1.1 Introduction
1.2 UNIX Domain Protocols
1.3 Socket Addresses

2.1 Socket()
2.2 Socketpair()
2.3 Bind()
2.4 Connect()
2.5 Listen()
2.6 Accept()
2.7 Send(), Sendto(), Recv() and Recvfrom()
2.8 Close()

5.1 Readv() and Writev()
5.2 Recvmsg() and Sendmsg()
5.3 Getpeername()
5.4 Getsockname()
5.5 Shutdown()

1.1 Introduction

Berkeley sockets are one of the most prevalent communication APIs in UNIX. It is developed for the C language. Some points to be considered for network programming are:

1. The network program should know what role it is playing, client or server.
2. The connection can be of two types, connection-oriented or connection-less.
3. A network application must know the names of its peer processes to verify that those processes have to authority to request services.
4. There are a lot of parameters required for a network connection like protocol, local address, local process, foreign address and foreign process.
5. Support for multiple protocols must be provided e.g. TCP/IP, XNS, etc.

1.2 UNIX Domain Protocols

1. The protocol is same as before PF_UNIX.
2. The local and foreign address are zero as they are in the same system.
3. The local and foreign processes are specified as pathnames.

1.3 Socket Addresses

2.1. Socket()

To do network I/O, the first thing a process must do is call the socket() system call specifying the type of communication protocol desired.

Syntax:
 

int socket(     int family,       // Family of protocols     int type,         // Type of connection     int protocol);    // Protocol to use

The family is defined as :

 

TYPE	DESCRIPTION
PF_UNIX	Unix internal protocols
PF_INET	ARPA internet protocols

The socket type is defined as:
 

TYPE	DESCRIPTION
SOCK_STREAM	Stream socket
SOCK_DGRAM	Datagram socket
SOCK_RAW	Raw socket
SOCK_RDM	Reliably delivered message socket
SOCK_SEQPACKET	Sequenced packet socket

The protocol argument specifies the particular protocol to be used with the socket. Generally only a single protocol exists to support a protocol family. This parameter is passed as 0.

This call returns an integer value similar to a file descriptor. This is the socket descriptor. A value of -1 indicates an error. The errno values are:

 

EACCES	Permission to create a socket of the specified type and/or protocol is denied.
EMFILE	The peer-process descriptor table is full.
ENOMEM	Insufficient user memory is available.
ENOSR	There were insufficient STREAMS resources available to complete the operation.
EPROTONOSUPPORT	The protocol type or the specified protocol is not supported within this domain.

In the 5 tuple association, this system call defines the protocol value. Before using the connection, we must specify the rest of the four values.

2.2. Socketpair()

1. SOCK_STREAM
2. SOCK_DGRAM

2.3. Bind()

This call assigns a name to an unnamed socket. It is used by a process to register itself to the system so any messages sent to its socket must be directed to it. This call fills the local address and local process fields of the 5 tuple.

Syntax:
 

int bind( int sockfd,              // Socket descriptor struct sockaddr *myaddr, // Pointer to a protocol specific structure int addrlen);            // Size of structure

This function returns 0 on success and -1 on error. The errno values are :

 

EACCES	Search permission is denied for a component of the path prefix of the pathname in name.
EIO	An I/O error occurred while making the directory entry or allocating the inode.
EISDIR	A null pathname was specified.
ELOOP	Too many symbolic links were encountered in translating the pathname in name.
ENOENT	A component of the path prefix of the pathname in name does not exist.
ENOTDIR	A component of the path prefix of the pathname in name is not a directory.
EROFS	The inode would reside on a read-only file system.

2.4. Connect()

A client process connects a socket descriptor following a socket call to establish a connection with the server.

Syntax:
 

int connect(     int sockfd,                // Socket descriptor     struct sockaddr *servaddr, // Pointer to structure with server address     int addrlen);              // Sizeof address structure

Stream sockets will use this call once while datagram sockets will use it every time they send a packet.

This function call returns 0 on success and -1 on error. The errno values are:

 

EACCES	Search permission is denied for a component of the path prefix of the path name in name.
EADDRINUSE	The address is already in use.
EADDRNOTAVAIL	The specified address is not available on the remote machine.
EAFNOSUPPORT	Addresses in the specified address family cannot be used with this socket.
EALREADY	The socket is non-blocking and a previous connection attempt has not yet been completed.
EBADF	sockfd is not a valid descriptor.
ECONNREFUSED	The attempt to connect was forcefully rejected. The calling program should close the socket descriptor, and issue another socket call to obtain a new descriptor before attempting another connect call.
EINPROGRESS	The socket is non-blocking and the connection cannot be completed immediately. It is possible to select for completion by selecting the socket for writing. However, this is only possible if the socket STREAMS module is the topmost module on the protocol stack with a write service procedure. This will be the normal case.
EINTR	The connection attempt was interrupted before any data arrived by the delivery of a signal.
EINVAL	addrlen is not the size of a valid address for the specified address family.
EIO	An I/O error occurred while reading from or writing to the file system.
EISCONN	The socket is already connected.
ELOOP	Too many symbolic links were encountered in translating the pathname in name.
ENETUNREACH	The network is not reachable from this host.
ENOENT	A component of the path prefix of the pathname in name does not exist.
ENOENT	The socket referred to by the pathname in name does not exist.
ENOSR	There were insufficient STREAMS resources available to complete the operation.
ETIMEDOUT	Connection establishment timed out

2.5. Listen()

This call is used by a connection oriented server to indicate that it is willing to receive connections. This call is executed immediately after the socket and the bind system calls and immediately before the accept system call. It is possible for some client requests to arrive between these two. So the system should store the pending messages for the server till it makes the accept system call.

