Python 3.3+Linux · macOSsocketfunction

socket.CMSG_LEN()

file descriptors and ancillary data

Returns the length of one ancillary data item without trailing padding, for use with sendmsg and recvmsg.

Syntax

socket.CMSG_LEN(length) -> int

The value is platform-specific (20 for 4 bytes on Linux x86-64), so compute it rather than hard-coding a number. For a recvmsg buffer size portable code uses CMSG_SPACE, not CMSG_LEN: with several items CMSG_LEN ignores padding, the data gets truncated (MSG_CTRUNC flag) and the passed descriptors are lost.

Examples

import socket
print(socket.CMSG_LEN(4))  # → ? 20 на Linux x86-64
print(socket.CMSG_LEN(4) <= socket.CMSG_SPACE(4))  # → True

See also

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