# semihosting
[](https://crates.io/crates/semihosting)
[](https://docs.rs/semihosting)
[](#license)
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Semihosting for AArch64, Arm, RISC-V, MIPS32, MIPS64, and Xtensa.
This library provides access to semihosting, a mechanism for programs running on the real or virtual (e.g., QEMU) target to communicate with I/O facilities on the host system. See the [Arm documentation](https://github.com/ARM-software/abi-aa/blob/2025Q1/semihosting/semihosting.rst) for more information on semihosting.
APIs are categorized into the following four types:
- The top-level API (`semihosting::{io,fs,..}`) provides a subset of the standard library's similar APIs.
- `io`: Provide no-std io traits and `std{in,out,err}`. (`std{in,out,err}` requires `stdio` feature, others are unconditionally provided)
- `fs`: Provide methods to manipulate the contents of the host filesystem. (requires `fs` feature)
- `process`: Provide `abort` and `exit`.
- `dbg!`/`print{,ln}!`/`eprint{,ln}!`: macros to output to stdout/stderr. (requires `stdio` feature)
Note that some APIs are not strictly a subset of the standard library.
- API that uses types not available in `core` such as `Path` (technically, the same thing could be implemented, but it makes sense to use `CStr` directly, because when converting a long `Path`/`OsStr` to `CStr`, it needs to either [do an allocation](https://github.com/rust-lang/rust/blob/1.84.0/library/std/src/sys/pal/common/small_c_string.rs#L25-L26) or return an error)
- API that panics on failure in `std` (in no-std it makes sense to return `Result` since `panic=abort` is default)
- Helpers that are useful when using this library.
- `c!`: `CStr` literal macro. (Since Rust 1.77, this macro is soft-deprecated in favor of C string literals (`c"..."`).)
- `semihosting::sys` module, which provides low-level access to platform-specific semihosting interfaces.
- `semihosting::experimental` module, which provides experimental APIs. See [optional features](#optional-features) for more.
Additionally, this library provides a panic handler for semihosting, `-C panic=unwind` support, backtrace support, via [optional features](#optional-features).
## Platform Support
The following target architectures are supported:
| target_arch | Specification | `semihosting::sys` module | Note |
| arm | [Semihosting for AArch32 and AArch64](https://github.com/ARM-software/abi-aa/blob/2025Q1/semihosting/semihosting.rst) | `sys::arm_compat` | use `SVC` on A+R profile by default based on Arm's recommendation but it can be changed by [`trap-hlt` feature](#optional-features-trap-hlt). |
| riscv32/riscv64 | [RISC-V Semihosting](https://github.com/riscv-non-isa/riscv-semihosting/blob/1.0/riscv-semihosting.adoc) | `sys::arm_compat` | |
| xtensa | [OpenOCD Semihosting](https://github.com/espressif/openocd-esp32/blob/HEAD/src/target/espressif/esp_xtensa_semihosting.c) | `sys::arm_compat` | requires [`openocd-semihosting` feature](#optional-features-openocd-semihosting) |
| mips/mips32r6/mips64/mips64r6 | Unified Hosting Interface (MD01069) | `sys::mips` | |
The host must be running an emulator or a debugger attached to the target.
The following targets have been tested on CI. (qemu-system has been tested on Linux, macOS, and Windows hosts, and qemu-user on Linux host.)
