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The latest version of this topic can be found at SAL Annotations.
If you examine the library header files, you may notice some unusual annotations, for example,
_Out_z_cap_(_Size). These are examples of the Microsoft source-code annotation language (SAL), which provides a set of annotations to describe how a function uses its parameters, for example, the assumptions it makes about them and the guarantees it makes on finishing. The header file <sal.h> defines the annotations.
For more information about using SAL annotations in Visual Studio, see Using SAL Annotations to Reduce C/C++ Code Defects.