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EBADMSG
EBADMSG is useful when an API expects a structured message, record, or protocol unit and the data cannot be interpreted as such. It is different from EMSGSIZE, which is about size, and from EILSEQ, which is about an invalid byte sequence for a character encoding.
The important diagnostic question is which layer declared the message invalid: kernel transport, remote filesystem, line discipline, message queue, cryptographic framing, or an application protocol parser. Retrying the same bytes rarely fixes EBADMSG; preserving the rejected frame and parser state is normally more valuable.
What to log
- The API that returned the errno and whether it works with datagrams, records, or typed messages.
- The length, flags, and peer identity for the rejected message.
- Protocol version, negotiated options, and checksum or authentication status when applicable.
- Whether the failure followed truncation, partial reads, or a decoder state reset.
POSIX errno.h · Linux errno(3) · Linux UAPI errno definitions
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