musl libc

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.

OpenHarmony 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, OpenHarmony provides a link to libm, and the interfaces are defined in libc.
libdl: provides dynamic linker interfaces such as dlopen. Currently, OpenHarmony provides a link to libdl, and the interfaces are defined in libc.

musl Version

1.2.0
1.2.3 since OpenHarmony 4.0
1.2.5 since OpenHarmony 5.0

Supported Capabilities

OpenHarmony provides header files and library interfaces that align with C99, C11, and POSIX standards, though not fully compliant. It supports Armv7-A, Arm64, and x86_64 architectures.

To better accommodate the basic features of OpenHarmony devices, such as high performance, low memory usage, high security, lightweight design, and multi-device adaptability, the musl library has been optimized and enhanced, with interfaces unsuitable for embedded devices removed according.

New Capabilities

The dynamic loader supports isolation by namespace. The dynamic libraries that can be loaded by dlopen() are restricted by the system namespace. For example, the system 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.

Debugging Capabilities

The libc provides dynamic enabling of debug logging (disabled by default). The debug logs help you learn about exceptions of the libc. With this function, you only need to set param, which eliminates the need for rebuilding the libc. However, you are advised not to enable debug logging in official versions because it affects the running performance.

musl.log

Set musl.log.enable to true to enable printing of musl debug logs. You need to enable musl.log before printing other logs.

param set musl.log.enable true

Loader Logging

The loader starts applications and invokes dlopen and dlclose in the libc. To view exceptions during the loading process, you need to enable the loader logging function. The following describes common operations.

Enable the loader logging for all applications. Exercise caution when enabling this function because a large number of logs will be generated.

param set musl.log.ld.app true

Enable the loader logging for an application specified by {app_name}.

param set musl.log.ld.all false
param set musl.log.ld.app.{app_name} true

Enable the loader logging for all applications except the specified application.

param set musl.log.ld.all true
param set musl.log.ld.app.{app_name} false

musl API Difference

API	Description
epoll_create	In OpenHarmony 5.0, this API uses the same logic as in musl 1.2.3. Specifically, the API does not verify input parameters or process them if the input parameter value is less than or equal to 0. In the next version, this API will adopt the logic from musl 1.2.5, which includes input parameter verification. If the input parameter value is less than or equal to 0, the error code EINVAL will be returned.

API

Description

epoll_create

In OpenHarmony 5.0, this API uses the same logic as in musl 1.2.3. Specifically, the API does not verify input parameters or process them if the input parameter value is less than or equal to 0. In the next version, this API will adopt the logic from musl 1.2.5, which includes input parameter verification. If the input parameter value is less than or equal to 0, the error code EINVAL will be returned.

Character Encoding Formats Supported by iconv

The following lists the character encoding formats and aliases supported by musl.

NOTE

The character conversion can be successful only when the source character encoding format is correct and the target character encoding format is supported. musl does not support the following target encoding formats: gb18030, gbk, gb2312, 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