Hello @DYenEmbedded ,
We will need more context concerning your issue.
Header files are only used during compilation (or cross compilation in your case), and are so not needed on the target. It is so like an "helper" for the compilation to say that this function exist and can be linked after the compilation.
When you work with OpenSTLinux, and when you try to cross compile an application, all the headers are provided by the SDK, the tool that you need to compile your app.
If you compile another software that does not come from ST, the needed headers are so outside of the SDK and you have to manage it yourself through putting them inside your source folder / header folder, and indicate their path into Makefile options.
That was to remind the basics. In your case, select.h is well inside the SDK, so you should not have issue with using the header. Please be sure that you well sourced your environment as mentioned in this article.
Kind regards,
Erwan.
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