What does errno 40 (ELOOP) mean?

Status / error code:		Category:

Constant name:
Source:		Operating System:
Description: Path resolution found a symbolic-link cycle or exceeded the link-traversal limit.

Could be also:
Constant	Type	OS
KERN_LOCK_OWNED	Kern return	Mac
ippStsWrongIntersectVOI	Intel Ipp Status	Any
EMSGSIZE	errno	Mac
EL3RST	errno	Solaris
ENOSYS	errno	Windows
CORRUPT_ACCESS_TOKEN	BugCheck Code	Windows

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ENOLCK	EMSGSIZE

ELOOP

Two meanings of the same errno

ELOOP usually means that resolving a pathname followed too many symbolic links, often because one link eventually points back to an earlier location. Linux limits the number of symlink traversals during path resolution, so a very long acyclic chain can fail as well. The error can also be intentional: open() with O_NOFOLLOW may use it to reject a symbolic link in the final path component.

This makes the error useful for security-sensitive code. A service that accepts path input should not “fix” it by silently following more links or stripping prefixes. That can turn a malformed path into a path traversal problem, especially in writable directories shared with less-trusted users.

What to inspect

Resolve links component by component and preserve the chain in logs without exposing sensitive path data unnecessarily.
Check deployment symlinks, release pointers, mount aliases, and generated cache paths for cycles.
Use directory file descriptors and explicit no-symlink resolution policies when an operation must remain beneath a trusted root.
Treat an O_NOFOLLOW failure as a policy result, not as a transient filesystem error.

References

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