Linux socket

Linux 系统的好工具： man

"man socket" or "man 2 socket"：

socket - create an endpoint for communication

int socket(int domain, int type, int protocol);

socket() creates an endpoint for communication and returns a descriptor.

1. The  "domain"  argument  specifies  a  communication domain; this selects the protocol family which will be used for communication:

       Name                Purpose                          Man page
       AF_UNIX, AF_LOCAL   Local communication              unix(7)
       AF_INET             IPv4 Internet protocols          ip(7)
       AF_INET6            IPv6 Internet protocols          ipv6(7)
       AF_IPX              IPX - Novell protocols
       AF_NETLINK          Kernel user interface device     netlink(7)
       AF_X25              ITU-T X.25 / ISO-8208 protocol   x25(7)
       AF_AX25             Amateur radio AX.25 protocol
       AF_ATMPVC           Access to raw ATM PVCs
       AF_APPLETALK        Appletalk                        ddp(7)
       AF_PACKET           Low level packet interface       packet(7)

2. The "socket" has the indicated type, which specifies the communication semantics:

       SOCK_STREAM     Provides sequenced, reliable, two-way, connection-based byte streams.  An out-of-band data transmission mechanism may be supported.


       SOCK_DGRAM      Supports datagrams (connectionless, unreliable messages of a fixed maximum length).


       SOCK_SEQPACKET  Provides a sequenced, reliable, two-way connection-based data transmission path for datagrams of fixed maximum length; a consumer is required to
                       read an entire packet with each input system call.


       SOCK_RAW        Provides raw network protocol access.


       SOCK_RDM        Provides a reliable datagram layer that does not guarantee ordering.


       SOCK_PACKET     Obsolete and should not be used in new programs; see packet(7).


       Some socket types may not be implemented by all protocol families; for example, SOCK_SEQPACKET is not implemented for AF_INET.


       Since  Linux  2.6.27,  the type argument serves a second purpose: in addition to specifying a socket type, it may include the bitwise OR of any of the following
       values, to modify the behavior of socket():


       SOCK_NONBLOCK   Set the O_NONBLOCK file status flag on the new open file description.  Using this flag saves extra calls to fcntl(2) to achieve the same result.


       SOCK_CLOEXEC    Set the close-on-exec (FD_CLOEXEC) flag on the new file descriptor.  See the description of the O_CLOEXEC flag in open(2) for reasons  why  this
                       may be useful.

3. The  "protocol"  specifies  a particular protocol to be used with the socket.  Normally only a single protocol exists to support a particular socket type within a
    given protocol family, in which case protocol can be specified as 0.  However, it is possible that many protocols may exist, in which case a particular protocol
    must  be  specified  in  this  manner.

4. man 7 unix

unix - sockets for local interprocess communication

The AF_UNIX (also known as AF_LOCAL) socket family is used to communicate between processes on the same machine efficiently.

Traditionally, UNIX domain sockets can be either unnamed, or bound to a filesystem pathname (marked as being of type socket).???

 Linux also supports an abstract namespace which  is  independent  of the filesystem.???

5. man 7 ip

ip - Linux IPv4 protocol implementation

Linux implements the Internet Protocol, version 4, described in RFC 791 and RFC 1122.

An IP socket is created by calling the socket(2) function as socket(AF_INET, socket_type, protocol).

6. man 7 netlink

netlink - communication between kernel and user space (AF_NETLINK)

Netlink is used to transfer information between kernel and user-space processes.

7. man 7 packet

packet - packet interface on device level

Packet  sockets  are  used  to  receive or send raw packets at the device driver (OSI Layer 2) level.

They allow the user to implement protocol modules in user space on top of the physical layer.

8.