See `ngx_write_channel` in `ngx_channel.c`[1]: cmsg.cm.cmsg_len = CMSG_LEN(sizeof(int)); cmsg.cm.cmsg_level = SOL_SOCKET; cmsg.cm.cmsg_type = SCM_R

See `ngx_write_channel` in `ngx_channel.c`[1]: cmsg.cm.cmsg_len = CMSG_LEN(sizeof(int)); cmsg.cm.cmsg_level = SOL_SOCKET; cmsg.cm.cmsg_type = SCM_RIGHTS; [...] ngx_memcpy(CMSG_DATA(&cmsg.cm), &ch->fd, sizeof(int)); [...] n = sendmsg(s, &msg, 0); edit: This article[2] also explains the way the master process sends commands to its workers. [1] https://github.com/nginx/nginx/blob/2a81e0556611188a1b9b3e1240a06928dcebc1a2/src/os/unix/ngx_channel.c#L41 https://github.com/nginx/nginx/blob/2a81e0556611188a1b9b3e12... [2] https://titanwolf.org/Network/Articles/Article?AID=6fb184f9-e06b-4b9a-bcf3-a62ce22cdd53#gsc.tab=0 https://titanwolf.org/Network/Articles/Article?AID=6fb184f9-...