Add function to redirect snd errors to an Output struct

Fixes: #138

Author: diwic

src/chmap.rs

@@ -139,6 +139,11 @@ fn chmap_for_first_pcm() {
     use super::*;
     use std::ffi::CString;
     use crate::device_name::HintIter;
+
+    use crate::Output;
+
+    let output = Output::local_error_handler().unwrap();
+
     let i = HintIter::new(None, &*CString::new("pcm").unwrap()).unwrap();
     for p in i.map(|n| n.name.unwrap()) {
         println!("Chmaps for {:?}:", p);
@@ -149,4 +154,10 @@ fn chmap_for_first_pcm() {
             Err(a) => println!("  {}", a) // It's okay to have entries in the name hint array that can't be opened
         }
     }
+
+    output.borrow_mut().buffer_string(|buf| {
+        let str = CString::new(buf).unwrap();
+        println!("Errors:\n{}", str.to_str().unwrap());
+    });
+
 }

src/io.rs

@@ -1,12 +1,19 @@
 use crate::alsa;
 use super::error::*;
 use std::{slice, ptr, fmt};
+use std::cell::RefCell;
+use std::rc::Rc;
+use libc::{c_char, c_int};
 
 /// [snd_output_t](http://www.alsa-project.org/alsa-doc/alsa-lib/group___output.html) wrapper
 pub struct Output(*mut alsa::snd_output_t);
 
 unsafe impl Send for Output {}
 
+thread_local! {
+    static ERROR_OUTPUT: RefCell<Option<Rc<RefCell<Output>>>> = RefCell::new(None);
+}
+
 impl Drop for Output {
     fn drop(&mut self) { unsafe { alsa::snd_output_close(self.0) }; }
 }
@@ -26,6 +33,14 @@ impl Output {
         };
         f(b)
     }
+
+    pub fn local_error_handler() -> Result<Rc<RefCell<Output>>> {
+        let output = Output::buffer_open()?;
+        let r = Rc::new(RefCell::new(output));
+        ERROR_OUTPUT.with_borrow_mut(|e| *e = Some(r.clone()));
+        unsafe { alsa::snd_lib_error_set_local(Some(our_error_handler)); }
+        Ok(r)
+    }
 }
 
 impl fmt::Debug for Output {
@@ -47,3 +62,19 @@ impl fmt::Display for Output {
 }
 
 pub fn output_handle(o: &Output) -> *mut alsa::snd_output_t { o.0 }
+
+unsafe extern "C" fn our_error_handler(file: *const c_char,
+    line: c_int,
+    func: *const c_char,
+    err: c_int,
+    fmt: *const c_char,
+    arg: *mut alsa::__va_list_tag,
+) {
+    ERROR_OUTPUT.with_borrow(|e| {
+        let b = e.as_ref().expect("ERROR_OUTPUT not set").borrow_mut();
+        alsa::snd_output_puts(b.0, func);
+        alsa::snd_output_puts(b.0, c": ".as_ptr());
+        alsa::snd_output_vprintf(b.0, fmt, arg);
+        alsa::snd_output_putc(b.0, '\n' as i32);
+    })
+}