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host-dependencies.md

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Host Dependencies

Tools used at build time by other ports to generate code or implement a custom build system can be packaged inside vcpkg.

Consuming

When consuming a port as a tool, you must set the dependency's "host" field to true. For example:

{
    "name": "contoso-http-library",
    "version-string": "1.0.0",
    "description": "Contoso's http runtime library",
    "dependencies": [
        "contoso-core-library",
        {
            "name": "contoso-code-generator",
            "host": true
        },
        {
            "name": "contoso-build-system",
            "host": true
        }
    ]
}

In this case, the contoso-code-generator and contoso-build-system (including any transitive dependencies) will be built and installed for the host triplet before contoso-http-library is built.

Note: Consumers must use vcpkg.json instead of CONTROL as their metadata format. You can easily convert an existing CONTROL file using vcpkg format-manifest /path/to/CONTROL.

Then, within the portfile of the consumer (contoso-http-library in the example), the CMake variable CURRENT_HOST_INSTALLED_DIR will be defined to installed/<host-triplet> and should be used to locate any required assets. In the example, contoso-code-generator might have installed tools/contoso-code-generator/ccg.exe which the consumer would add to its local path via

# ports/contoso-http-library/portfile.cmake
vcpkg_add_to_path(${CURRENT_HOST_INSTALLED_DIR}/tools/contoso-code-generator)

Specifying the Host Triplet

The default host triplets are chosen based on the host architecture and operating system, for example x64-windows, x64-linux, or x64-osx. They can be overridden via:

  1. In CMake-based manifest mode, calling set(VCPKG_HOST_TRIPLET "<triplet>" CACHE STRING "") before the first project() directive
  2. In MSBuild-based manifest mode, setting the VcpkgHostTriplet property
  3. On the command line, via the flag --host-triplet=...
  4. The VCPKG_DEFAULT_HOST_TRIPLET environment variable

Producing

Producing a tool has no special requirements; tools should be authored as a standard port, following all the normal policies and practices. Notably, they should build against TARGET_TRIPLET, not HOST_TRIPLET within the context of their portfile.

Sometimes, it can be useful to determine whether the current context is a cross-compiling one or not. This should be done by comparing the strings TARGET_TRIPLET and HOST_TRIPLET. For example:

string(COMPARE EQUAL "${TARGET_TRIPLET}" "${HOST_TRIPLET}" I_AM_NOT_CROSSCOMPILING)

if(TARGET_TRIPLET STREQUAL HOST_TRIPLET)
    # This is a native build
else()
    # This is a cross build
endif()

Host-only ports

Some ports are host-only: script ports and tool ports are common examples. In this case, you can use the "native" supports expression to describe this. This supports expression is true when TARGET_TRIPLET == HOST_TRIPLET.