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ElemList: avoid heap allocation for buffer used to query size #131

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ElemList: avoid heap allocation for buffer used to query size #131

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15 changes: 10 additions & 5 deletions src/ctl_int.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -18,6 +18,7 @@ use libc::{c_uint, c_void, size_t, c_long, c_int, pollfd, c_short};
const ELEM_ID_SIZE: usize = 64;
// const ELEM_VALUE_SIZE: usize = 1224;
// const ELEM_INFO_SIZE: usize = 272;
const ELEM_LIST_SIZE: usize = 80;

/// [snd_ctl_pcm_next_device](https://www.alsa-project.org/alsa-doc/alsa-lib/control_8c.html#accbb0be6e5ca7361ffec0ea304ed1b05) wrapper.
/// Iterate over devices of a card.
Expand Down Expand Up @@ -106,10 +107,10 @@ impl Ctl {
}

pub fn elem_list(&self) -> Result<ElemList> {
// populate an empty list to get the number of elements
let empty_list = elem_list_new(0)?;
acheck!(snd_ctl_elem_list(self.0, empty_list.0))?;
let required_elements = empty_list.get_count();
// initialize an empty list to get the number of elements
let empty_list = elem_list_empty_buffer();
acheck!(snd_ctl_elem_list(self.0, empty_list.get() as *mut _))?;
let required_elements = unsafe { alsa::snd_ctl_elem_list_get_count(empty_list.get() as *const _) };

// obtain the list of all the elements now that we know how many there are
let full_list = elem_list_new(required_elements)?;
Expand Down Expand Up @@ -470,6 +471,11 @@ impl Drop for ElemList {
}
}

pub(crate) fn elem_list_empty_buffer() -> UnsafeCell<[u8; ELEM_LIST_SIZE]> {
assert!(unsafe { alsa::snd_ctl_elem_list_sizeof() } as usize <= ELEM_LIST_SIZE);
UnsafeCell::new(unsafe { mem::zeroed() })
}

pub fn elem_list_new(count: u32) -> Result<ElemList> {
let mut p = ptr::null_mut();
let list = acheck!(snd_ctl_elem_list_malloc(&mut p)).map(|_| ElemList(p))?;
Expand All @@ -489,7 +495,6 @@ impl ElemList {
}
}

pub(crate) fn get_count(&self) -> u32 { unsafe { alsa::snd_ctl_elem_list_get_count(self.0) } }
pub fn get_used(&self) -> u32 { unsafe { alsa::snd_ctl_elem_list_get_used(self.0) } }
pub fn get_id(&self, index: u32) -> Result<ElemId> {
self.ensure_valid_index(index)?;
Expand Down
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