C++ Compatibility

In HarmonyOS, both the system library and application native library use C++ standard library (see libc++). However, the C++ standard library used by the system library is updated with the image version, and the C++ standard library used by the application native library is updated with the SDK used for compilation. Since both the C++ standard libraries have multiple major versions, the different update approach may cause ABI compatibility issues. To solve this problem, HarmonyOS differentiates the two C++ standard libraries as follows:

System library: uses libc++.so, which is released with the system image.
Application native library: uses libc++_shared.so, which is released with the application.

The two libraries use different C++ namespaces. libc++.so uses __h, and libc++_shared.so uses __n1.

The system and application cannot use the same C++ standard library. Native APIs are C APIs, which are used to isolate the C++ running environment. If the libc++_shared.so version in the HAR file is different from that of the application, incompatibility issues may occur. To solve this problem, use the same SDK version to update the HAR file.

Known C++ Compatibility Issues

If "symbol not found, s=__emutls_get_address" is reported when an application starts or when dlopen() is called, update the SDK of the application or HAR. This symbol is not provided by libc++_shared.so in the API version 9 or earlier, and is available since API version 11.

musl libc Dynamic Linker

[h2]Linker Namespace

Unlike the namespace in C++, a linker namespace (also referred to as ns) is a mechanism provided by the dynamic linker to isolate shared libraries within different namespaces. For example, the system native library can load the native library in the system directories, such as /system/lib64 or /vendor/lib64; a common application can load only the common application native library and NDK library, but cannot directly load the system native library.

The dynamic linker must be associated with a specific namespace when it loads the shared library specified by DT_NEEDED in compilation or calls dlopen to load a shared library.

HarmonyOS has the following types of linker namespaces:

Default ns: default namespace created for locating the .so files in /system/lib{abi} and /vendor/lib{abi} when the dynamic linker stats.
NDK ns: default namespace created for exposing the NDK interfaces in .so files in /system/lib{abi}/ndk when the dynamic linker stats.
App ns: namespace created when the application starts. Its search path is generally the application installation path (possibly a sandbox path), so that the application's .so files can be loaded.

The namespace mechanism restricts the invocation between the application native library and the system native library, as shown in the following figure.

The default ns and NDK ns can access all .so files of each other, but cannot access the .so files of the app ns.
The app ns can access all .so files of the NDK ns, but cannot access the .so files of the default ns.

[h2]rpath

Run-time path (rpath) is a mechanism for specifying the runtime search path of a shared library. This mechanism allows a runtime search path to be embedded in an executable file or shared library. The dynamic linker uses this path to find required libraries.

The namespace mechanism allows an application to load only the native libraries in its installation directory (for example, libs/arm64 on the ARM64 platform). If an application needs to load multiple native libraries, the native library in a new directory cannot be loaded if multiple loading paths are created. In this case, you can use the rpath mechanism to specify the search path during compilation.

For example, libhello.so in the application installation directory lib/arm64 depends on libworld.so in the newly created directory lib/arm64/module. Set rpath in the CMakeLists.txt file of the application and compile the file, and run readelf to view the rpath of libhello.so. As shown in the following figure, $ORIGIN indicates the path of libhello.so. The application can load libworld.so in the module directory during running.

SET(CMAKE_BUILD_WITH_INSTALL_RPATH TRUE)
SET(CMAKE_INSTALL_RPATH "\${ORIGIN}/module")

[h2]dlclose

You can use dlclose to truly unload a dynamic library.

[h2]symbol-version

symbol-version is a symbol retrieval mechanism provided by libc for symbol relocation in dynamic linking. It supports relocation of the symbols of different versions and helps solve the problem of duplicate symbols. For details, see LD Version Scripts (GNU Gnulib).

[h2]Fortified Check of the FD in select()

If the file descriptor (fd) passed in FD_SET or FD_CLR is not within the value range [0, 1024), abort crash will be triggered.

If the fd value passed in FD_ISSET is not within the value range [0, 1024), false will be returned.

[h2]Globalization

Since API version 12, locale in newlocale() and setlocale() can be set to C, C.UTF-8, en_US, en_US.UTF-8, zh_CN, or zh_CN.UTF-8. strtod_l, wcstod_l, and localeconv support the locale values zh_CN and zh_CN.UTF-8. Note that strtod_l() and wcstod_l() do not support conversion of hexadecimal numbers and hexadecimal floating-point numbers.

