Run time (program lifecycle phase)
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In computer science, run time, run-time, runtime, or execution time is the time during which a program is running (executing), in contrast to other phases of a program's lifecycle such as compile time, link time, load time, etc.
A run-time error is detected after or during the execution of a program, whereas a compile-time error is detected by the compiler before the program is ever executed. Type checking, storage allocation, code generation, and code optimization are typically done at compile time, but may be done at run time depending on the particular language and compiler.
Implementation details 
In certain cases, the execution of a program begins after a loader performs the necessary memory setup and links the program with any dynamically linked libraries it needs. In some cases a language or implementation will have these tasks done by the language runtime instead, though this is unusual in mainstream languages on common consumer operating systems.
Some program debugging can only be performed (or is more efficient or accurate) when performed at runtime. Logic errors and array bounds checking are examples. For this reason, some programming bugs are not discovered until the program is tested in a "live" environment with real data, despite sophisticated compile-time checking and pre-release testing. In this case, the end user may encounter a runtime error message.
Application errors — exceptions 
Exception handling is one language feature designed to handle runtime errors, providing a structured way to catch completely unexpected situations as well as predictable errors or unusual results without the amount of inline error checking required of languages without it. More recent advancements in runtime engines enable automated exception handling which provides 'root-cause' debug information for every exception of interest and is implemented independent of the source code, by attaching a special software product to the runtime engine.
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See also 
- Name binding
- Compile time and compiling
- Interpreter (computing)
- Runtime Type Information
- Run-time system
- Runtime library