What is Undefined Behaviour in C? [duplicate]

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The definition of undefined behavior:

C11(ISO/IEC 9899:201x) §3.4.3

1 undefined behavior

behavior, upon use of a non portable or erroneous program construct or of erroneous data, for which this International Standard imposes no requirements

2 NOTE Possible undefined behavior ranges from ignoring the situation completely with unpredictable results, to behaving during translation or program execution in a documented manner characteristic of the environment (with or without the issuance of a diagnostic message), to terminating a translation or execution (with the issuance of a diagnostic message).

3 EXAMPLE An example of undefined behavior is the behavior on integer overflow.

There is also a list of undefined behaviors in C11 §J.2 Undefined behavior

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