Berkeley sockets

Berkeley sockets (or BSD sockets) is a computing library with an application programming interface (API) for internet sockets and Unix domain sockets, used for inter-process communication (IPC).

As the API has evolved with little modification from a de facto standard into part of the POSIX specification, POSIX sockets are basically Berkeley sockets.

Contents

History, Implementations

Berkeley sockets originated with the 4.2BSD Unix operating system (released in 1983) as an API. Only in 1989, however, could UC Berkeley release versions of its operating system and networking library free from the licensing constraints of AT&T's copyright-protected Unix. This interface implementation is the original API of the Internet Protocol Suite (TCP/IP).

All modern operating systems now have some implementation of the Berkeley socket interface, as it became the standard interface for connecting to the Internet. Even the Winsock implementation for MS Windows, developed by unaffiliated developers, closely follows the Berkeley standard.

BSD vs POSIX

As the Berkeley socket API evolved over time, and ultimately into the POSIX socket API,[1] certain functions were deprecated or even removed and replaced by others. The POSIX API is also designed to be reentrant. These features now set the classic BSD API from the POSIX API apart.

Action BSD POSIX
Conversion from text address to packed address inet_aton inet_pton
Conversion from packed address to text address inet_ntoa inet_ntop
Forward lookup for host name/service gethostbyname, gethostbyaddr, getservbyname, getservbyport getaddrinfo
Reverse lookup for host name/service gethostbyaddr, getservbyport getnameinfo

C and other programming languages

The BSD sockets API is written in the programming language C. Most other programming languages use provide similar interfaces, typically written as a wrapper library based on the C API.[2]

Alternatives

The STREAMS-based Transport Layer Interface (TLI) API offers an alternative to the socket API. However, recent systems that provide the TLI API also provide the Berkeley socket API.

Header files

The Berkeley socket interface is defined in several header files. The names and content of these files differ slightly between implementations. In general, they include:

<sys/socket.h>
Core BSD socket functions and data structures.
<netinet/in.h>
AF_INET and AF_INET6 address families and their corresponding protocol families PF_INET and PF_INET6. Widely used on the Internet, these include IP addresses and TCP and UDP port numbers.
<sys/un.h>
PF_UNIX/PF_LOCAL address family. Used for local communication between programs running on the same computer. Not used on networks.
<arpa/inet.h>
Functions for manipulating numeric IP addresses.
<netdb.h>
Functions for translating protocol names and host names into numeric addresses. Searches local data as well as DNS.

Socket API functions

This list is a summary of functions or methods provided by the Berkeley sockets API library:

Further details are given below.

socket()

socket() creates an endpoint for communication and returns a file descriptor for the socket. socket() takes three arguments:

The function returns -1 if an error occurred. Otherwise, it returns an integer representing the newly-assigned descriptor.

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

bind()

bind() assigns a socket to an address. When a socket is created using socket(), it is only given a protocol family, but not assigned an address. This association with an address must be performed with the bind() system call before the socket can accept connections to other hosts. bind() takes three arguments:

Bind() returns 0 on success and -1 if an error occurs.

Prototype
int bind(int sockfd, const struct sockaddr *my_addr, socklen_t addrlen);

listen()

After a socket has been associated with an address, listen() prepares it for incoming connections. However, this is only necessary for the stream-oriented (connection-oriented) data modes, i.e., for socket types (SOCK_STREAM, SOCK_SEQPACKET). listen() requires two arguments:

Once a connection is accepted, it is dequeued. On success, 0 is returned. If an error occurs, -1 is returned.

Prototype
int listen(int sockfd, int backlog);

accept()

When an application is listening for stream-oriented connections from other hosts, it is notified of such events (cf. select() function) and must initialize the connection using the accept() function. The accept() function creates a new socket for each connection and removes the connection from the listen queue. It takes the following arguments:

The accept() function returns the new socket descriptor for the accepted connection, or -1 if an error occurs. All further communication with the remote host now occurs via this new socket.

Datagram sockets do not require processing by accept() since the receiver may immediately respond to the request using the listening socket.

Prototype
int accept(int sockfd, struct sockaddr *cliaddr, socklen_t *addrlen);

connect()

The connect() system call connects a socket, identified by its file descriptor, to a remote host specified by that host's address in the argument list.

