Undefined behavior

In computer programming, undefined behavior (UB) is the result of executing computer code the behaviour of which is not prescribed by the language specification to which the code adheres, for the current state of the program (e.g. memory). This happens when the translator of the source code makes certain assumptions, but these assumptions are not satisfied during execution.

The behavior of some programming languages - most famously C and C++ - is undefined in some cases.^[1] In the standards for these languages, the semantics of certain operations is undefined. An implementation is allowed to assume that such operations never occur in standard-conforming program code; the implementation will be considered correct whatever it does in such cases, analogous to don't-care terms in digital logic. This assumption can make various program transformations valid or simplify their proof of correctness, giving flexibility to the implementation. As a result, the compiler can often make more optimizations. It is the responsibility of the programmer to write code that never invokes undefined behavior, although compiler implementations are allowed to issue diagnostics when this happens.

Undefined behavior is often unpredictable and a frequent cause of software bugs. In the C community, undefined behavior may be humorously referred to as "nasal demons", after a comp.std.c post that explained undefined behavior as allowing the compiler to do anything it chooses, even "to make demons fly out of your nose".^[2] Under some circumstances there can be specific restrictions on undefined behavior. For example, the instruction set specifications of a CPU might leave the behavior of some forms of an instruction undefined, but if the CPU supports memory protection then the specification will probably include a blanket rule stating that no user-accessible instruction may cause a hole in the operating system's security; so an actual CPU would be permitted to corrupt user registers in response to such an instruction, but would not be allowed to, for example, switch into supervisor mode.

Benefits from undefined behavior

Documenting an operation as undefined behavior allows compilers to assume that this operation will never happen in a conforming program. This gives the compiler more information about the code and this information can lead to more optimization opportunities.

An example for the C language:

int foo(unsigned x)
{
     int value = 5;
     value += x;
     if (value < 5)
        bar();
     return value;
}

The value of x cannot be negative and, given that signed integer overflow is undefined behavior in C, the compiler can assume that at the line of the if check value >= 5. Thus the if and the call to the function bar can be ignored by the compiler since the if has no side effects and its condition will never be satisfied. The code above is therefore semantically equivalent to:

int foo(unsigned x)
{
     int value = 5;
     value += x;
     return value;
}

Had the compiler been forced to assume that signed integer overflow has wraparound behavior, then the transformation above would not have been legal.

Such optimizations become hard to spot by humans when the code is more complex and other optimizations, like inlining, take place.

Another benefit from allowing signed integer overflow to be undefined is that it makes it possible to store and manipulate a variable's value in a processor register that is larger than the size of the variable in the source code. For example, if the type of a variable as specified in the source code is narrower than the native register width (such as "int" on a 64-bit machine, a common scenario), then the compiler can safely use a signed 64-bit integer for the variable in the machine code it produces, without changing the defined behaviour of the code. If the behaviour of a 32-bit integer under overflow conditions was depended upon by the program, then a compiler would have to insert additional logic when compiling for a 64-bit machine, because the overflow behaviour of most machine code instructions depends on the register width.^[3]

Risks of undefined behavior

C and C++ standards are littered with undefined behavior throughout, which offer increased liberty in compiler implementations at the expense of limiting what users of the language can do. In particular, there is an entire appendix section dedicated to a non-exhaustive listing of common sources of undefined behavior in C.^[4] Moreoever, compilers are not required to diagnose code that relies on undefined behavior. Hence, it is common for programmers, even experienced ones, to unintentionally rely on undefined behavior either by mistake, or simply because they are not well-versed in the rules of the language that can span over hundreds of pages. This can result in bugs that occur when optimizations are enabled on the compiler, or when a compiler of a different vendor or version is used.

In scenarios where security is critical, undefined behavior can lead to security vulnerabilities in software. When GCC's developers changed their compiler in 2008 such that it omitted certain overflow checks that relied on undefined behavior, CERT issued a warning against the newer versions of the compiler.^[5] Linux Weekly News pointed out that the same behavior was observed in PathScale C, Microsoft Visual C++ 2005 and several other compilers;^[6] the warning was later amended to warn about various compilers.^[7]

HTML versions 4 and earlier left error handling undefined. Over time pages started relying on unspecified error-recovery implemented in popular browsers. This caused difficulties for vendors of less-popular browsers who were forced to reverse-engineer and implement bug compatible error recovery. This has led to de facto standard (i.e. HTML5) that was much more complicated than it could have been if this behavior was specified from the start.

Examples in C and C++

In C the use of any automatic variable before it has been initialized yields undefined behavior, as does integer division by zero or indexing an array outside of its defined bounds (see buffer overflow). In general, any instance of undefined behavior leaves the abstract execution machine in an unknown state, and any subsequent behavior is also undefined. If it is not required that the compiler diagnose undefined behavior, programs invoking undefined behavior may compile and run producing correct results, incorrect results, or have any other behavior. Because of this, undefined behavior can create errors that are difficult to detect.

Attempting to modify a string literal causes undefined behavior:^[8]

char *p = "wikipedia"; // valid C, ill-formed C++11, deprecated C++98/C++03
p[0] = 'W'; // undefined behavior

Integer division by zero results in undefined behavior:^[9]

int x = 1;
return x / 0; // undefined behavior

Certain pointer operations may result in undefined behavior:^[10]

int arr[4] = {0, 1, 2, 3};
int *p = arr + 5;  // undefined behavior

Reaching the end of a value-returning function (other than main()) without a return statement results in undefined behavior if the value of the function call is used by the caller:^[11]

int f()
{
}  /* undefined behavior if the value of the function call is used*/

Modifying an object between two sequence points more than once produces undefined behavior.^[12] It is worth mentioning that there are considerable changes in what causes undefined behavior in relation to sequence points as of C++11.^[13] The following example will however cause undefined behavior in both C++ and C.

i = i++ + 1; // undefined behavior

When modifying an object between two sequence points, reading the value of the object for any other purpose than determining the value to be stored is also undefined behavior.^[14]

a[i] = i++; // undefined behavior
printf("%d %d\n", ++n, power(2, n)); // also undefined behavior

See also

• Implementation-defined behavior
• Halt and Catch Fire

References

Further reading

• Peter van der Linden, Expert C Programming. ISBN 0-13-177429-8
• UB Canaries (April 2015), John Regehr (University of Utah, USA)

External links

• Corrected version of the C99 standard. Look at section 6.10.6 for #pragma