Berkeley sockets

As the Berkeley socket API evolved and ultimately yielded the POSIX socket API,[2] certain functions were deprecated or removed and replaced by others. The POSIX API is also designed to be reentrant and supports IPv6.

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

Non-Unix systems often expose the Berkeley socket API with a translation layer to a native networking API. Plan 9[3] and Genode[4] use file-system APIs with control files rather than file-descriptors.

The function socket() creates an endpoint for communication and returns a file descriptor for the socket. It uses three arguments:

• domain, which specifies the protocol family of the created socket. For example:
  • AF_INET for network protocol IPv4 (IPv4-only)
  • AF_INET6 for IPv6 (and in some cases, backward compatible with IPv4)
  • AF_UNIX for local socket (using a special filesystem node)
• type, one of:
  • SOCK_STREAM (reliable stream-oriented service or Stream Sockets)
  • SOCK_DGRAM (datagram service or Datagram Sockets)
  • SOCK_SEQPACKET (reliable sequenced packet service)
  • SOCK_RAW (raw protocols atop the network layer)
• protocol specifying the actual transport protocol to use. The most common are IPPROTO_TCP, IPPROTO_SCTP, IPPROTO_UDP, IPPROTO_DCCP. These protocols are specified in file netinet/in.h. The value 0 may be used to select a default protocol from the selected domain and type.

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

bind() associates a socket with an address. When a socket is created with socket(), it is only given a protocol family, but not assigned an address. This association must be performed before the socket can accept connections from other hosts. The function has three arguments:

• sockfd, a descriptor representing the socket
• my_addr, a pointer to a sockaddr structure representing the address to bind to.
• addrlen, a field of type socklen_t specifying the size of the sockaddr structure.

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

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:

• sockfd, a valid socket descriptor.
• backlog, an integer representing the number of pending connections that can be queued up at any one time. The operating system usually places a cap on this value.

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

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 function accept(). It creates a new socket for each connection and removes the connection from the listening queue. The function has the following arguments:

• sockfd, the descriptor of the listening socket that has the connection queued.
• cliaddr, a pointer to a sockaddr structure to receive the client's address information.
• addrlen, a pointer to a socklen_t location that specifies the size of the client address structure passed to accept(). When accept() returns, this location contains the size (in bytes) of the structure.

accept() returns the new socket descriptor for the accepted connection, or the value -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.

connect() establishes a direct communication link to a specific remote host identified by its address via a socket, identified by its file descriptor.

When using a connection-oriented protocol, this establishes a connection. Certain types of protocols are connectionless, most notably the User Datagram Protocol. When used with connectionless protocols, connect defines the remote address for sending and receiving data, allowing the use of functions such as send and recv. In these cases, the connect function prevents reception of datagrams from other sources.

connect() returns an integer representing the error code: 0 represents success, while –1 represents an error. Historically, in BSD-derived systems, the state of a socket descriptor is undefined if the call to connect fails (as it is specified in the Single Unix Specification), thus, portable applications should close the socket descriptor immediately and obtain a new descriptor with socket(), in the case the call to connect() fails.[5]

The functions gethostbyname() and gethostbyaddr() 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 use the following arguments:

• name specifies the name of the host.
• addr specifies a pointer to a struct in_addr containing the address of the host.
• len specifies the length, in bytes, of addr.
• type specifies the address family type (e.g., AF_INET) of the host address.

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 for looking up a host. 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 functions are getaddrinfo() and getnameinfo(), and are based on a new addrinfo data structure.[6]

This pair of functions appeared at the same time as the BSD socket API proper in 4.2BSD (1983),[7] the same year DNS was first created. Early versions did not query DNS and only performed /etc/hosts lookup. The 4.3BSD (1984) version added DNS in a crude way. The current implementation using Name Service Switch derives Solaris and later NetBSD 1.4 (1999).[8] Initially defined for NIS+, NSS makes DNS only one of the many options for lookup by these functions and its use can be disabled even today.[9]

Raw sockets provide a simple interface that bypasses the processing by the host's TCP/IP stack. They permit implementation of networking protocols in user space and aid in debugging of the protocol stack.[13] Raw sockets are used by some services, such as ICMP, that operate at the Internet Layer of the TCP/IP model.