Certain types of sockets are connectionless, most commonly user datagram protocol sockets. For these sockets, connect takes on a special meaning: the default target for sending and receiving data gets set to the given address, allowing the use of functions such as send() and recv() on connectionless sockets.

connect() returns an integer representing the error code: 0 represents success, while -1 represents an error.

Prototype
int connect(int sockfd, const struct sockaddr *serv_addr, socklen_t addrlen);

gethostbyname() and gethostbyaddr()

The gethostbyname() and gethostbyaddr() functions are used to resolve host names and addresses in the domain name system or the local host's other resolver mechanisms (e.g., /etc/hosts lookup). They return a pointer to an object of type struct hostent, which describes an Internet Protocol host. The functions take the following arguments:

The functions return a NULL pointer in case of error, in which case the external integer h_errno may be checked to see whether this is a temporary failure or an invalid or unknown host. Otherwise a valid struct hostent * is returned.

These functions are not strictly a component of the BSD socket API, but are often used in conjunction with the API functions. Furthermore, these functions are now considered legacy interfaces for querying the domain name system. New functions that are completely protocol-agnostic (supporting IPv6) have been defined. These new function are getaddrinfo() and getnameinfo(), and are based on a new addrinfo data structure.

Prototypes
struct hostent *gethostbyname(const char *name);
struct hostent *gethostbyaddr(const void *addr, int len, int type);

Protocol and address families

The socket API is a general interface for Unix networking and allows the use of various network protocols and addressing architectures.

The following lists a sampling of protocol families (preceded by the standard symbolic identifier) defined in a modern Linux or BSD implementation:

PF_LOCAL, PF_UNIX, PF_FILE
                Local to host (pipes and file-domain)
PF_INET         IP protocol family
PF_AX25         Amateur Radio AX.25
PF_IPX          Novell Internet Protocol
PF_APPLETALK    Appletalk DDP
PF_NETROM       Amateur radio NetROM
PF_BRIDGE       Multiprotocol bridge
PF_ATMPVC       ATM PVCs
PF_X25          Reserved for X.25 project
PF_INET6        IP version 6
PF_ROSE         Amateur Radio X.25 PLP
PF_DECnet       Reserved for DECnet project
PF_NETBEUI      Reserved for 802.2LLC project
PF_SECURITY     Security callback pseudo AF
PF_KEY          PF_KEY key management API
PF_NETLINK, PF_ROUTE
                routing API
PF_PACKET       Packet family
PF_ASH          Ash
PF_ECONET       Acorn Econet
PF_ATMSVC       ATM SVCs
PF_SNA          Linux SNA Project
PF_IRDA         IRDA sockets
PF_PPPOX        PPPoX sockets
PF_WANPIPE      Wanpipe API sockets
PF_BLUETOOTH    Bluetooth sockets

A socket for communications using any family is created with the socket() function (see above), by specifying the desired protocol family (PF_-identifier) as an argument.

The original design concept of the socket interface distinguished between protocol types (families) and the specific address types that each may use. It was envisioned that a protocol family may have several address types. Address types were defined by additional symbolic constants, using the prefix AF_ instead of PF_. The AF_-identifiers are intended for all data structures that specifically deal with the address type and not the protocol family. However, this concept of separation of protocol and address type has not found implementation support and the AF_-constants were simply defined by the corresponding protocol identifier, rendering the distinction between AF_ versus PF_ constants a technical argument of no significant practical consequence. Indeed, much confusion exists in the proper usage of both forms.[3]

However, the current POSIX.1—2008 specification doesn't specify any of PF_-constants, but only AF_-constants[4]

Raw sockets

Main article: raw socket

The most detailed and powerful method is control at the raw socket level. Very few applications need the degree of control over communications that this provides, so raw sockets support was intended to be available only on computers used for developing Internet-related technologies. Nevertheless, most operating systems have implemented support for it.

Options for sockets

After creating a socket, it is possible to set options on it. Some of the more common options are:

Blocking vs. non-blocking mode

Berkeley sockets can operate in one of two modes: blocking or non-blocking. A blocking socket will not return control until it has sent (or received) some or all data specified for the operation. It is normal for a blocking socket not to send all data. The application must check the return value to determine how many bytes have been sent or received and it must resend any data not already processed [1]. It also may cause problems if a socket continues to listen: a program may hang as the socket waits for data that may never arrive. When using blocking sockets, special consideration should be given to accept() as it may still block after indicating readability if a client disconnects during the connection phase.

