1# OpenHarmony ABIs
2
3
4OpenHarmony supports diverse device forms, instruction sets, and operating system kernels. To ensure application compatibility on different OpenHarmony devices, follow the basic OHOS Application Binary Interface (ABI) standards provided in this topic.
5
6
7## Byte Order and Word Width
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9OHOS ABIs always use little-endian, ILP32 for 32-bit systems, and LP64 for 64-bit systems.
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11
12## Procedure Call Standards
13
14The parameter transfer mode in function calls, register usage rules, and stack operation rules are defined. Different C++ compilers, operating systems, and architectures may use different calling rules. For details, see [Calling conventions for different C++ compilers and operating systems](https://www.agner.org/optimize/calling_conventions.pdf). For details about the architecture-specific call standards, see the following:
15
16- [Procedure Call Standard for the Arm<sup>®</sup> 32-bit Architecture (AArch32)](https://github.com/ARM-software/abi-aa/tree/main/aapcs32)
17
18- [Procedure Call Standard for the Arm<sup>®</sup> 64-bit Architecture (AArch64)](https://github.com/ARM-software/abi-aa/tree/main/aapcs64)
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20
21## C++ ABI
22
23OpenHarmony uses libc++ in the LLVM project as the C++ runtime library. It uses the libc++.so library for the underlying system and the libc++_shared.so library for applications, with the same set of code but different C++ namespaces. For details about the symbol mangling rules of C++, see [Itanium C++ ABI](https://itanium-cxx-abi.github.io/cxx-abi/).
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25
26## Floating-Point Format
27
28OpenHarmony uses IEE754 as the floating-point encoding format. For details about the definition of the long double format, see [Supported ABIs](#supported-abis).
29
30
31## Executable File Format
32
33OpenHarmony uses ELF as the binary file format of the entire system. For details about the format, see [System V Application Binary Interface](https://refspecs.linuxfoundation.org/elf/gabi4+/contents.html). For details about the format definition related to the CPU architecture, see the following:
34
35- [ELF for the Arm<sup>®</sup> Architecture](https://github.com/ARM-software/abi-aa/tree/main/aaelf32)
36
37- [ELF for the Arm<sup>®</sup> 64-bit Architecture (AArch64)](https://github.com/ARM-software/abi-aa/tree/main/aaelf64)
38
39
40## Supported ABIs
41
42This section describes the ABIs supported by OpenHarmony and their differences.
43
44
45### armeabi-v7a
46
47armeabi-v7a is developed on the [Application Binary Interface](https://developer.arm.com/Architectures/ABI) and applies to 32-bit ARMv7-A CPUs. It supports ARM processors Cortex-A5, Cortex-A7, Cortex-A8, Cortex-A9, Cortex-A12, Cortex-A15, and Cortex-A17. It also supports ARM32, Thumb-2, and VFPv3-D16 instructions.
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49
50This ABI uses **-mfloat-cpu=softfp** to enforce the floating-point rule in function calls. The compiler still uses hardware floating point instructions. Other extensions including Neon are optional in this ABI. For better compatibility, you are advised to use **-mfpu=softvfp** to compile the native libraries.
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52This ABI uses 64-bit long double (IEEE binary64).
53
54
55### arm64-v8a
56
57arm64-v8a is developed on the *Application Binary Interface (ABI) for the ARM Architecture*. It supports the AArch64 instruction set and the Neon feature by default.
58
59This ABI uses **-mfloat-cpu=softfp** to enforce the floating-point rule in function calls.
60
61This ABI uses 128-bit long double (IEEE binary128).
62
63
64### x86_64
65
66x86_64 is developed on Intel 64 and IA-32 ABI and supports MMX, SSE, SSE2, SSE3, SSSE3, and SSE4.1 instructions. For details about x86 specifications, see **System V Application Binary Interface** and **AMD64 Architecture Processor Supplement**.
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68This ABI uses 128-bit long double (IEEE binary128). Note that many x86 platforms use the float80 format, whereas OpenHarmony uses the 128-bit format.
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70
71## Specifying the ABI in the Architecture at Build Time
72
73
74### Setting in DevEco Studio
75
76In the C++ project of OpenHarmony, find the **buildOption/externalNativeOptions** field in the **build-profile.json5** file of the project where the C++ code is located, and add the **abiFilters** field.
77
78```json
79{
80    "abiType": 'stageMode',
81    "buildOption": {
82        "externalNativeOptions": {
83            "path": "./src/main/cpp/CMakeLists.txt",
84            "arguments": "",
85            "abiFilters": [
86                "armeabi-v7a",
87                "arm64-v8a"
88            ]
89        }
90    }
91}
92```
93
94
95### Setting in .cmake
96
97When you develop native code using the SDK, some common environment variables for cross compilation of OpenHarmony are defined in **build/cmake/ohos.toolchain.cmake**. The **OHOS_ARCH** variable defines the target ABI for build, which can be **arm64-v8a**, **armeabi-v7a**, and **x86_64**.
98