Crate cortex_m_semihosting [] [src]

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


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

And since the most used semihosting operation is writing to the host's stdout, convenience hprint and hprintln macros are provided.


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


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

Target program:

fn main() {
    // File descriptor (on the host)
    const STDOUT: usize = 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()) };

    // (Or you could have just written `hprintln!("Hello, world!")`)
    // ..

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 watch + tail

$ watch 'tail /tmp/openocd.log'
Every 2.0s: tail /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

Then, we run the program:

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

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

# Flash the program
(gdb) load

# Run the program
(gdb) continue

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

# openocd -f $INTERFACE -f $TARGET -l /tmp/openocd.log
Hello, world!


For documentation about the semihosting operations, check:

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



Host I/O


Semihosting operations



Macro for printing to the host's standard stderr


Macro for printing to the host's standard error, with a newline.


Macro for printing to the host's standard output


Macro for printing to the host's standard output, with a newline.


Variable argument version of syscall