System dependencies

System dependencies are external libraries that Cabin does not fetch, build, or install. Users provide them through their OS package manager (or any other local installation mechanism) and Cabin discovers their compile / link flags by querying pkg-config.

See docs/dependency-kinds.md for the dependency kinds and the declaration syntax; this page covers the build-time behavior of system-sourced (system = true) entries specifically.

Manifest declaration

A system-sourced dependency lives in one of the regular dependency tables ([dependencies], [dev-dependencies]) with system = true set:

[dependencies]
zlib    = { version = ">=1.2", system = true }
fmt     = { version = "^9.0",  system = true }

Every declared system = true dependency is required. There is no required field; declaring required on any dependency entry is rejected with a clear "unknown field" diagnostic.

Per-kind activation matches the Cabin-package rule for the table the entry lives in:

Table	Probed by `cabin build` / `run` / `metadata`	Probed by `cabin test`
`[dependencies]`	always	always
`[dev-dependencies]`	no	selected primaries

A test-only system dep (e.g. gtest) is declared under [dev-dependencies] with system = true, so an ordinary cabin build does not require it to be installed.

Conditional declarations work the same way as the other dependency kinds:

[target.'cfg(os = "linux")'.dependencies]
systemd = { version = ">=240", system = true }

A conditional system dependency is probed only when its condition matches the host platform.

What Cabin does with system-sourced entries

When the selected primary packages declare at least one active system dependency, Cabin invokes pkg-config (once per dependency) to:

verify the library is present (pkg-config --exists 'name op version'),
retrieve compile-time flags (pkg-config --cflags name),
retrieve link-time flags (pkg-config --libs name).

Discovered include directories from --cflags are added to the package's include-dir set. All other --cflags tokens are appended verbatim to the package's language-neutral compile argument bucket. All --libs tokens are appended verbatim to the package's ldflags list, preserving the order pkg-config emitted them so C / C++ link semantics are not disturbed.

Probe scope is intentionally primary-only. For a single-package project, the root package is primary; for a workspace, the members selected by the command are primary. Local path dependencies and registry / extracted dependencies may preserve system = true declarations in metadata, package archives, and index entries, but those dependency manifests do not trigger pkg-config probing for the downstream build.

Probing happens for every command that produces compile or link commands (cabin build, cabin run, cabin test, cabin tidy) and for cabin metadata and cabin explain build-config so the build-configuration fingerprint stays consistent across those commands. Other cabin explain subcommands (package, target, source, feature) do not invoke pkg-config because they never materialize a build configuration.

Version requirements

Cabin tries to convert each Cabin / npm-flavored SemVer requirement into a list of pkg-config-native operator pairs:

Cabin requirement	pkg-config form
`^1.2`	`>= 1.2`, `< 2.0.0`
`~1.2.3`	`>= 1.2.3`, `< 1.3.0`
`>=1.2 <2`	`>= 1.2`, `< 2`
`=1.0.0`	`= 1.0.0`

Requirements that do not parse as SemVer are forwarded to pkg-config verbatim provided they include an explicit operator. A bare non-SemVer token (e.g. vendor-special) produces a clear error rather than being silently dropped.

pkg-config itself does the version comparison.

Missing dependency semantics

A declared system = true dependency that fails to probe stops the build with an actionable diagnostic that names the package, the requirement Cabin attempted, and the installed version when available. Cabin never silently drops a missing system dependency from the build graph.

Executable resolution

Cabin uses the pkg-config executable from PATH by default. The CABIN_PKG_CONFIG environment variable overrides the default with a verbatim executable path or command name:

CABIN_PKG_CONFIG=/opt/pkgconf/bin/pkg-config cabin build

If a project declares no system = true entries, Cabin never spawns pkg-config and the executable is not required. When the project does declare system dependencies and the resolved executable is missing, Cabin produces a single diagnostic suggesting the install / override fix.

Pkg-config environment

Cabin does not sanitize the standard pkg-config environment. Variables PKG_CONFIG_PATH, PKG_CONFIG_LIBDIR, and PKG_CONFIG_SYSROOT_DIR are inherited by the child process, so users can point Cabin at custom .pc search paths the same way they would for any other pkg-config consumer.

Fingerprint and rebuild

Discovered compile / link flags participate in cabin_core::BuildConfiguration::fingerprint through the per-package ResolvedProfileFlags. When pkg-config reports different flags (for example because the user upgraded the system library or changed PKG_CONFIG_PATH), the build configuration fingerprint moves; Ninja then rebuilds from the changed command lines.

Output / diagnostics

Normal mode: Cabin prints no probe-specific output on successful runs.

Verbose mode (cabin -v ...): Cabin prints one line per probed dependency naming the resolved version, and one summary line per workspace identifying the pkg-config executable that was used.

Very-verbose mode (cabin -vv ...): Cabin also prints the per-dependency probe request (name, version requirement) before the probe runs.

All probe-related output is written to stderr so that machine-readable stdout (cabin metadata, cabin explain --format json) stays a parseable document.

Quiet mode (cabin -q ...): probe status output is suppressed. Errors are not.

Diagnostics

Code	When
`cabin::system_deps::executable_not_found`	Cabin tried to spawn pkg-config but the binary was missing.
`cabin::system_deps::package_not_found`	`pkg-config --exists` reported the package as absent.
`cabin::system_deps::version_mismatch`	The installed version does not satisfy the manifest requirement.
`cabin::system_deps::invalid_version_requirement`	The requirement string could not be interpreted.
`cabin::system_deps::pkg_config_failed`	`pkg-config` exited non-zero for `--cflags` / `--libs`.
`cabin::system_deps::invocation_failed`	A `pkg-config` invocation failed for a reason other than not-found.
`cabin::system_deps::malformed_output`	`pkg-config` produced output Cabin could not split into argv tokens.

What Cabin does not do

Cabin does not install system packages. Use your OS package manager (apt, dnf, brew, pacman, ...), an external toolchain manager, or a manual install. Cabin does not bundle or wrap any third-party package manager.

Cabin does not integrate with vcpkg, Conan, or Cargo-style system probes beyond the pkg-config query path described above.