1# musl libc
2
3## Overview
4The C standard library (libc) provides standard header files and common library implementations (such as input/output processing and string handling) in C language programming.
5
6OpenHarmony uses musl as the libc. musl is a lightweight, fast, simple, and free open-source libc. For details, see [musl libc Reference Manual](http://musl.libc.org/manual.html).
7
8For details about the differences between musl and glibc, see [Functional differences from glibc](https://wiki.musl-libc.org/functional-differences-from-glibc.html).
9
10## libc Components
11
12C11 is implemented by [libc, libm, and libdl](https://en.cppreference.com/w/c/header).
13
14- libc: provides thread-related interfaces and a majority of standard interfaces.
15
16- libm: provides mathematical library interfaces. Currently, OpenHarmony provides a link to libm, and the interfaces are defined in libc.
17
18- libdl: provides dynamic linker interfaces such as dlopen. Currently, OpenHarmony provides a link to libdl, and the interfaces are defined in libc.
19
20## musl Version
21
22- 1.2.0
23- 1.2.3 since OpenHarmony 4.0
24
25- 1.2.5 since OpenHarmony 5.0
26
27## Supported Capabilities
28OpenHarmony provides header files and library interfaces that are compatible (not fully compatible) with C99, C11, and POSIX, and supports Armv7-A, Arm64, and x86_64 architectures.
29
30To better adapt to the basic features of OpenHarmony devices, such as high performance, low memory, high security, lightweight, and multi-form support, OpenHarmony has optimized the musl library and removed the interfaces that are not applicable to embedded devices.
31
32### New Capabilities
331. 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.
342. **dlclose()** can be used to unload a dynamic library. This capability is not supported by musl.
353. The symbol-versioning is supported.
364. **dlopen()** can directly load uncompressed files in a .zip package.
37
38### Debugging Capabilities
39The 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.
40
41#### musl.log
42Set **musl.log.enable** to **true** to enable printing of musl debug logs. You need to enable musl.log before printing other logs.
43```
44param set musl.log.enable true
45```
46
47#### Loader Logging
48The 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.
49* Enable the loader logging for all applications. Exercise caution when enabling this function because a large number of logs will be generated.
50```
51param set musl.log.ld.app true
52```
53* Enable the loader logging for an application specified by {app_name}.
54```
55param set musl.log.ld.all false
56param set musl.log.ld.app.{app_name} true
57```
58* Enable the loader logging for all applications except the specified application.
59```
60param set musl.log.ld.all true
61param set musl.log.ld.app.{app_name} false
62```
63## musl API Difference
64
65| API         | Description                                                                                        |
66|:--               |    :--                                                                                       |
67| 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.|
68
69## Character Encoding Formats Supported by iconv
70
71The following lists the character encoding formats and aliases supported by musl.
72> **NOTE**
73>
74> The character conversion can be successful only when the source character encoding format is correct and the target character encoding format is supported.
75> musl does not support the following target encoding formats: gb18030, gbk, gb2312, big5, and euckr.
76
77| Encoding Format| Alias                  | Supported by musl|
78|:--      |    :--                   |  :--:  |
79|utf8     |                          |  Yes  |
80|wchart   |                          |  Yes  |
81|ucs2be   |                          |  Yes  |
82|ucs2le   |                          |  Yes  |
83|utf16be  |                          |  Yes  |
84|utf16le  |                          |  Yes  |
85|ucs4be   |utf32be                   |  Yes  |
86|ucs4le   |utf32le                   |  Yes  |
87|ascii    |usascii, iso646, iso646us |  Yes  |
88|utf16    |                          |  Yes  |
89|ucs4     |utf32                     |  Yes  |
90|ucs2     |                          |  Yes  |
91|eucjp    |                          |  Yes  |
92|shiftjis |sjis, cp932               |  Yes  |
93|iso2022jp|                          |  Yes  |
94|gb18030  |                          |  Yes  |
95|gbk      |                          |  Yes  |
96|gb2312   |                          |  Yes  |
97|big5     |bigfive, cp950, big5hkscs |  Yes  |
98|euckr    |ksc5601, ksx1001, cp949   |  Yes  |
99|iso88591 |latin1                    |  Yes  |
100|iso88592 |                          |  Yes  |
101|iso88593 |                          |  Yes  |
102|iso88594 |                          |  Yes  |
103|iso88595 |                          |  Yes  |
104|iso88596 |                          |  Yes  |
105|iso88597 |                          |  Yes  |
106|iso88598 |                          |  Yes  |
107|iso88599 |                          |  Yes  |
108|iso885910|                          |  Yes  |
109|iso885911|tis620                    |  Yes  |
110|iso885913|                          |  Yes  |
111|iso885914|                          |  Yes  |
112|iso885915|latin9                    |  Yes  |
113|iso885916|                          |  Yes  |
114|cp1250   |windows1250               |  Yes  |
115|cp1251   |windows1251               |  Yes  |
116|cp1252   |windows1252               |  Yes  |
117|cp1253   |windows1253               |  Yes  |
118|cp1254   |windows1254               |  Yes  |
119|cp1255   |windows1255               |  Yes  |
120|cp1256   |windows1256               |  Yes  |
121|cp1257   |windows1257               |  Yes  |
122|cp1258   |windows1258               |  Yes  |
123|koi8r    |                          |  Yes  |
124|koi8u    |                          |  Yes  |
125|cp437    |                          |  Yes  |
126|cp850    |                          |  Yes  |
127|cp866    |                          |  Yes  |
128|cp1047   |ibm1047                   |  Yes  |
129
130## musl APIs Not Supported
131
132[Native API Symbols Not Exported](musl-peculiar-symbol.md)
133
134[Restricted musl libc APIs](guidance-on-ndk-libc-interfaces-affected-by-permissions.md)
135
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