UWP Apps, the Windows Runtime, and the C Run-Time
Universal Windows Platform (UWP) apps are programs that run in the Windows Runtime that executes on Windows 8. The Windows Runtime is a trustworthy environment that controls the functions, variables, and resources that are available to a UWP app. However, by design, Windows Runtime restrictions prevent the use of most C Run-Time Library (CRT) features in UWP apps.
The Windows Runtime does not support the following CRT features:
Most CRT functions that are related to unsupported functionality.
For example, a UWP app cannot create a process by using the exec and spawn families of routines.
When a CRT function is not supported in a UWP app, that fact is noted in its reference article.
Most multibyte character and string functions.
However, both Unicode and ANSI text are supported.
The concept of a current working directory.
UWP apps and DLLs that are statically linked to the CRT and built by using the /MT or
That is, an app that uses a multithread, static version of the CRT.
An app that's built by using the /MDd compiler option.
That is, a debug, multithread, and DLL-specific version of the CRT. Such an app is not supported on the Windows Runtime.
For a complete list of CRT functions that are not available in a UWP app and suggestions for alternative functions, see CRT functions not supported in Universal Windows Platform apps.