| `aarch64-unknown-none{,-softfloat}` | ✓ | ✓ | ✓ | ✓ | |
| `{arm,thumb}v4t-none-eabi` | ✓ | | ✓ | | |
| `{arm,thumb}v5te-none-eabi` | ✓ | ✓ | ✓ | | |
| `armv7a-none-eabi{,hf}` | ✓ | ✓ | ✓ | | |
| `armv7r-none-eabi{,hf}` | ✓ | ✓ | ✓ | | |
| `armebv7r-none-eabi{,hf}` | ✓ | | ✓ | | |
| `armv8r-none-eabihf` | ✓ | ✓ | ✓ | | |
| `thumbv6m-none-eabi` | ✓ | ✓ | N/A | | |
| `thumbv7m-none-eabi` | ✓ | ✓ | N/A | | |
| `thumbv7em-none-eabi{,hf}` | ✓ | ✓ | N/A | | |
| `thumbv8m.base-none-eabi` | ✓ | ✓ | N/A | | |
| `thumbv8m.main-none-eabi{,hf}` | ✓ | ✓ | N/A | | |
| `riscv32*-unknown-none-elf` | ✓ | ✓ | ✓ | ✓ | |
| `riscv64*-unknown-none-elf` | ✓ | ✓ | ✓ | ✓ | |
| `mips{,el}-unknown-none` | ✓ | ✓ | N/A | | \[3] \[4] |
| `mips64{,el}-unknown-none` | ✓ | ✓ | N/A | | \[3] \[4] |
| `mipsisa32r6{,el}-unknown-none` | ✓ | ✓ | N/A | | \[3] \[4] |
| `mipsisa64r6{,el}-unknown-none` | ✓ | ✓ | N/A | | \[3] \[4] |
\[1] `stdio`, `fs`, `time`, and `args`.<br>
\[2] I'm not sure how to test panic-unwind on qemu-user.<br>
\[3] Requires nightly due to `#![feature(asm_experimental_arch)]`.<br>
\[4] It seems [unsupported on QEMU 8.0+](https://qemu-project.gitlab.io/qemu/about/removed-features.html#mips-trap-and-emulate-kvm-support-removed-in-8-0).<br>
## Optional features
All features are disabled by default.
In general use cases, you probably only need the `stdio` feature that enables print-related macros and/or the `panic-handler` feature that exits with a non-zero error code on panic.
```toml
[dependencies]
semihosting = { version = "0.1", features = ["stdio", "panic-handler"] }
```
- **`alloc`**<br>
Use `alloc`.
- **`stdio`**<br>
Enable `semihosting::io::{stdin,stdout,stderr}` and `semihosting::{print*,eprint*,dbg}`.
- **`fs`**<br>
Enable `semihosting::fs`.
- **`panic-handler`**<br>
Provide panic handler based on `semihosting::process::exit`.
If the `stdio` feature is also enabled, this attempt to output panic message and
location to stderr.
- <a name="optional-features-trap-hlt"></a>**`trap-hlt`**<br>
Arm-specific: Use HLT instruction on A+R profile.
[Arm documentation](https://github.com/ARM-software/abi-aa/blob/2025Q1/semihosting/semihosting.rst#the-semihosting-interface) says:
> The `HLT` encodings are new in version 2.0 of the semihosting specification.
> Where possible, have semihosting callers continue to use the previously existing
> trap instructions to ensure compatibility with legacy semihosting implementations.
> These trap instructions are `HLT` for A64, `SVC` on A+R profile A32 or T32, and
> `BKPT` on M profile. However, it is necessary to change from SVC to HLT instructions
> to support AArch32 semihosting properly in a mixed AArch32/AArch64 system.
>
> ARM encourages semihosting callers to implement support for trapping using `HLT`
> on A32 and T32 as a configurable option. ARM strongly discourages semihosting
> callers from mixing the `HLT` and `SVC` mechanisms within the same executable.
Based on the Arm's recommendation, this is implemented as an optional feature.
Enabling this feature on architectures other than Arm A+R profile will result in a compile error.
- <a name="optional-features-openocd-semihosting"></a>**`openocd-semihosting`**<br>
Xtensa-specific: Use OpenOCD Semihosting.
Xtensa has two semihosting interfaces:
- Tensilica ISS SIMCALL used in Cadence tools and [QEMU](https://www.qemu.org/docs/master/about/emulation.html#supported-targets).
- Arm-semihosting-compatible semihosting interface used in [OpenOCD](https://github.com/espressif/openocd-esp32/blob/HEAD/src/target/espressif/esp_xtensa_semihosting.c) and [probe-rs](https://github.com/probe-rs/probe-rs/pull/2303). (This crate calls it "OpenOCD Semihosting", which is the same as the option name in [newlib-esp32](https://github.com/espressif/newlib-esp32/blob/esp-4.3.0_20240530/libgloss/xtensa/syscalls.c#L21).)
