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+# `libpam-sys-consts`: Constants for LibPAM
+
+This crate does two primary things:
+
+- Detects which implementation of LibPAM the current machine uses (as part of the build script), and exports that information to downstream crates.
+- Exports the constants specific to that version of LibPAM.
+  These are located in the `constants` module.
+
+## Handling different PAM implmementations
+
+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
+
+The current PAM implementation is available in `PamImpl::CURRENT`.
+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_helpers::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
+
+By default, this crate automatically detects your libpam version.
+Known implementations are listed in the `PamImpl` enum.
+
+You can override this **at build time** by setting the `LIBPAMSYS_IMPL` environment variable to one of those names.
+For example, `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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