C standards define UsB, UB and IDB in a way that can be summarized as follows:

Unspecified Behavior (UsB)

This is a behavior for which the standard gives some alternatives among which the implementation must choose, but it doesn’t mandate how and when the choice is to be made. In other words, the implementation must accept user code triggering that behavior without erroring out and must comply with one of the alternatives given by the standard.

Be aware that the implementation is not required to document anything about the choices made. These choices may also be non-deterministic or dependent (in an undocumented way) on compiler options.

To summarize: the standard gives some possibilities among which to choose, the implementation chooses when and how the specific alternative is selected and applied.

Note that the standard may provide a really large number of alternatives. The typical example is the initial value of local variables that are not explicitly initialized. The standard says that this value is unspecified as long as it is a valid value for the variable’s data type.

To be more specific consider an int variable: an implementation is free to choose any int value, and this choice can be completely random, non-deterministic or be at the mercy of the whims of the implementation, which is not required to document anything about it. As long as the implementation stays within the limits stated by the standard this is ok and the user cannot complain.

Undefined Behavior (UB)

As the naming indicates this is a situation in which the C standard doesn’t impose or guarantee what the program would or should do. All bets are off. Such a situation:

• renders a program either erroneous or nonportable

• doesn’t require absolutely anything from the implementation

This is a really nasty situation: as long as there is a piece of code that has undefined behavior, the entire program is considered erroneous and the implementation is allowed by the standard to do everything.

In other words, the presence of a cause of UB allows the implementation to completely ignore the standard, as long as the program triggering the UB is concerned.

Note that the actual behavior in this case may cover an unlimited range of possibilities, the following is by no means an exhaustive list:

• A compile-time error may be issued.
• A run-time error may be issued.
• The problem is completely ignored (and this may lead to program bugs).
• The compiler silently throws the UB-code away as an optimization.
• Your hard disk may be formatted.
• Your computer may erase your bank account and ask your girlfriend for a date.

I hope the last two (half-serious) items can give you the right gut-feeling about the nastiness of UB. And even though most implementations will not insert the necessary code to format you hard drive, real compilers do optimize!

Terminology Note: Sometimes people argue that some piece of code which the standard deems a source of UB in their implementation/system/environment work in a documented way, therefore it cannot be really UB. This reasoning is wrong, but it is a common (and somewhat understandable) misunderstanding: when the term UB (and also UsB and IDB) is used in a C context it is meant as a technical term whose precise meaning is defined by the standard(s). In particular the word “undefined” loses its everyday meaning. Therefore it doesn’t make sense to show examples where erroneous or nonportable programs produce “well-defined” behavior as counterexamples. If you try, you really miss the point. UB means that you lose all the guarantees of the standard. If your implementation provides an extension then your guarantees are only those of your implementation. If you use that extension your program is no more a conforming C program (in a sense, it is no more a C program, since it doesn’t follow the standard any longer!).

Usefulness of undefined behavior

A common question about UB is something on these lines: “If UB is so nasty, why does not the standard mandate that an implementation issues an error when faced with UB?”

First, optimizations. Allowing implementations not to check for possible causes of UB allows lots of optimizations that make a C program extremely efficient. This is one of the features of C, although it makes C a source of many pitfalls for beginners.

Second, the existence of UB in the standards allows a conforming implementation to provide extensions to C without being deemed non-conforming as a whole.

As long as an implementation behaves as mandated for a conforming program, it is itself conforming, although it may provide non-standard facilities that may be useful on specific platforms. Of course the programs using those facilities will be nonportable and will rely on documented UB, i.e. behavior that is UB according to the standard, but that an implementation documents as an extension.

Implementation-defined Behavior (IDB)

This is a behavior that can be described in a way similar to UsB: the standard provides some alternatives and the implementation choose one, but the implementation is required to document exactly how the choice is made.

This means that a user reading her compiler’s documentation must be given enough information to predict exactly what will happen in the specific case.

Note that an implementation that doesn’t fully document an IDB cannot be deemed conforming. A conforming implementation must document exactly what happens in any case that the standard declares IDB.

Examples of unspecified behavior

Order of evaluation

Function arguments

The order of evaluation for function arguments is unspecified EXP30-C.

For instance, in c(a(), b()); it is unspecified whether the function a is called before or after b. The only guarantee is that both are called before the c function.

Examples of undefined behavior

Pointers

Dereferencing of null pointer

Null pointers are used to signal that a pointer does not point to valid memory. As such, it does not make much sense to try to read or write to memory via a null pointer.

Technically, this is undefined behaviour. However, since this is a very common source of bugs, most C-environments ensure that most attempts to dereference a null pointer will immediately crash the program (usually killing it with a segmentation fault). This guard is not perfect due to the pointer arithmetic involved in references to arrays and/or structures, so even with modern tools, dereferencing a null pointer may format your hard drive.

Dereferencing of uninitialized pointer

Just like null pointers, dereferencing a pointer before explitely setting its value is UB. Unlike for null pointers, most environments do not provide any safety net against this sort of error, except that compiler can warn about it. If you compile your code anyway, you’ll are likely to experience the whole nastiness of UB.

Dereferencing of invalid pointers

An invalid pointer is a pointer that contains an address that is not within any allocated memory area. Common ways to create invalid pointers is to call free() (after the call, the pointer will be invalid, which is pretty much the point of calling free()), or to use pointer arithmetic to get an address that is beyond the limits of an allocated memory block.

This is the most evil variant of pointer dereferencing UB: There is no safety net, there is no compiler warning, there is just the fact that the code may do anything. And commonly, it does: Most malware attacks use this kind of UB behaviour in programs to make the programs behave as they want them to behave (like installing a trojan, keylogger, encrypting your hard drive etc.). The possibility of a formatted hard drive becomes very real with this kind of UB!

Casting away constness

If we declare an object as const, we give a promise to the compiler that we will never change the value of that object. In many contexts compilers will spot such an invalid modification and shout at us. But if we cast the constness away as in this snippet:

int const a = 42;
...
int* ap0 = &a;      //< error, compiler will tell us
int* ap1 = (int*)&a; //< silences the compiler
...
*ap1 = 43;          //< UB ==> program crash?

the compiler might not be able to track this invalid access, compile the code to an executable and only at run time the invalid access will be detected and lead to a program crash.

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Examples of implementation-defined behavior

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