This crate does not currently support SIMCALL-based semihosting, but users need to explicitly enable the feature to avoid accidentally selecting a different one than one actually want to use.
Enabling this feature on architectures other than Xtensa will result in a compile error.
- **`portable-atomic`**<br>
Use [portable-atomic]'s atomic types.
portable-atomic provides atomic CAS on targets where the standard library does not provide atomic CAS.
To use the `panic-unwind` feature on such targets (e.g., RISC-V without A-extension), you need to enable this feature.
See [its documentation](https://github.com/taiki-e/portable-atomic#optional-features-critical-section) for details.
- **`args`**<br>
Enable `semihosting::experimental::env::args`.
Note:
- This feature is experimental (tracking issue: [#1](https://github.com/taiki-e/semihosting/issues/1))
and outside of the normal semver guarantees and minor or patch versions of semihosting may make
breaking changes to them at any time.
- **`time`**<br>
Enable `semihosting::experimental::time`.
Note:
- This feature is experimental (tracking issue: [#2](https://github.com/taiki-e/semihosting/issues/2))
and outside of the normal semver guarantees and minor or patch versions of semihosting may make
breaking changes to them at any time.
- **`panic-unwind`**<br>
Provide `-C panic=unwind` support for panic handler and enable
`semihosting::experimental::panic::catch_unwind`.
This currently uses [unwinding] crate to support unwinding.
See its documentation for supported platforms and requirements.
Note:
- This feature is experimental (tracking issue: [#3](https://github.com/taiki-e/semihosting/issues/3))
and outside of the normal semver guarantees and minor or patch versions of semihosting may make
breaking changes to them at any time.
- This requires nightly compiler.
- This implicitly enables the `alloc` and `panic-handler` features.
- This uses atomic CAS. You need to use `portable-atomic` feature together if your target doesn't support atomic CAS (e.g., RISC-V without A-extension).
- When enabling this feature, you may need to rebuild the standard library with
`-C panic=unwind` for `catch_unwind` to work properly. The recommended way to
rebuild the standard library is passing `-Z build-std="core,alloc"` option to cargo.
- **`backtrace`**<br>
Provide backtrace support for panic handler.
This currently uses [unwinding] crate to support backtrace.
See its documentation for supported platforms and requirements.
Note:
- This feature is experimental (tracking issue: [#3](https://github.com/taiki-e/semihosting/issues/3))
and outside of the normal semver guarantees and minor or patch versions of semihosting may make
breaking changes to them at any time.
- This requires nightly compiler.
- This implicitly enables the `stdio` feature.
- When enabling this, it is recommended to also enable the `panic-unwind` feature. Otherwise, a decent backtrace will not be displayed at this time. (Using [`-C force-unwind-tables`](https://doc.rust-lang.org/rustc/codegen-options/index.html#force-unwind-tables) may work, but has not been tested yet.)
- Currently, the backtrace generated is not human-readable.
```text
panicked at 'a', src/main.rs:86:13
stack backtrace:
0x84dc0
0x8ed80
0x8332c
0x83654
0x80644
0x803cc
0x809dc
0x800bc
```
You can use `addr2line` to resolve the addresses and [rustfilt] to demangle Rust symbols.
For example, run the following command (please replace `<path/to/binary>` with your binary path), then paste the addresses:
```sh
llvm-addr2line -fipe <path/to/binary> | rustfilt
```
[portable-atomic]: https://github.com/taiki-e/portable-atomic
[rustfilt]: https://github.com/luser/rustfilt
[unwinding]: https://github.com/nbdd0121/unwinding
## License
Licensed under either of [Apache License, Version 2.0](LICENSE-APACHE) or
[MIT license](LICENSE-MIT) at your option.
Unless you explicitly state otherwise, any contribution intentionally submitted
for inclusion in the work by you, as defined in the Apache-2.0 license, shall
be dual licensed as above, without any additional terms or conditions.