Undefined Behavior - Examples in C and C++

Examples in C and C++

Attempting to modify a string literal causes undefined behavior:

char * p = "wikipedia"; // ill-formed C++11, deprecated C++98/C++03 p = 'W'; // undefined behaviour

One way to prevent this is defining it as an array instead of a pointer.

char p = "wikipedia"; /* RIGHT */ p = 'W';

In C++ one can use STL string as follows.

std::string s = "wikipedia"; /* RIGHT */ s = 'W';

Division by zero results in undefined behavior; under IEEE 754, the division of float, double, and long double values by zero results in an infinity or NaN:

return x/0; // undefined behavior

Certain pointer operations may result in undefined behavior:

int arr = {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 may result in undefined behavior:

int f { } /* undefined behavior */

The C Programming Language cites the following examples of code that have undefined behavior in Section 2.12:

printf("%d %d\n", ++n, power(2, n)); /* WRONG */

and

a = i++;