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+# `libpam-sys-impls`: LibPAM library detection
+
+This crate detects what implementation of LibPAM should be used, as part of the build script, and exports that information to downstream crates.
+
+This is mostly a backend crate for [libpam-sys](https://crates.io/crates/libpam-sys/).
+That crate re-exports pretty much everything we provide.
+In most cases, you can just use that instead of depending upon this directly.
+
+## Usage
+
+Different PAM implementations have different constants and some different behaviors.
+If you need to change your library's behavior based on PAM implementation, there are a few ways to do so.
+
+### Constants
+
+You can match on the current PAM implementation at runtime.
+All known PAM implementations are in the `PamImpl` enumeration, and `PamImpl::CURRENT` is set to the current implementation.
+This is present as a string literal macro in `pam_impl_name!`.
+
+### Conditional compilation
+
+This package provides custom `#[cfg]`s to compile based on the current PAM implementation.
+
+First, **enable custom `#[cfg]`s in your build.rs**:
+
+```rust
+// build.rs
+use libpam_sys_impls::pam_impl;
+
+fn main() {
+    pam_impl::enable_pam_impl_cfg();
+    
+    // everything else you do at build time
+}
+```
+
+This will then allow you to use the `pam_impl` configuration variable at compile time:
+
+```rust
+#[cfg(pam_impl = "LinuxPam")]
+fn handle_pam() {
+    // do things in a Linux-PAM specific way
+}
+
+#[cfg(not(pam_impl = "LinuxPam"))]
+fn handle_pam() {
+    // do things in another, more different way
+}
+```
+
+## Configuration
+
+Known implementations of PAM are listed in the `PamImpl` enum, and your currently installed implementation is automatically detected.
+
+If you need to configure this, you can override it **at build time** with the `LIBPAMSYS_IMPL` environment variable:
+
+- Unset or empty (the default): Use the version of PAM most commonly found on the target OS.
+  If we don't know what kind of PAM is usually installed on this OS, we fall back to `__installed__`.
+- `__installed__`: Looks at the PAM library installed on the current machine.
+  If none is recognized, falls back to `XSso`.
+- The name of a `PamImpl` entry: The named PAM implementation.
+  For instance, `LIBPAMSYS_IMPL=OpenPam cargo build` will build this library for OpenPAM.
+
+## MSRV
+
+This library supports **Rust 1.75**, as the version currently (July 2025) available in Debian Trixie and Ubuntu 24.04 LTS.
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