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In computer programming, a runtime library is a special program library used by a compiler to implement functions built into a programming language during the runtime (execution) of a computer program. This often includes functions for input and output, memory management or mathematical functions.

When the source code of a computer program is translated into the respective target language by a compiler, it would cause an extreme enlargement of program code if each command in the program and every call to a built-in function would cause the in-place generation of the complete respective program code in the target language every time. Instead the compiler often uses compiler-specific auxiliary functions that are mostly not accessible to application programmers and which are implemented in the runtime library. Depending on the compiler manufacturer, the runtime library will sometimes also contain the standard library of the respective compiler or be contained in it.

Also some functions that can be performed only (or are more efficient or accurate) at runtime are implemented in the runtime library, e.g. some logic errors, array bounds checking, dynamical type checking, exception handling and possibly debugging functionality. 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.

Usually the runtime library realizes many functions by accessing the operating system. Many programming languages have built-in functions that do not necessarily have to be realized in the compiler, but can be implemented in the runtime library. So the border between runtime library and standard library is up to the compiler manufacturer. Therefore a runtime library is always compiler-specific and platform-specific.

The concept of a runtime library should not be confused with an ordinary program library like created by an application programmer or delivered by a third party or a dynamic library, meaning a program library linked at run time. For example, the programming language C requires only a very minimal runtime library, but defines a large standard library (called C Standard Library) that each implementation has to deliver.