[h2]fdsan

File descriptor sanitizer (fdsan) helps detect the user-after-close and double-close issues of FDs.

[h2]Network Usage

Using RTM_GETLINK (the core message type for network interface information) through the send, sendto, sendmsg, getifaddrs, and if_nameindex APIs on sockets of the route netlink type poses security risks and is therefore not recommended. You are advised to use the C library ioctl API and the corresponding cmd values to query network interface information.

The meanings of HarmonyOS internal ioctl cmd values are as follows:

Name	Description
SIOCGIFINDEX	Obtains the network interface index.
SIOCGIFFLAGS	Obtains the flags (status) of a network interface.
SIOCGIFADDR	Obtains the address of a network interface.
SIOCGIFBRDADDR	Obtains the broadcast address of a network interface.
SIOCGIFNAME	Obtains the network interface name by index.
SIOCGIFCONF	Obtains the list of all network interfaces.

Signal Usage

To avoid conflicts with system reserved signals, comply with the following rules when using signals:

Signals 1 to 34 are reserved for the system and cannot be used.
Signals 35 to 45 have been occupied by internal system modules (such as memory, DFX, runtime, and system services) by API version 19. To avoid conflicts with system behaviors and unexpected problems, do not use these signals.
The values of SIGRTMIN and __libc_current_sigrtmin are 35, indicating the start number of real-time signals available to applications (applications can actually use only signals 46 and later).

The HarmonyOS internal signals are as follows.

ID	Name	Description	ID	Name	Description
1	SIGHUP	Terminal suspension control.	24	SIGXCPU	CPU time limit exceeded.
2	SIGINT	Interrupted.	25	SIGXFSZ	Maximum file size exceeded.
3	SIGQUIT	Keyboard exit.	26	SIGVTALRM	Virtual timer.
4	SIGILL	Invalid instruction.	27	SIGPROF	Profiling timer expired.
5	SIGTRAP	Debug breakpoint.	28	SIGWINCH	Terminal window size changed.
6	SIGABRT	Aborted.	29	SIGIO	I/O availability notification.
7	SIGBUS	Bus error.	30	SIGPWR	Power fault.
8	SIGFPE	Arithmetic exception.	31	SIGSYS	Invalid system call.
9	SIGKILL	Forcible termination.	32	SIGTIMER	Timer.
10	SIGUSR1	Custom signal 1.	33	SIGCANCEL	Thread canceled.
11	SIGSEGV	Invalid memory access.	34	SIGSYNCCALL	Synchronous call.
12	SIGUSR2	Custom signal 2.	35	MUSL_SIGNAL_NATIVE_REMOTE (SIGRTMIN + 0)	System reserved.
13	SIGPIPE	Pipe damaged.	36	MUSL_SIGNAL_HOOK (SIGRTMIN + 1)	System reserved.
14	SIGALRM	Alarm.	37	MUSL_SIGNAL_UNHOOK (SIGRTMIN + 2)	System reserved.
15	SIGTERM	Application termination request.	38	MUSL_SIGNAL_NATIVE_LOCAL (SIGRTMIN + 3)	System reserved.
16	SIGSTKFLT	Coprocessor stack error.	39	MUSL_SIGNAL_JSHEAP (SIGRTMIN + 4)	System reserved.
17	SIGCHLD	Child process exit/stop.	40	MUSL_SIGNAL_JSHEAP_PRIV (SIGRTMIN + 5)	System reserved.
18	SIGCONT	Continue the execution.	41	MUSL_SIGNAL_SAMPLE_STACK (SIGRTMIN + 6)	System reserved.
19	SIGSTOP	Forcible stop.	42	MUSL_SIGNAL_LEAK_STACK (SIGRTMIN + 7)	System reserved.
20	SIGTSTP	Stop inputting on the terminal.	43	MUSL_SIGNAL_RECYCLE_JEMALLOC (SIGRTMIN + 8)	System reserved.
21	SIGTTIN	Background terminal reading.	44	MUSL_SIGNAL_MEMCHECK (SIGRTMIN + 9)	System reserved.
22	SIGTTOU	Background terminal writing.	45	MUSL_SIGNAL_FDTRACK (SIGRTMIN + 10)	System reserved.
23	SIGURG	Socket has urgent data.	-	-	-

目录

C/C++ Library Mechanisms