Returns the total number of fds (nervosnetwork#118)

* Returns the total number of fds

* Use ///

* Add example

src/high_level.rs

@@ -713,3 +713,16 @@ pub fn spawn_cell(
     syscalls::spawn(index, Source::CellDep, 0, 0, &mut spgs)?;
     Ok(process_id)
 }
+
+/// Get inherited file descriptors.
+///
+/// # Example
+///
+/// ```
+/// let fds = inherited_fds();
+/// ```
+pub fn inherited_fds() -> Vec<u64> {
+    let mut fds = [0u64; 64];
+    let l = syscalls::inherited_fds(&mut fds);
+    fds[..l as usize].to_vec()
+}

src/syscalls/native.rs

@@ -720,7 +720,7 @@ pub fn write(fd: u64, buffer: &[u8]) -> Result<usize, SysError> {
 /// This syscall retrieves the file descriptors available to the current process, which are passed in from the parent
 /// process. These results are copied from the inherited_fds parameter of the Spawn syscall.
 /// Note: available after ckb2023.
-pub fn inherited_fds(fds: &mut [u64]) {
+pub fn inherited_fds(fds: &mut [u64]) -> u64 {
     let mut l: u64 = fds.len() as u64;
     unsafe {
         syscall(
@@ -734,6 +734,7 @@ pub fn inherited_fds(fds: &mut [u64]) {
             SYS_INHERITED_FDS,
         )
     };
+    l
 }
 
 /// This syscall manually closes a file descriptor. After calling this, any attempt to read/write the file descriptor

src/syscalls/simulator.rs

@@ -300,7 +300,7 @@ pub fn write(_fd: u64, _buffer: &[u8]) -> Result<usize, SysError> {
     panic!("This is not supported in the native-simulator!");
 }
 
-pub fn inherited_fds(_fds: &mut [u64]) {
+pub fn inherited_fds(_fds: &mut [u64]) -> u64 {
     panic!("This is not supported in the native-simulator!");
 }