errno 38 (ENOSYS): meaning and diagnosis

Status / error code:		Category:

Constant name:
Source:		Operating System:
Description: the requested Linux system call number or kernel facility is not implemented.

Could be also:
Constant	Type	OS
ERROR_HANDLE_EOF	Win32 error	Windows
KERN_LOCK_SET_DESTROYED	Kern return	Mac
ippStsSymKernelExpected	Intel Ipp Status	Any
ENOTSOCK	errno	Mac
EL2NSYNC	errno	Solaris
ENAMETOOLONG	errno	Windows
CDFS_FILE_SYSTEM	BugCheck Code	Windows

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ENOLCK	ENOTEMPTY

ENOSYS

ENOSYS is not just a high-level “feature missing” result. The Linux UAPI header comments describe it as the value returned by architecture syscall entry code when a user program attempts to call a syscall number that does not exist. The same name can also be exposed by compatibility layers when a requested kernel facility is unavailable.

For a real syscall failure, preserve the syscall number, architecture, libc wrapper, kernel version, container/seccomp environment, and whether the process is running under an emulator. A retry loop will not make an absent syscall appear. The useful fix is normally to choose the supported syscall/API path for that kernel and ABI, or to detect the feature at runtime before using it.

What to distinguish

ENOSYS: the syscall/facility is not implemented for the current ABI or environment.
EINVAL: the syscall exists, but the particular arguments violate its contract.
ENOTSUP or EOPNOTSUPP: an operation exists but is unsupported for this object or socket.

Linux errno(3) · Linux UAPI errno definitions · Linux syscall(2)

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