Undefined, Unspecified and Implementation-Defined Behaviour
The C standard does not pin down the result of every program. It leaves some choices to the compiler on purpose, so that C can run efficiently on very different machines.
Programming in C · topic
Defined, undefined, unspecified and implementation-defined behaviour
How to tell standard behaviour from implementation-defined, unspecified and undefined behaviour, sequencing and side effects, short-circuit evaluation, constraint violations that fail to compile, and how to answer GATE options when a program has no defined output.
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Defined, undefined, unspecified and implementation-defined behaviour cheat sheets
One-page visual sheets for this topic, in the sheet library on the store. Previews are free; the full sheets come with the subject pack.