Merge pull request #31 from the8472/printf-debugging

How to printf-debug alloc/core

Author: yaahc

src/SUMMARY.md

@@ -9,6 +9,11 @@
   - [Membership](./membership.md)
   - [Reviewing](./reviewing.md)
 
+---
+
+- [Building and debugging libraries](./development/building-and-debugging.md)
+
+
 ---
 
 - [The feature lifecycle](./feature-lifecycle/summary.md)

src/development/building-and-debugging.md

@@ -0,0 +1,38 @@
+
+# Building and Debugging the library crates
+
+Most of the [instructions from the rustc-dev-guide][rustc-guide] also apply to the standard library since
+it is built with the same build system, so it is recommended to read it first.
+
+[rustc-guide]: https://rustc-dev-guide.rust-lang.org/building/how-to-build-and-run.html
+
+# Println-debugging alloc and core
+
+Since logging and IO APIs are not available in `alloc` and `core` advice meant for the rest of the compiler
+is not applicable here.
+
+Instead one can either extract the code that should be tested to a normal crate and add debugging statements there or
+on POSIX systems one can use the following hack:
+
+```rust
+extern "C" {
+    fn dprintf(fd: i32, s: *const u8, ...);
+}
+
+macro_rules! dbg_printf {
+    ($s:expr) => {
+        unsafe { dprintf(2, "%s\0".as_ptr(), $s as *const u8); }
+    }
+}
+
+fn function_to_debug() {
+    let dbg_str = format!("debug: {}\n\0", "hello world");
+    dbg_printf!(dbg_str.as_bytes().as_ptr());
+}
+```
+
+Then one can run a test which exercises the code to debug and show the error output via
+
+```shell,ignore
+./x.py test library/alloc --test-args <test_name> --test-args --no-capture
+```