errno 93 (EPROTONOSUPPORT): meaning and diagnosis

Status / error code:		Category:

Constant name:
Source:		Operating System:
Description: the chosen protocol is not supported for the requested socket domain or socket type.

Could be also:
Constant	Type	OS
ENOATTR	errno	Mac
ENOTEMPTY	errno	Solaris
UNSUPPORTED_PROCESSOR	BugCheck Code	Windows

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ESTRPIPE	ENOATTR

EPROTONOSUPPORT

EPROTONOSUPPORT means the system cannot provide the protocol requested for the selected socket domain. It commonly appears during socket() creation when the code specifies a protocol number that is unavailable or incompatible with that domain and type. It can also appear later where a protocol-specific operation is attempted against a socket that cannot support it.

Keep this separate from related errors. EAFNOSUPPORT concerns the address family itself. EPROTOTYPE describes a protocol or peer type mismatch. ENOPROTOOPT says that a particular socket option is unknown at the specified level. The distinctions matter because they point to different configuration fields.

What to validate

Log the requested domain, type, and protocol numeric value at socket creation.
Prefer the tuple returned by getaddrinfo() instead of hard-coding a protocol number that may not exist in every environment.
Check container, kernel, or sandbox restrictions when code works on the host but not in the deployment target.
For protocol options, verify the option level and option name against the selected protocol rather than retrying the same call.

POSIX socket() · Linux socket(2) · Linux getsockopt(2)

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