EC
EOPNOTSUPP
Linux / POSIXERRORCommonNetworkHIGH confidence

Operation Not Supported

Production Risk

Programming error; operation not valid for this socket/file type.

What this means

EOPNOTSUPP (errno 95) is returned when an operation is not supported for the given object type. Commonly seen on sockets (e.g., accept() on SOCK_DGRAM) and filesystems (e.g., fcntl on a FIFO).

Why it happens
  1. 1Calling accept() on a SOCK_DGRAM socket
  2. 2listen() on a SOCK_DGRAM socket
  3. 3Requesting an unsupported filesystem operation (e.g., FALLOC_FL_COLLAPSE_RANGE on ext3)
How to reproduce

accept() called on a UDP socket.

trigger — this will error
trigger — this will error
int sockfd = socket(AF_INET, SOCK_DGRAM, 0);
// accept() is only valid for connection-oriented sockets
accept(sockfd, NULL, NULL);
// Returns -1, errno = EOPNOTSUPP

expected output

accept: Operation not supported (EOPNOTSUPP)

Fix

Use the correct socket type for the operation

WHEN When accept()/listen() return EOPNOTSUPP

Use the correct socket type for the operation
// Use SOCK_STREAM for connection-oriented operations
int sockfd = socket(AF_INET, SOCK_STREAM, 0);
bind(sockfd, (struct sockaddr*)&addr, sizeof(addr));
listen(sockfd, 10);
int client = accept(sockfd, NULL, NULL);

Why this works

accept() and listen() require SOCK_STREAM (TCP) or SOCK_SEQPACKET; not valid for SOCK_DGRAM.

Sources
Official documentation ↗

Linux Programmer Manual accept(2)

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