EC
ENOTSOCK
Linux / POSIXERRORCommonNetworkHIGH confidence

Socket Operation on Non-Socket

Production Risk

Programming error; usually a wrong-variable or scope bug.

What this means

ENOTSOCK (errno 88) is returned when a socket operation (bind, connect, accept, send, recv, etc.) is called on a file descriptor that is not a socket.

Why it happens
  1. 1Passing a regular file fd to connect(), bind(), or recv()
  2. 2Using a closed socket fd that was reused for another file type
  3. 3Wrong variable holding the fd — off-by-one or scope error
How to reproduce

Calling recv() on a file descriptor opened as a regular file.

trigger — this will error
trigger — this will error
int fd = open("/tmp/myfile", O_RDWR);
// Mistakenly used fd where sockfd is expected
recv(fd, buf, sizeof(buf), 0);
// Returns -1, errno = ENOTSOCK

expected output

recv: Socket operation on non-socket (ENOTSOCK)

Fix

Verify the fd is a socket

WHEN When debugging ENOTSOCK

Verify the fd is a socket
// Check if fd is a socket
struct stat st;
fstat(sockfd, &st);
if (!S_ISSOCK(st.st_mode)) {
  // sockfd is not a socket — trace how it was opened
}

Why this works

S_ISSOCK() checks the file type; use fstat() to diagnose which fd was passed incorrectly.

What not to do

✕ Reuse the same variable name for both socket and file fds

This is a common source of ENOTSOCK — use distinct names like sockfd and filefd.

Sources
Official documentation ↗

Linux Programmer Manual recv(2)

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