A socket is typically set to blocking or nonblocking mode using the fcntl() or ioctl() functions.

Terminating sockets

The operating system does not release the resources allocated to a socket until a close() call occurs on the socket descriptor. This is especially important if the connect() call fails and may be retried. Each successful call to socket() must have a matching call to close() in all possible execution paths. The header file <unistd.h> defines the close function.

When the close() system call is initiated by an application, only the interface to the socket is destroyed, not the socket itself. It is the kernel's responsibility to destroy the socket internally. Sometimes, a socket may enter a TIME_WAIT state, on the server side, for up to 4 minutes.[5]

On SVR4 systems use of close() may discard data. The use of shutdown() or SO_LINGER may be required on these systems to guarantee delivery of all data.[6]

Client-server example using TCP

The Transmission Control Protocol (TCP) is a connection-oriented protocol that provides a variety of error correction and performance features for transmission of byte streams. A process creates a TCP socket by calling the socket() function with the parameters for the protocol family (PF_INET, PF_INET6), the socket mode for Stream Sockets (SOCK_STREAM), and the IP protocol identifier for TCP (IPPROTO_TCP).

Server

Setting up a simple TCP server involves the following steps:

  /* Server code in C */
 
  #include <sys/types.h>
  #include <sys/socket.h>
  #include <netinet/in.h>
  #include <arpa/inet.h>
  #include <stdio.h>
  #include <stdlib.h>
  #include <string.h>
  #include <unistd.h>
 
  int main(void)
  {
    struct sockaddr_in stSockAddr;
    int SocketFD = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP);
 
    if(-1 == SocketFD)
    {
      perror("can not create socket");
      exit(EXIT_FAILURE);
    }
 
    memset(&stSockAddr, 0, sizeof(stSockAddr));
 
    stSockAddr.sin_family = AF_INET;
    stSockAddr.sin_port = htons(1100);
    stSockAddr.sin_addr.s_addr = INADDR_ANY;
 
    if(-1 == bind(SocketFD,(struct sockaddr *)&stSockAddr, sizeof(stSockAddr)))
    {
      perror("error bind failed");
      close(SocketFD);
      exit(EXIT_FAILURE);
    }
 
    if(-1 == listen(SocketFD, 10))
    {
      perror("error listen failed");
      close(SocketFD);
      exit(EXIT_FAILURE);
    }
 
    for(;;)
    {
      int ConnectFD = accept(SocketFD, NULL, NULL);
 
      if(0 > ConnectFD)
      {
        perror("error accept failed");
        close(SocketFD);
        exit(EXIT_FAILURE);
      }
 
     /* perform read write operations ... 
     read(ConnectFD,buff,size)*/
 
      shutdown(ConnectFD, SHUT_RDWR);
 
      close(ConnectFD);
    }
return 0;  
}

Client

Programming a TCP client application involves the following steps:

  /* Client code in C */
 
  #include <sys/types.h>
  #include <sys/socket.h>
  #include <netinet/in.h>
  #include <arpa/inet.h>
  #include <stdio.h>
  #include <stdlib.h>
  #include <string.h>
  #include <unistd.h>
 
  int main(void)
  {
    struct sockaddr_in stSockAddr;
    int Res;
    int SocketFD = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
 
    if (-1 == SocketFD)
    {
      perror("cannot create socket");
      exit(EXIT_FAILURE);
    }
 
    memset(&stSockAddr, 0, sizeof(stSockAddr));
 
    stSockAddr.sin_family = AF_INET;
    stSockAddr.sin_port = htons(1100);
    Res = inet_pton(AF_INET, "192.168.1.3", &stSockAddr.sin_addr);
 
    if (0 > Res)
    {
      perror("error: first parameter is not a valid address family");
      close(SocketFD);
      exit(EXIT_FAILURE);
    }
    else if (0 == Res)
    {
      perror("char string (second parameter does not contain valid ipaddress)");
      close(SocketFD);
      exit(EXIT_FAILURE);
    }
 
    if (-1 == connect(SocketFD, (struct sockaddr *)&stSockAddr, sizeof(stSockAddr)))
    {
      perror("connect failed");
      close(SocketFD);
      exit(EXIT_FAILURE);
    }
 
