How to pass target specific flags when cross compiling?

[abique]

Hi,

I wonder how to pass target specific flags when cross compiling?
Some programs have to be compiled for the machine currently building (host), like tablegen.exe in llvm, and some programs have to be built for the target like ffmpeg.exe.

How to specify different CFLAGS for the host and for the target?

Thanks!

[Osyotr]

Just set different values for VCPKG_C_FLAGS in host and target triplet.
https://learn.microsoft.com/en-us/vcpkg/users/triplets#vcpkg_cxx_flags

[abique]

This isn't my question.
When cross compiling, you have sometime to build intermediary programs that you have to execute during the build process. And you can't have ARM specific flags there if you're building on an intel machine while you still want to be able to have ARM specific flags for the target libraries.

[Osyotr]

When cross compiling, you have sometime to build intermediary programs that you have to execute during the build process.

These are host dependencies and should be built with the host triplet.

[abique]

So you're saying that every ports that requires host tools during the build process needs to be built for the host as well?

It might be a tough job for libx11 or even llvm 🤔

[Osyotr]

llvm probably doesn't need host tools, but libx11 does:

vcpkg/ports/libx11/vcpkg.json

Lines 10 to 13 in 6ab3314

	{
	"name": "libx11",
	"host": true
	},

[dg0yt]

It might be a tough job for libx11 or even llvm 🤔

It is a tough job, and it causes quite some breakage and limitations (qt5-base, icu, luajit, ...).

[dg0yt]

Target specific flags are actually triplet specific flags.
Ports are only allowed to build for the target triplet.
Host binaries must be built in the host triplet, and then be used from the target triplet via a dependency.
This usually requires patching.