about virtual variable

[zhouyuanqing8 at outlook.com]

Hi everyone,

     I would like to ask, is virtual a variable defined by bitbake? I did not find it in the bitbake manual. What is the use of the virtual variable? What would be the problem without this variable?
2.4. Preferences¶<https://www.yoctoproject.org/docs/latest/bitbake-user-manual/bitbake-user-manual.html#bb-bitbake-preferences>

The PROVIDES list is only part of the solution for figuring out a target's recipes. Because targets might have multiple providers, BitBake needs to prioritize providers by determining provider preferences.

A common example in which a target has multiple providers is "virtual/kernel", which is on the PROVIDES list for each kernel recipe. Each machine often selects the best kernel provider by using a line similar to the following in the machine configuration file:

     PREFERRED_PROVIDER_virtual/kernel = "linux-yocto"

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