    /* perform read write operations ... */
 
    shutdown(SocketFD, SHUT_RDWR);
 
    close(SocketFD);
    return 0;
  }

Client-server example using UDP

The User Datagram Protocol (UDP) is a connectionless protocol with no guarantee of delivery. UDP packets may arrive out of order, multiple times, or not at all. Because of this minimal design, UDP has considerably less overhead than TCP. Being connectionless means that there is no concept of a stream or permanent connection between two hosts. Such data are referred to as datagrams (Datagram Sockets).

UDP address space, the space of UDP port numbers (in ISO terminology, the TSAPs), is completely disjoint from that of TCP ports.

Server

Code may set up a UDP server on port 7654 as follows:

#include <stdio.h>
#include <errno.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <unistd.h> /* for close() for socket */ 
#include <stdlib.h>
 
int main(void)
{
  int sock = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP);
  struct sockaddr_in sa; 
  char buffer[1024];
  ssize_t recsize;
  socklen_t fromlen;
 
  memset(&sa, 0, sizeof sa);
  sa.sin_family = AF_INET;
  sa.sin_addr.s_addr = INADDR_ANY;
  sa.sin_port = htons(7654);
  fromlen = sizeof(sa);
 
  if (-1 == bind(sock,(struct sockaddr *)&sa, sizeof(sa)))
  {
    perror("error bind failed");
    close(sock);
    exit(EXIT_FAILURE);
  } 
 
  for (;;) 
  {
    printf ("recv test....\n");
    recsize = recvfrom(sock, (void *)buffer, 1024, 0, (struct sockaddr *)&sa, &fromlen);
    if (recsize < 0) {
      fprintf(stderr, "%s\n", strerror(errno));
      exit(EXIT_FAILURE);
    }
    printf("recsize: %z\n ", recsize);
    sleep(1);
    printf("datagram: %.*s\n", (int)recsize, buffer);
  }
}

This infinite loop receives any UDP datagrams to port 7654 using recvfrom(). It uses the parameters:

Client

A simple demonstration of sending a UDP packet containing the string "Hello World!" to address 127.0.0.1, port 7654 might look like this:

#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <unistd.h>
#include <arpa/inet.h>
 
 
int main(int argc, char *argv[])
{
  int sock;
  struct sockaddr_in sa;
  int bytes_sent;
  char buffer[200];
 
  strcpy(buffer, "hello world!");
 
  sock = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP);
  if (-1 == sock) /* if socket failed to initialize, exit */
    {
      printf("Error Creating Socket");
      exit(EXIT_FAILURE);
    }
 
  memset(&sa, 0, sizeof sa);
  sa.sin_family = AF_INET;
  sa.sin_addr.s_addr = inet_addr("127.0.0.1");
  sa.sin_port = htons(7654);
 
  bytes_sent = sendto(sock, buffer, strlen(buffer), 0,(struct sockaddr*)&sa, sizeof sa);
  if (bytes_sent < 0) {
    printf("Error sending packet: %s\n", strerror(errno));
    exit(EXIT_FAILURE);
  }
 
  close(sock); /* close the socket */
  return 0;
}

In this code, buffer is a pointer to the data to be sent, and buffer_length specifies the size of the data.

References

  1. ^ http://www.opengroup.org/onlinepubs/9699919799/functions/contents.html — POSIX.1-2008 specification
  2. ^ E. g. in the Ruby programming language: ruby-doc::Socket
  3. ^ UNIX Network Programming Volume 1, Third Edition: The Sockets Networking API, W. Richard Stevens, Bill Fenner, Andrew M. Rudoff, Addison Wesley, 2003.
  4. ^ http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/sys_socket.h.html
  5. ^ terminating sockets
  6. ^ ntua.gr - Programming UNIX Sockets in C - Frequently Asked Questions: Questions regarding both Clients and Servers (TCP/SOCK_STREAM)

The "de jure" standard definition of the Sockets interface is contained in the POSIX standard, known as:

Information about this standard and ongoing work on it is available from the Austin website.

The IPv6 extensions to the base socket API are documented in RFC 3493 and RFC 3542.

External links

This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.