Syntax:
 

int listen(     int sockfd,   // Socket descriptor     int backlog); // No of messages that the system should track

The backlog parameter cannot be more than 5.

2.6. Accept()

1. -1 on error.
2. New socket descriptor on success.
3. The peer address.
4. The size of peer address.

2.7. Send(), Sendto(), Recv() and Recvfrom()

Used to send and receive messages. The sendto and recvfrom calls are for connection-less processes while the other two are for connection-oriented.

Syntax:
 

int send(     int sockfd,            // Socket descriptor     const char *buff,      // Message buffer     int nbytes,            // No of bytes to send     int flags);            // Flags int sendto(     int sockfd,            // Socket descriptor     const char *buff,      // Message buffer     int nbytes,            // No of bytes to send     int flags,             // Flags     struct sockaddr *to,   // Address of destination     int addrlen);          // Length of address structure int revc(     int sockfd,            // Socket descriptor     const char *buff,      // Message buffer     int nbytes,            // No of bytes to receive     int flags);            // Flags int recvfrom(     int sockfd,            // Socket descriptor     const char *buff,      // Message buffer     int nbytes,            // No of bytes to receive     int flags,             // Flags     struct sockaddr *from, // Address of source filled     int *addrlen);         // Length of address structure filled

The flags argument is either zero or is or?red using the following constants:
 

MSG_OOB	Send or receive out-of-band data.
MSG_PEEK	Peek at incoming message. Used by recv and recvfrom.

This function returns the length of data that was sent or received.

2.8. Close()

These routines handle the potential byte order difference between different computer architectures and different network protocols.

Syntax:
 

#include <sys/types.h> #include <netinet/in.h> ulong htonl(u_long hostlong); u_short htons(u_short hostshort); u_long ntohl(u_long netlong); u_short ntohs(u_short netshort);

The following table describes the functions:

 

FUNCTION	DESCRIPTION
htonl()	Convert host to network, long integer
htons()	Convert host to network, short integer
ntohl()	Convert network to host, long integer
ntohs()	Convert network to host, short integer

Implicit to these functions are that a short integer occupies 16 bits and a long integer occupies 32 bits.

Address conversion routines are used for internet address conversions.

Syntax:
 

#include <netinet/in.h> #include <arpa/inet.h> unsigned long inet_addr(const char *cp); char *inet_ntoa(const struct in_addr in);

The first function takes the stringified IP address and returns the 32 bit value.

The second function takes the in_addr structure and returns a pointer to the stringified IP address form.

5.1 Readv() and Writev()

These calls are called scatter read and gather write. They can be used to write or read multiple buffers in a single operation. They are atomic calls.

Syntax:
 

#include <sys/types.h> #include <sys/uio.h> int writev( int fd, struct iovec iov[], int iovcount ); int readv ( int fd, struct iovec iov[], int iovcount );

The structure iovec is defined as:
 

struct iovec {     caddr_t iov_base; // Starting address of buffer     int iovlen;       // Size of buffer in bytes };

Each element of the iovec array specifies a base address of a buffer and the amount of data to be written or read from it. The parameter iovcount specifies the number of separate buffers. In case of a datagram system, each write will generate separate datagrams but if combined in one, a writev will generate only one.

5.2 Recvmsg() and Sendmsg()

These are the most general of the read and write system calls.

Syntax:
 

int sendmsg( int sockfd, struct msghdr msg[], int flags ); int recvmsg( int sockfd, struct msghdr msg[], int flags );

The structure msghdr is defined as:
 

struct msghdr{     caddr_t msg_name;      // Optional address     int msg_namelen;       // Size of address     struct iovec *msg_iov; // Scatter or gather array     int msg_iovlen;        // Elements in msg_iov     caddr_t msg_accrights; // Access rights sent or received     int msg_accrightslen;  // Length of the access rights buffer };

The fields msg_name and msg_namelen are not used for connected processes.

5.3 Getpeername()

This system call returns the name of the peer process that is connected to a given socket.

Syntax:
 

int getpeername( int sockfd, struct sockaddr *peer, int *addrlen );

This function returns the foreign address and the foreign process elements of the 5 tuple association of sockfd.

5.4 Getsockname()

This function call returns the name associated with the socket.

Syntax:
 

int getsockname( int sockfd, struct sockaddr *peer, int *addrlen );

This call returns the local address and the local process of the association.

5.5 Shutdown()

This call provides more control over a connection termination.

Syntax:
 

int shutdown( int sockfd, int howto );

The control is defined using different values of howto. They are:
 

VALUE	FUNCTION
0	No more data can be received on the socket
1	No more data can be sent through the socket
2	Both sends and receives are disallowed

1. The process can itself specify the port number.
2. The process can leave it to the system to assign it any available port.