Sockets: Network IPC

Internet Socket UNIX Domain Socket

Sockets Allows processes to communicate

Process on the same machine Process on different machines connect via a

network Sockets are identified by socket

descriptors Implemented with file descriptors in UNIX Not all functions that work with file

descriptors work with sockets. See page 548 for which ones do work

Sockets

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

domain defines the address family AF_INET – IPv4 AF_INET6 – IPv6 AF_UNIX – UNIX domain (covered

later) AF_UNSPEC – unspecified

Sockets type defines the type of socket

SOCK_DGRAM SOCK_RAW SOCK_SEQPACKET SOCK_STREAM

protocol defines which of the available protocols to use for this socket We usually use 0 for the default protocol AF_INET + SOCK_STREAM TCP AF_INET + SOCK_DGRAM UDP

Byte Ordering

In order to connect to a remote computer and use a socket, we need to use its address

LINUX is little-endian but TCP/IP uses big-endian byte ordering

Byte Ordering 4 conversion functions for TCP/IPuint32_t htonl(uint32_t hostlong);uint16_t htons(uint16_t hostshort);uint32_t ntohl(uint32_t netlong);uint16_t ntohs(uint16_t netshort);

h – host n - network

Binding a Socket

Binds a network address to a socket Used by server so a remote

computer can connect to the socketint bind(int sockfd, const struct sockaddr

*my_addr, socklen_t addrlen); sockfd socket descriptor addrlen size in bytes of struct pointed

to by my_addr

Binding a Socket my_addr points to a struct that

represents an address on the network Format of struct depends on specified

domain. So that we can use a single bind function, it is typecast to the generic

struct sockaddr { sa_family_t sa_family; char sa_data[]; …};

Binding a Socket IPv4 addresses are represented bystruct sockaddr_in { sa_family_t sin_family; /*addr family*/ in_port_t sin_port; /* port number */ struct in_addr sin_addr; /*IPv4 addr*/ unsigned char sin_zero[8]; /*LINUX only*/};struct in_addr { in_addr_t s_addr; /*IPv4 address*/};

listen

int listen(int sockfd, int backlog); Causes the server to wait for an

incoming connection sockfd socket descriptor of an open

socket backlog suggested max connection

requests to queue

acceptint accept(int sockfd, struct sockaddr *addr, socklen_t *addrlen);

Creates a new socket from those queued by listen function

Returns new socket descriptor on success or -1 on failure

Newly created socket used for send and recv

Old socket that we called listen on still exists waiting for more incoming connections

accept If we don’t care about the identity of the

client, we can pass in NULL for addr and addrlen

Otherwise, addr should point to a buffer and addrlen should be the size of the buffer in bytes

accept function blocks if no pending connect requests unless the socket is created non-blocking

connect

Used by client to connect to a serverint connect(int sockfd, const struct sockaddr *serv_addr,

socklen_t addrlen);

sockfd – socket descriptor of an open socket

sockaddr – address of server we wish to connect to

addrlen – size in bytes of the struct pointed to by sockaddr

Summery of Connecting Server

socket bind listen accept

Client socket connect

send and recvssize_t send(int s, const void *buf, size_t len, int flags);ssize_t recv(int s, void *buf, size_t len, int flags);

s is the socket descriptor for an open and connected socket

buf is a buffer of information to send or an empty buffer to receive information

len is the size of the buffer in bytes flags – zero or an OR of MSG_EOR (end of

record), MSG_OOB (out-of-band data) See page 565 for full list

sendto and recvfrom

sendto and recvfrom used with connectionless sockets (SOCK_DGRAM)

Server socket, bind, recvfrom

Client socket, sendto

shutdown and close close(sfd); will not deallocate the socket

until we close the last descriptor that references it (we may have several)

int shutdown(int s, int how); Use shutdown to force a full or partial

closure of a socket s is a socket descriptor how can be SHUT_RD, SHUT_WR or

SHUT_RDWR

Network Address Functions

inet_addr - converts an IP address in numbers-and-dots notation into an unsigned long (in_addr_t). Note: this function is deprecated!

inet_aton - converts an IP address in numbers-and-dots notation into an in_addr struct

int inet_aton(const char *cp, struct in_addr *inp);

Network Address Functions

inet_ntoa - converts an IP address in an in_addr struct into dots-and-numbers notation

char *inet_ntoa(struct in_addr in); inet_ntop - converts a network

address into a dots-and-numbers address

const char *inet_ntop(int af, const void *src, char *dst, socklen_t cnt);

Network Address Functions

inet_pton - converts a network address in numbers-and-dots notion into a network address structure

int inet_pton(int af, const char *src, void *dst);

Network Addresses From Host Names

To get an address from a host name we need to obtain it from DNS

struct hostent *gethostbyname(const char *name);struct hostent { char *h_name; /* official name of host */ char **h_aliases; /* alias list */ int h_addrtype; /* host address type */ int h_length; /* length of address */ char **h_addr_list; /* list of addresses */};#define h_addr h_addr_list[0] /* for backward

compatibility */ This function marked obsolete by POSIX.1 See page 555 for getaddrinfo function

Peer Name

int getpeername(int s, struct sockaddr *name, socklen_t *namelen);

Sets sockaddr with the name of the connect peer. Can be used by hosts after calling accept to get the name of a client

Hostname

int gethostname(char *name, size_t len);

Retrieves the hostname of the system running the process

UNIX Domain Sockets

Socket for communicating with another process on the same machine only

Provides an optimization since there is no network overhead

UNIX Domain Sockets

Uses sockaddr_un structure instead of sockaddr_in

struct sockaddr_un { sa_family_t sun_family; /* AF_UNIX

*/ char sun_path[108]; /* pathname */};

UNIX Domain Sockets

When the socket is bound a new special file (type “s”) corresponding to sun_path is created

This file is NOT automatically deleted, so we should be careful to unlink it

If bind finds the file already exists, it will fail