detects presence of udev_hwdb_* functions in build script

build.rs

@@ -1,5 +1,48 @@
 extern crate pkg_config;
 
+use std::fs::{self,File};
+use std::path::Path;
+use std::process::Command;
+
+use std::io::prelude::*;
+
+fn check_func(function_name: &str) -> bool {
+    let test_file_path = format!("target/build/check_{}.rs", function_name);
+    let test_file_name = Path::new(&test_file_path[..]);
+
+    fs::create_dir_all("target/build").unwrap();
+
+    {
+        let mut test_file = File::create(test_file_name).unwrap();
+
+        writeln!(&mut test_file, "extern \"C\" {{").unwrap();
+        writeln!(&mut test_file, "    fn {}();", function_name).unwrap();
+        writeln!(&mut test_file, "}}").unwrap();
+        writeln!(&mut test_file, "").unwrap();
+        writeln!(&mut test_file, "fn main() {{").unwrap();
+        writeln!(&mut test_file, "    unsafe {{").unwrap();
+        writeln!(&mut test_file, "        {}();", function_name).unwrap();
+        writeln!(&mut test_file, "    }}").unwrap();
+        writeln!(&mut test_file, "}}").unwrap();
+    }
+
+    let output = Command::new("rustc").
+        arg(test_file_name).
+        arg("--out-dir").arg("target/build/").
+        arg("-l").arg("udev").
+        output().unwrap();
+
+    output.status.success()
+}
+
 fn main() {
-  pkg_config::find_library("libudev").unwrap();
+    pkg_config::find_library("libudev").unwrap();
+
+    if check_func("udev_hwdb_new") {
+        println!("cargo:rustc-cfg=hwdb");
+        println!("cargo:hwdb=true");
+    }
+    else {
+        println!("cargo:hwdb=false");
+    }
 }

examples/hwdb_query.rs

@@ -1,14 +1,15 @@
 extern crate libudev_sys as udev;
 extern crate libc;
 
-use std::env;
-use std::str;
 
-use std::ffi::{CString,CStr};
-
-use libc::c_char;
+#[cfg(hwdb)] use std::env;
+#[cfg(hwdb)] use std::str;
+#[cfg(hwdb)] use std::ffi::{CString,CStr};
+#[cfg(hwdb)] use libc::c_char;
 
+#[cfg(hwdb)]
 fn main() {
+
     let args = env::args().collect::<Vec<_>>();
 
     if args.len() != 2 {
@@ -34,6 +35,7 @@ fn main() {
     }
 }
 
+#[cfg(hwdb)]
 unsafe fn query_hwdb(hwdb: *mut udev::udev_hwdb, query: &CString) {
     println!("{:>30}: {:?}", "query", query);
 
@@ -42,6 +44,7 @@ unsafe fn query_hwdb(hwdb: *mut udev::udev_hwdb, query: &CString) {
     udev::udev_hwdb_unref(hwdb);
 }
 
+#[cfg(hwdb)]
 unsafe fn print_results(list_entry: *mut udev::udev_list_entry) {
     if list_entry.is_null() {
         return;
@@ -55,6 +58,13 @@ unsafe fn print_results(list_entry: *mut udev::udev_list_entry) {
     print_results(udev::udev_list_entry_get_next(list_entry));
 }
 
+#[cfg(hwdb)]
 unsafe fn unwrap_str<'a>(ptr: *const c_char) -> &'a str {
     str::from_utf8(CStr::from_ptr(ptr).to_bytes()).unwrap()
 }
+
+
+#[cfg(not(hwdb))]
+fn main() {
+    println!("hwdb is not supported by native libudev");
+}

src/lib.rs

@@ -2,7 +2,10 @@
 
 extern crate libc;
 
-use libc::{c_void,c_int,c_uint,c_char,c_ulonglong,size_t,dev_t};
+use libc::{c_void,c_int,c_char,c_ulonglong,size_t,dev_t};
+
+#[cfg(hwdb)]
+pub use hwdb::*;
 
 
 #[repr(C)]
@@ -23,9 +26,6 @@ pub struct udev_enumerate;
 #[repr(C)]
 pub struct udev_queue;
 
-#[repr(C)]
-pub struct udev_hwdb;
-
 extern "C" {
     // udev
     pub fn udev_new() -> *mut udev;
@@ -116,12 +116,23 @@ extern "C" {
     pub fn udev_queue_get_fd(udev_queue: *mut udev_queue) -> c_int;
     pub fn udev_queue_flush(udev_queue: *mut udev_queue) -> c_int;
 
-    // udev_hwdb
-    pub fn udev_hwdb_ref(hwdb: *mut udev_hwdb) -> *mut udev_hwdb;
-    pub fn udev_hwdb_unref(hwdb: *mut udev_hwdb) -> *mut udev_hwdb;
-    pub fn udev_hwdb_new(udev: *mut udev) -> *mut udev_hwdb;
-    pub fn udev_hwdb_get_properties_list_entry(hwdb: *mut udev_hwdb, modalias: *const c_char, flags: c_uint) -> *mut udev_list_entry;
-
     // udev_util
     pub fn udev_util_encode_string(str: *const c_char, str_enc: *mut c_char, len: size_t) -> c_int;
 }
+
+
+#[cfg(hwdb)]
+mod hwdb {
+    use super::libc::{c_uint,c_char};
+    use super::{udev,udev_list_entry};
+
+    #[repr(C)]
+    pub struct udev_hwdb;
+
+    extern "C" {
+        pub fn udev_hwdb_ref(hwdb: *mut udev_hwdb) -> *mut udev_hwdb;
+        pub fn udev_hwdb_unref(hwdb: *mut udev_hwdb) -> *mut udev_hwdb;
+        pub fn udev_hwdb_new(udev: *mut udev) -> *mut udev_hwdb;
+        pub fn udev_hwdb_get_properties_list_entry(hwdb: *mut udev_hwdb, modalias: *const c_char, flags: c_uint) -> *mut udev_list_entry;
+    }
+}