Sockets: Network IPC
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Internet Socket
UNIX Domain Socket
Sockets
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Allows processes to communicate
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Process on the same machine
Process on different machines connect via a
network
Sockets are identified by socket descriptors
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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);
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domain defines the address family
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AF_INET – IPv4
AF_INET6 – IPv6
AF_UNIX – UNIX domain (covered later)
AF_UNSPEC – unspecified
Sockets
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type defines the type of socket
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SOCK_DGRAM
SOCK_RAW
SOCK_SEQPACKET
SOCK_STREAM
protocol defines which of the available
protocols to use for this socket
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We usually use 0 for the default protocol
AF_INET + SOCK_STREAM  TCP
AF_INET + SOCK_DGRAM  UDP
Byte Ordering
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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
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4 conversion functions for TCP/IP
uint32_t
uint16_t
uint32_t
uint16_t
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htonl(uint32_t hostlong);
htons(uint16_t hostshort);
ntohl(uint32_t netlong);
ntohs(uint16_t netshort);
h – host
n - network
Binding a Socket
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Binds a network address to a socket
Used by server so a remote computer
can connect to the socket
int bind(int sockfd, const struct sockaddr
*my_addr, socklen_t addrlen);
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sockfd socket descriptor
addrlen size in bytes of struct pointed to by
my_addr
Binding a Socket
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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 by
struct 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*/
};
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listen
int listen(int sockfd, int backlog);
 Causes the server to wait for an
incoming connection
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sockfd socket descriptor of an open socket
backlog suggested max connection
requests to queue
accept
int accept(int sockfd, struct sockaddr *addr, socklen_t *addrlen);
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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
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If we don’t care about the identity of the
client, we can pass in NULL for addr and
addrlen
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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 nonblocking
connect
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Used by client to connect to a server
int connect(int sockfd, const struct sockaddr *serv_addr,
socklen_t addrlen);
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sockfd – socket descriptor of an open
socket
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sockaddr – address of server we wish to
connect to
addrlen – size in bytes of the struct pointed
to by sockaddr
Summery of Connecting
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Server
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socket
bind
listen
accept
Client
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socket
connect
send and recv
ssize_t send(int s, const void *buf, size_t len, int flags);
ssize_t recv(int s, void *buf, size_t len, int flags);
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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
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sendto and recvfrom used with
connectionless sockets (SOCK_DGRAM)
Server
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socket, bind, recvfrom
Client
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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
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s is a socket descriptor
how can be SHUT_RD, SHUT_WR or SHUT_RDWR
Network Address Functions
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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-andnumbers notation
char *inet_ntoa(struct in_addr in);
 inet_ntop - converts a network address
into a dots-and-numbers address
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const char *inet_ntop(int af, const void *src,
char *dst, socklen_t cnt);
Network Address Functions
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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 */
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This function marked obsolete by POSIX.1
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See page 555 for getaddrinfo function
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Peer Name
int getpeername(int s, struct sockaddr *name,
socklen_t *namelen);
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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
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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 */
};
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UNIX Domain Sockets
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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