[][src]Crate cortex_m_semihosting

Semihosting for ARM Cortex-M processors

What is semihosting?

"Semihosting is a mechanism that enables code running on an ARM target to communicate and use the Input/Output facilities on a host computer that is running a debugger." - ARM


This crate provides implementations of core::fmt::Write, so you can use it, in conjunction with core::format_args! or the write! macro, for user-friendly construction and printing of formatted strings.

Since semihosting operations are modeled as system calls, this crate exposes an untyped syscall! interface just like the sc crate does.


Semihosting operations are very slow. Like, each WRITE operation can take hundreds of milliseconds.


Using hio::HStdout

This example will demonstrate how to print formatted strings.

use cortex_m_semihosting::hio;
use core::fmt::Write;

// This function will be called by the application
fn print() -> Result<(), core::fmt::Error> {
    let mut stdout = match hio::hstdout() {
        Ok(fd) => fd,
        Err(()) => return Err(core::fmt::Error),

    let language = "Rust";
    let ranking = 1;

    write!(stdout, "{} on embedded is #{}!", language, ranking)?;


On the host side:

$ openocd -f $INTERFACE -f $TARGET -l /tmp/openocd.log
Open On-Chip Debugger 0.9.0 (2016-04-27-23:18)
Licensed under GNU GPL v2
For bug reports, read
# the command will block at this point

The OpenOCD logs will be redirected to /tmp/openocd.log. You can view those logs in "real time" using tail

$ tail -f /tmp/openocd.log
Info : Unable to match requested speed 1000 kHz, using 950 kHz
Info : Unable to match requested speed 1000 kHz, using 950 kHz
Info : clock speed 950 kHz
Info : STLINK v1 JTAG v11 API v2 SWIM v0 VID 0x0483 PID 0x3744
Info : using stlink api v2
Info : nrf51.cpu: hardware has 4 breakpoints, 2 watchpoints

Alternatively you could omit the -l flag from the openocd call, and the tail -f command but the OpenOCD output will have intermingled in it logs from its normal operation.

Then, we run the program:

$ arm-none-eabi-gdb hello-world
(gdb) # Connect to OpenOCD
(gdb) target remote :3333

(gdb) # Enable OpenOCD's semihosting support
(gdb) monitor arm semihosting enable

(gdb) # Flash the program
(gdb) load

(gdb) # Run the program
(gdb) continue

And you'll see the output under OpenOCD's terminal

# openocd -f $INTERFACE -f $TARGET -l /tmp/openocd.log
Rust on embedded is #1!

Using the syscall interface

This example will show how to print "Hello, world!" on the host.

Target program:

use cortex_m_semihosting::syscall;

// This function will be called by the application
fn print() {
    // File descriptor (on the host)
    const STDOUT: usize = 1; // NOTE the host stdout may not always be fd 1
    static MSG: &'static [u8] = b"Hello, world!\n";

    // Signature: fn write(fd: usize, ptr: *const u8, len: usize) -> usize
    let r = unsafe { syscall!(WRITE, STDOUT, MSG.as_ptr(), MSG.len()) };

Output and monitoring proceed as in the above example.

The dbg! macro

Analogous to std::dbg the macro dbg! returns a given expression and prints it using heprintln! including context for quick and dirty debugging.

Panics if heprintln! returns an error.


const UUID: *mut u32 = 0x0009_FC70 as *mut u32;
let mut uuid: [u32; 4] = [0; 4];
for i in 0..4 {
    uuid[i] = unsafe { dbg!(UUID.offset(i as isize).read_volatile()) };


[examples/semihosting.rs:37] UUID = 0x0009fc70
[examples/semihosting.rs:40] i = 0
[examples/semihosting.rs:41] UUID.offset(i as isize).read_volatile() = 3370045464
[examples/semihosting.rs:40] i = 1
[examples/semihosting.rs:41] UUID.offset(i as isize).read_volatile() = 1426218275
[examples/semihosting.rs:40] i = 2
[examples/semihosting.rs:41] UUID.offset(i as isize).read_volatile() = 2422621116
[examples/semihosting.rs:40] i = 3
[examples/semihosting.rs:41] UUID.offset(i as isize).read_volatile() = 1044138593

Optional features


When this feature is enabled semihosting is implemented using inline assembly (llvm_asm!) and compiling this crate requires nightly.

When this feature is disabled semihosting is implemented using FFI calls into an external assembly file and compiling this crate works on stable and beta.

When this feature is enabled, return values above 0xfffffff0 from semihosting operation SYS_WRITE (0x05) are interpreted as if the entire buffer had been written. The current latest version 6.48b of J-Link exhibits such behaviour, causing a panic if this feature is not enabled.


When this feature is enabled, the underlying system calls to bkpt are patched out.


For documentation about the semihosting operations, check:

'Chapter 8 - Semihosting' of the 'ARM Compiler toolchain Version 5.0' manual.



Interacting with debugging agent


Host I/O


Semihosting operations



Macro that prints and returns the value of a given expression for quick and dirty debugging. Works exactly like dbg! in the standard library, replacing eprintln with heprintln, which it unwraps.


Macro for printing to the HOST standard error


Macro for printing to the HOST standard error, with a newline.


Macro for printing to the HOST standard output


Macro for printing to the HOST standard output, with a newline.


Variable argument version of syscall


Macro version of syscall1



Performs a semihosting operation, takes a pointer to an argument block


Performs a semihosting operation, takes one integer as an argument