errno 40 (EMSGSIZE): meaning and diagnosis

Status / error code:		Category:

Constant name:
Source:		Operating System:
Description: a message could not be sent because this socket requires it to fit atomically.

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

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ELOOP	EL3RST

EMSGSIZE

The message cannot be represented as one required unit

EMSGSIZE is significant for sockets and protocols that require an atomic message. The send operation cannot split the supplied payload into acceptable units under the socket's rules, so the kernel rejects it instead of silently sending only a prefix.

That makes this different from a stream write that merely completes partially. It is also different from ENOBUFS: EMSGSIZE is about the size or framing constraint of this message, while ENOBUFS concerns available buffering resources.

Useful measurements

Record the payload length, socket type, protocol, destination family, and whether the call used sendmsg() with several I/O vectors.
For a datagram protocol, compare the attempted payload with the path and protocol limits rather than assuming that an application-level chunk size is portable.
Check whether the application accidentally added headers, encryption overhead, or encapsulation after it calculated a maximum payload size.

What not to conclude

The error does not show that the peer received a malformed application message. The local kernel rejected the attempted send before it could form a permitted transmission. Retrying unchanged data only repeats the same size constraint.

References

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