Atomic write on an unix socket?

Pipes\n\nYes the non-blocking capacity is usually 4KB, but for maximum portability you'd probably be better off using the PIPE_BUF constant. An alternative is to use non-blocking I/O.\n\nMore information than you want to know in man 7 pipe.\n\nUnix datagram sockets\n\nWrites using the send family of functions on datagram sockets are indeed guaranteed to be atomic. In the case of Linux, they're reliable as well, and preserve ordering. (which makes the recent introduction of SOCK_SEQPACKET a bit confusing to me) Much information about this in man 7 unix.\n\nThe maximum datagram size is socket-dependent. It's accessed using getsockopt/setsockopt on SO_SNDBUF. On Linux systems, it ranges between 2048 and wmem_max, with a default of wmem_default. For example on my system, wmem_default = wmem_max = 112640. (you can read them from /proc/sys/net/core) Most relevant documentation about this is in man 7 socket around the SO_SNDBUF option. I recommend you read it yourself, as the capacity doubling behavior it describes can be a bit confusing at first.\n\nPractical differences between stream and datagram\n\nStream sockets only work connected. This mostly means they can only communicate with one peer at a time. As streams, they're not guaranteed to preserve \"message boundaries\".\n\nDatagram sockets are disconnected. They can (theoretically) communicate with multiple peers at a time. They preserve message boundaries.\n\n[I suppose the new SOCK_SEQPACKET is in between: connected and boundary-preserving.]\n\nOn Linux, both are reliable and preserve message ordering. If you use them to transmit stream data, they tend to perform similarly. So just use the one that matches your flow, and let the kernel handle buffering for you.\n\nCrude benchmark comparing stream, datagram, and pipes:\n\n# unix stream 0:05.67\nsocat UNIX-LISTEN:u OPEN:/dev/null &\nuntil [[ -S u ]]; do :;done\ntime socat OPEN:large-file UNIX-CONNECT:u\n\n# unix datagram 0:05.12\nsocat UNIX-RECV:u OPEN:/dev/null &\nuntil [[ -S u ]]; do :;done\ntime socat OPEN:large-file UNIX-SENDTO:u\n\n# pipe 0:05.44\nsocat PIPE:p,rdonly=1 OPEN:/dev/null &\nuntil [[ -p p ]]; do :;done\ntime socat OPEN:large-file PIPE:p\n\n\nNothing statistically significant here. My bottleneck is likely reading large-file.",