Overview
The C standard library (libc) provides standard header files and common library implementations (such as input/output processing and string handling) in C language programming.
HarmonyOS uses musl as the libc. musl is a lightweight, fast, simple, and free open-source libc. For details, see musl libc Reference Manual.
For details about the differences between musl and glibc, see Functional differences from glibc.
libc Components
C11 is implemented by libc, libm, and libdl.
libc: provides thread-related interfaces and a majority of standard interfaces.
libm: provides mathematical library interfaces. Currently, HarmonyOS provides a link to libm, and the interfaces are defined in libc.
libdl: provides dynamic linker interfaces such as dlopen. Currently, HarmonyOS provides a link to libdl, and the interfaces are defined in libc.
Supported Capabilities
Provides header files and library function APIs that are compatible with the C99, C11, and POSIX standards, but not fully compatible. Currently, it supports three architectures: armv7a, arm64, and x86_64.
To better adapt to the basic characteristics of HarmonyOS devices, including high performance, low memory usage, high security, lightweight design, and support for multiple device forms, the musl open-source library has been optimized and enhanced, and APIs that are not suitable for embedded devices have been trimmed.
[h2]New Capabilities
- The dynamic loader supports namespace isolation. Dynamic libraries loaded by an application through dlopen are restricted by the system namespace (for example, system-side dynamic libraries cannot be opened).
- dlclose() can be used to unload a dynamic library. This capability is not supported by musl.
- The symbol-versioning is supported.
- dlopen() can directly load uncompressed files in a .zip package.
- Supports C API compatibility version protection.
APIAVAILABLE Compatibility Version Protection
To use the APIAVAILABLE feature, you must be thoroughly familiar with mechanisms such as strong symbols, weak symbols, and weak libraries, and strictly follow the steps in the guide. Otherwise, runtime crashes may occur.
The ease of use is being optimized. Before the optimization is complete, it is recommended that you use dlopen and dlsym to resolve compatibility issues.
| Macro Definition Name | Description |
|---|---|
| APIAVAILABLE(maj, min, patch) | Used together with the compiler to ensure the compatibility and stability of applications across different system versions. Through compile-time and runtime condition checks, it prevents crashes caused by calling nonexistent APIs on lower system versions. When using an API higher than the target distribution version (compatibleSdkVersion), you must use this macro for compatibility protection and provide a reasonable fallback solution. The input parameter maj is the major version number (value range 0-99), min is the minor version number (value range 0-99), and patch is the patch version number (value range 0-99). Since: 22 |
musl API Difference
| API | Description |
|---|---|
| epoll_create | In the current HarmonyOS musl repository, epoll_create does not validate the size input parameter and does not distinguish whether size is less than or equal to 0. The actual behavior is as follows: when size <= 0, the call may still create an epoll instance successfully. This behavior differs from musl community v1.2.5 and later versions. The musl community requires that size be greater than 0, and the API behaves as follows: when size <= 0, epoll_create should fail, return -1, and set errno to EINVAL. If developers need to be compatible with the musl community v1.2.5+ behavior, they are advised to ensure that size > 0 before calling epoll_create, or to use epoll_create1 instead. |
Character Encoding Formats Supported by iconv
The character set encoding formats supported by musl, and the supported aliases.
When performing character set encoding format conversion, use the correct source character set encoding format, and the target character set encoding format must support the characters to be converted. Otherwise, the conversion fails.
In musl, converting the source character set encoding format into the following two target character set encoding formats is not supported: big5 and euckr.
| Encoding Format | Alias | Supported by musl |
|---|---|---|
| utf8 | Yes | |
| wchart | Yes | |
| ucs2be | Yes | |
| ucs2le | Yes | |
| utf16be | Yes | |
| utf16le | Yes | |
| ucs4be | utf32be | Yes |
| ucs4le | utf32le | Yes |
| ascii | usascii, iso646, iso646us | Yes |
| utf16 | Yes | |
| ucs4 | utf32 | Yes |
| ucs2 | Yes | |
| eucjp | Yes | |
| shiftjis | sjis, cp932 | Yes |
| iso2022jp | Yes | |
| gb18030 | Yes | |
| gbk | Yes | |
| gb2312 | Yes | |
| big5 | bigfive, cp950, big5hkscs | Yes |
| euckr | ksc5601, ksx1001, cp949 | Yes |
| iso88591 | latin1 | Yes |
| iso88592 | Yes | |
| iso88593 | Yes | |
| iso88594 | Yes | |
| iso88595 | Yes | |
| iso88596 | Yes | |
| iso88597 | Yes | |
| iso88598 | Yes | |
| iso88599 | Yes | |
| iso885910 | Yes | |
| iso885911 | tis620 | Yes |
| iso885913 | Yes | |
| iso885914 | Yes | |
| iso885915 | latin9 | Yes |
| iso885916 | Yes | |
| cp1250 | windows1250 | Yes |
| cp1251 | windows1251 | Yes |
| cp1252 | windows1252 | Yes |
| cp1253 | windows1253 | Yes |
| cp1254 | windows1254 | Yes |
| cp1255 | windows1255 | Yes |
| cp1256 | windows1256 | Yes |
| cp1257 | windows1257 | Yes |
| cp1258 | windows1258 | Yes |
| koi8r | Yes | |
| koi8u | Yes | |
| cp437 | Yes | |
| cp850 | Yes | |
| cp866 | Yes | |
| cp1047 | ibm1047 | Yes |