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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 behavior of the code. If a program ...
An integer overflow can cause the value to wrap and become negative, which violates the program's assumption and may lead to unexpected behavior (for example, 8-bit integer addition of 127 + 1 results in −128, a two's complement of 128).
The problem is, he chose a sample of code that might be considered to be "here's how you can use C's efficiency (it doesn't try to detect overflow) and undefined behavior on a known local architecture (therefore defined behavior) to detect overflow if you insert the code appropriate for your semantics" which is illustrating the positive side of ...
C and C++ distinguish implementation-defined behavior from unspecified behavior. For implementation-defined behavior, the implementation must choose a particular behavior and document it. An example in C/C++ is the size of integer data types. The choice of behavior must be consistent with the documented behavior within a given execution of the ...
An example, suppose we add 127 and 127 using 8-bit registers. 127+127 is 254, but using 8-bit arithmetic the result would be 1111 1110 binary, which is the two's complement encoding of −2, a negative number. A negative sum of positive operands (or vice versa) is an overflow.
Another example can be when dealing with structs. In the code snippet below, we have a struct student which contains some variables describing the information about a student. The function register_student leaks memory contents because it fails to fully initialize the members of struct student new_student .
The types float and int32_t are not compatible, therefore this code's behavior is undefined. Although on GCC and LLVM this particular program compiles and runs as expected, more complicated examples may interact with assumptions made by strict aliasing and lead to unwanted behavior.
John Regehr is a computer scientist specializing in compiler correctness and undefined behavior.As of 2016, he is a professor at the University of Utah.He is best known for the integer overflow sanitizer which was merged into the Clang C compiler, [1] the C compiler fuzzer Csmith, [2] [3] and his widely read blog Embedded in Academia.