Restricted musl libc APIs

Overview

You may need to use musl libc APIs when developing your app with the NDK. However, some musl libc APIs cannot be used due to system or environment restrictions. Errors may occur when some musl libc APIs are used with DevEco Studio. This topic helps you learn about the musl libc APIs that cannot be used with the NDK.

Scenarios

The use of musl libc APIs may be restricted by the following factors:

The use of the musl libc API is restricted by the secure computing (seccomp) mechanism. For details about the seccomp mechanism, see Seccomp Policy Development.

A common error caused by seccomp is as follows:
- Top function is the musl function in stacktrace.
- The reason is "signal:SIGSYS", as shown in the following example:
```
Process name:com.example.myapplication
Reason:Signal:SIGSYS(UNKNOWN)
Fault thread Info:
Tid:13893, Name:e.myapplication
#00 pc 000a5d30 /system/lib/ld-musl-arm.so.1(sethostname+16)(584c9d0a0e9000497bb0d66799a9526a)
#01 pc 00002f68 /data/storage/el1/bundle/libs/arm/libentry.so(test()+64)
```
The API cannot be used because it is a kernel API that is not exposed.
The use of the musl libc API is restricted by SELinux. For details about SELinux, see OpenHarmony SELinux Overview.
The use of the musl libc API is restricted by the sandbox mechanism. For details about the sandbox mechanism, see Application Sandbox.
The musl libc API is a null implementation or failed by default.
The API can be executed after special permissions are granted.

Restricted musl libc APIs

The following table lists the musl libc APIs that cannot be used.

Restricted By	Header File	Symbol
seccomp	sys/fsuid.h	setfsgid
seccomp	sys/fsuid.h	setfsuid
seccomp	unistd.h	setgid
seccomp	unistd.h	setegid
seccomp	unistd.h	setreuid
seccomp	unistd.h	setregid
seccomp	unistd.h	setresgid
seccomp	unistd.h	setgroups
seccomp	unistd.h	chroot
seccomp	unistd.h	sethostname
seccomp	unistd.h	setdomainname
seccomp	unistd.h	acct
seccomp	sys/xattr.h	setxattr
seccomp	time.h	clock_settime
seccomp	time.h	settimeofday
seccomp	sys/stat.h	mknod
seccomp	sys/stat.h	mknodat
seccomp	sys/stat.h	mkfifo
seccomp	fcntl.h	name_to_handle_at
seccomp	fcntl.h	open_by_handle_at
seccomp	sys/klog.h	klogctl
seccomp	sys/swap.h	swapon
seccomp	sys/swap.h	swapoff
seccomp	sys/sem.h	semctl
seccomp	sys/sem.h	semop
seccomp	sys/sem.h	semtimedop
seccomp	sys/mount.h	mount
seccomp	sys/mount.h	umount2
seccomp	sys/mount.h	umount
seccomp	sys/msg.h	msgctl
seccomp	sys/msg.h	msgget
seccomp	sys/msg.h	msgrcv
seccomp	sys/msg.h	msgsnd
seccomp	grp.h	initgroups
seccomp	grp.h	init_module
seccomp	sched.h	unshare
seccomp	sched.h	setns
seccomp	None	pivot_root
Kernel API not exposed	sys/fanotify.h	fanotify_init
Kernel API not exposed	sys/fanotify.h	fanotify_mark
Kernel API not exposed	syslog.h	syslog
Kernel API not exposed	syslog.h	vsyslog
SELinux	pty.h	forkpty
SELinux	pty.h	openpty
SELinux	stdlib.h	ptsname
SELinux	stdlib.h	ptsname_r
SELinux	stdlib.h	posix_openpt
SELinux	stdlib.h	unlockpt
SELinux	sys/shm.h	shmget
SELinux	sys/shm.h	shmat
SELinux	sys/shm.h	shmdt
SELinux	sys/shm.h	shmctl
SELinux	sys/sem.h	semget
SELinux	stdio.h	popen
SELinux	stdio.h	pclose
SELinux	unistd.h	tcgetpgrp
SELinux	unistd.h	tcsetpgrp
SELinux	unistd.h	link
SELinux	unistd.h	linkat
SELinux	unistd.h	readlink
SELinux	unistd.h	readlinkat
SELinux	unistd.h	symlink
SELinux	unistd.h	symlinkat
SELinux	sys/stat.h	mkfifoat
SELinux	termios	tcgetattr
SELinux	termios	tcsetattr
SELinux	termios	tcsendbreak
SELinux	termios	tcdrain
SELinux	termios	tcflush
SELinux	termios	tcflow
SELinux	termios	tcgetsid
SELinux	net/if.h	if_indextoname
SELinux	net/if.h	if_nametoindex
Sandbox	stdio.h	tmpfile
Sandbox	stdio.h	tmpfile64
Sandbox	nl_type.h	catgets
Sandbox	nl_type.h	catclose
Null implementation or failed by default	utmp.h	setutent
Null implementation or failed by default	utmp.h	pututline
Null implementation or failed by default	utmp.h	getutent
Null implementation or failed by default	utmp.h	utmpname
Null implementation or failed by default	unistd.h	brk
Null implementation or failed by default	stdio_ext.h	__fsetlocking
Null implementation or failed by default	netdb.h	getnetbyaddr
Null implementation or failed by default	netdb.h	getnetbyname
CAP_SYS_ADMIN permission required	None	pivot_root