declared within (barring those with static keyword) are garbage collected. But when I am trying out the following code, it still prints the value after the function has returned. Can anybody explain why?
int *fun();
main() {
int *p;
p = fun();
printf("%d",*p); //shouldn't print 5, for the variable no longer exists at this address
}
int *fun() {
int q;
q = 5;
return(&q);
}
There's no garbage collection in C. Once the scope of a variable cease to exist, accessing it in any means is illegal. What you see is UB(Undefined behaviour).
It's undefined behavior, anything can happen, including appearing to work. The memory probably wasn't overwritten yet, but that doesn't mean you have the right to access it. Yet you did! I hope you're happy! :)
If you really want it to loose the value, perhaps call another function with at least a few lines of code in it, before doing the printf by accessing the location. Most probably your value would be over written by then.
But again as mentioned already this is undefined behavior. You can never predict when (or if at all) it crashes or changes. But you cannot rely upon it 'changing or remaining the same' and code an application with any of these assumptions.
What i am trying to illustrate is, when you make another function call after returning from previous one, another activation record is pushed on to the stack, most likely over writing the previous one including the variable whose value you were accessing via pointer.
No body is actually garbage collecting or doing a say memset 0 once a function and it's data goes out of scope.
C doesn't support garbage collection as supported by Java. Read more about garbage collection here
Logically, q ceases to exist when fun exits.
Physically (for suitably loose definitions of "physical"), the story is a bit more complicated, and depends on the underlying platform. C does not do garbage collection (not that garbage collection applies in this case). That memory cell (virtual or physical) that q occupied still exists and contains whatever value was last written to it. Depending on the architecture / operating system / whatever, that cell may still be accessible by your program, but that's not guaranteed:
6.2.4 Storage durations of objects
2 The lifetime of an object is the portion of program execution during which storage is
guaranteed to be reserved for it. An object exists, has a constant address,33)
and retains
its last-stored value throughout its lifetime.34)
If an object is referred to outside of its
lifetime, the behavior is undefined. The value of a pointer becomes indeterminate when
the object it points to (or just past) reaches the end of its lifetime.
33) The term ‘‘constant address’’ means that two pointers to the object constructed at possibly different
times will compare equal. The address may be different during two different executions of the same
program.
34) In the case of a volatile object, the last store need not be explicit in the program.
"Undefined behavior" is the C language's way of dealing with problems by not dealing with them. Basically, the implementation is free to handle the situation any way it chooses to, up to ignoring the problem completely and letting the underlying OS kill the program for doing something naughty.
In your specific case, accessing that memory cell after fun had exited didn't break anything, and it had not yet been overwritten. That behavior is not guaranteed to be repeatable.
This question already has answers here:
Closed 11 years ago.
Possible Duplicate:
How an uninitialised variable gets a garbage value?
So when an undefined (but declared) variable is used, it contains strange value each time. How does it have such value? Is it randomly generated purposely?
It is not randomly generated, it is just residual memory.
It would be highly inefficient to clear all unused memory every time. So the memory is released to the OS and made available. When you request new memory, you get some of this memory, which is not owned by anyone but still has garbage in it since it was just freed, but not cleared.
When a local variable is declared, the compiler allocates a slot within the enclosing function's stack frame in which the variable will live. Whatever value was in that particular spot in memory before that stack frame was setup (usually from a previous function call who's stack frame occupied that space) becomes the initial contents of that variable.
In some cases, uninitialised variables are in fact deliberately set to some value, but it's rarely random. For instance, a debug malloc() might set every word of a newly allocated block to 0xbadf00d, to serve as a marker that the memory hasn't been allocated. Thus, struct members might be initialised to something other than whatever was there before. I don't know of any compilers that do this for stack variables, but they might exist.
C is not cleaning the memory it allocates. These values are the 'leftover' in the allocated location in memory.
C as a principle is not hiding operations from the programmer. It does only what asked to.Since you did not ask to initialize the variable, it doesn't do it for you, so the same bits that were 'up'/'down' in the destination memory - do not change.
Variable can have undefined values to avoid overhead of initializing with some sensible values, so it is definitely not generated randomly (which is itself nontrivial operation). The value variable initially holds is just what happens to be in variable's memory location at that time.
The value of a unitialized value in c depends of whatever the value stored at the memory address was. It isn't randomized on purpose.
Whenever you declare a variable, it will already have a memory space to hold it's value. If you don't set anything explicitly, it will contain whatever value that was previously stored on that location. So it's not randomly generated purposely by the program, just the value that happens to be there.
Declaration of a variable is a indication to the compiler saying that there is one such variable of so and so type. Definition of a variable allocates memory for that. The allocated memory could be anywhere from stack (auto variables), heap (dynamically allocated memory), etc. Unless it is a static variable, it will be allocated memory from an un-initialized data segment. So the random values that you are seeing is nothing but the values that are stored in that memory locations earlier! So it is advised to initialize the variables before using them (for the first time) or in other words, do not use/de-reference un-initialized variables/pointers.
More information on a structure of a program in memory can be found here.
Hope it helps!
When we create a variable and don't initialize it, then some (random) number called garbage value is assigned to it.
How this value is assigned to the variable?
What is whole concept/mechanism behind this?
Does this happen only in C?
The garbage value is not assigned, rather the value is already there. When you allocate a variable you are reserving a piece of memory - until you overwrite it that memory will contain whatever "random" information was there before.
As a metaphor, think of allocating a variable like buying a piece of land - until you do something with it (like build a house) the land will just have whatever trash was already sitting there (like an old crumbling house).
Some languages will automatically fill newly allocated variables with zeros - this takes time to do. In more "do-it-yourself" languages like C this extra behavoir is not guarenteed (though on some systems memory is cleared regardless of language, for example as a security measure)
Memory is used and reused at various points in your application. For example, as the call stack of your application grows and shrinks the same location in memory may be overwritten many, many times. The thing to remember is that as a piece of memory is abandoned it is not zeroed out, so if you do not specify a new initial value for that place in memory when you use it again you will get the old, "garbage" value.
Some languages and structure implementations do default-initialize memory as it is used. Others do not, so it is important to read the documentation of your language carefully to know what to expect.
Nobody explicitly assigns a grabage value. If you create a variable, only location of the variable is determined, and not its value. Thats why we are initializing it. The garbage value might come from some previous operations over the same memory by old processes! So it can hold anything. I think it holds to pretty good number of languages. I am not sure about the list! :)
When we create a variable and don't initialize it, then nothing happens. When you read value from that variable you get data from memory where variable located now. It could looks like garbage/random value only because variables are placed in memory with some degree of randomness.
C standards say:
undefined behavior for local variables: (Why) is using an uninitialized variable undefined behavior? (e.g. segfault is legal)
zero for global variables: What happens to a declared, uninitialized variable in C? Does it have a value?
Implementation: detailed examination of an implementation at: https://stackoverflow.com/a/36725211/895245 Summary:
local: the address is never written to, so whatever was there previously gets used
global: .bss