make it work

test/Makefile

@@ -1,5 +1,12 @@
 CC = clang -Wall -Wpedantic -target riscv32-unknown-linux-gnu -fuse-ld=lld -march=rv32i
 CC_STATIC = $(CC) -static -nostdlib -nodefaultlibs
 
+RUSTC = rustc --target riscv32i-unknown-none-elf
+
+all: init init1
+
 init: init.c
 	$(CC_STATIC) init.c -o init
+
+init1: init1.rs
+	$(RUSTC) init1.rs

test/init.c

@@ -1,11 +1,20 @@
-void _start(void)
+// <https://jborza.com/post/2021-05-11-riscv-linux-syscalls/>
+
+_Noreturn void __attribute__ ((noinline)) exit(int code)
 {
-    __asm__ volatile (
+    __asm__ volatile(
         "li a7, 93;" // exit
-        "li a0, 0;" // code 0
+        "mv a0, %0;"  // code
         "ecall"
         :
-        :
+        : "r"(code)
         : "a7", "a0", "memory"
     );
+    __builtin_unreachable();
+}
+
+_Noreturn void _start(void)
+{
+    
+    exit(10);
 }

test/init1.rs

@@ -0,0 +1,73 @@
+#![no_std]
+#![no_main]
+
+use core::fmt::Write;
+
+fn exit(code: i32) -> ! {
+    unsafe {
+        core::arch::asm!(
+            "li a7, 93",
+            "ecall",
+            in("a0") code,
+            options(noreturn),
+        );
+    }
+}
+
+fn write(fd: i32, data: &[u8]) -> isize {
+    let mut out;
+    unsafe {
+        core::arch::asm!(
+            "li a7, 64",
+            "ecall",
+            in("a0") fd,
+            in("a1") data.as_ptr(),
+            in("a2") data.len(),
+            lateout("a0") out,
+            out("a7") _,
+        )
+    }
+    out
+}
+
+fn read(fd: i32, buf: &mut [u8]) -> isize {
+    let mut out;
+    unsafe {
+        core::arch::asm!(
+            "li a7, 63",
+            "ecall",
+            in("a0") fd,
+            in("a1") buf.as_mut_ptr(),
+            in("a2") buf.len(),
+            lateout("a0") out,
+            out("a7") _,
+        )
+    }
+    out
+}
+
+struct Stderr;
+
+impl core::fmt::Write for Stderr {
+    fn write_str(&mut self, s: &str) -> core::fmt::Result {
+        let ret = write(2, s.as_bytes());
+        if ret < 0 { Err(core::fmt::Error) } else { Ok(()) }
+    }
+}
+
+#[panic_handler]
+fn handle(info: &core::panic::PanicInfo<'_>) -> ! {
+    let _ = writeln!(Stderr, "panicked: {}", info.message());
+    unsafe { core::arch::asm!("unimp", options(noreturn)) }
+}
+
+#[no_mangle]
+fn _start() {
+    write(1, b"enter a number: ");
+    let mut buf = [0; 10];
+    let len = read(0, &mut buf);
+    let buf = &buf[..(len as usize)];
+    let n = str::from_utf8(buf).unwrap().trim().parse::<i32>().unwrap();
+
+    exit(n);
+}