Establishing a TCP server involves the following basic steps:

• Creating a TCP socket with a call to socket().
• Binding the socket to the listening port (bind()) after setting the port number.
• Preparing the socket to listen for connections (making it a listening socket), with a call to listen().
• Accepting incoming connections (accept()). This blocks the process until an incoming connection is received, and returns a socket descriptor for the accepted connection. The initial descriptor remains a listening descriptor, and accept() can be called again at any time with this socket, until it is closed.
• Communicating with the remote host with the API functions send() and recv(), as well as with the general-purpose functions write() and read().
• Closing each socket that was opened after use with function close()

The following program creates a TCP server listening on port number 1100:

  #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 sa;
    int SocketFD = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP);
    if (SocketFD == -1) {
      perror("cannot create socket");
      exit(EXIT_FAILURE);
    }
  
    memset(&sa, 0, sizeof sa);
  
    sa.sin_family = AF_INET;
    sa.sin_port = htons(1100);
    sa.sin_addr.s_addr = htonl(INADDR_ANY);
  
    if (bind(SocketFD,(struct sockaddr *)&sa, sizeof sa) == -1) {
      perror("bind failed");
      close(SocketFD);
      exit(EXIT_FAILURE);
    }
  
    if (listen(SocketFD, 10) == -1) {
      perror("listen failed");
      close(SocketFD);
      exit(EXIT_FAILURE);
    }
  
    for (;;) {
      int ConnectFD = accept(SocketFD, NULL, NULL);
  
      if (ConnectFD == -1) {
        perror("accept failed");
        close(SocketFD);
        exit(EXIT_FAILURE);
      }
  
      /* perform read write operations ... 
      read(ConnectFD, buff, size)
      */
  
      if (shutdown(ConnectFD, SHUT_RDWR) == -1) {
        perror("shutdown failed");
        close(ConnectFD);
        close(SocketFD);
        exit(EXIT_FAILURE);
      }
      close(ConnectFD);
    }

    close(SocketFD);
    return EXIT_SUCCESS;  
}

Programming a TCP client application involves the following steps:

• Creating a TCP socket.
• Connecting to the server (connect()), by passing a sockaddr_in structure with the sin_family set to AF_INET, sin_port set to the port the endpoint is listening (in network byte order), and sin_addr set to the IP address of the listening server (also in network byte order).
• Communicating with the remote host with the API functions send() and recv(), as well as with the general-purpose functions write() and read().
• Closing each socket that was opened after use with function close().

  #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 sa;
    int res;
    int SocketFD;

    SocketFD = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP);
    if (SocketFD == -1) {
      perror("cannot create socket");
      exit(EXIT_FAILURE);
    }
  
    memset(&sa, 0, sizeof sa);
  
    sa.sin_family = AF_INET;
    sa.sin_port = htons(1100);
    res = inet_pton(AF_INET, "192.168.1.3", &sa.sin_addr);

    if (connect(SocketFD, (struct sockaddr *)&sa, sizeof sa) == -1) {
      perror("connect failed");
      close(SocketFD);
      exit(EXIT_FAILURE);
    }
  
    /* perform read write operations ... */
  
    close(SocketFD);
    return EXIT_SUCCESS;
  }

An application may set up a UDP server on port number 7654 as follows. The programs contains an infinite loop that receives UDP datagrams with function recvfrom().

#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;
  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 = htonl(INADDR_ANY);
  sa.sin_port = htons(7654);
  fromlen = sizeof sa;

  sock = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP);
  if (bind(sock, (struct sockaddr *)&sa, sizeof sa) == -1) {
    perror("error bind failed");
    close(sock);
    exit(EXIT_FAILURE);
  }

  for (;;) {
    recsize = recvfrom(sock, (void*)buffer, sizeof buffer, 0, (struct sockaddr*)&sa, &fromlen);
    if (recsize < 0) {
      fprintf(stderr, "%s\n", strerror(errno));
      exit(EXIT_FAILURE);
    }
    printf("recsize: %d\n ", (int)recsize);
    sleep(1);
    printf("datagram: %.*s\n", (int)recsize, buffer);
  }
}

The following is a client program for sending a UDP packet containing the string "Hello World!" to address 127.0.0.1 at port number 7654.

#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(void)
{
  int sock;
  struct sockaddr_in sa;
  int bytes_sent;
  char buffer[200];
 
  strcpy(buffer, "hello world!");
 
  /* create an Internet, datagram, socket using UDP */
  sock = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP);
  if (sock == -1) {
      /* if socket failed to initialize, exit */
      printf("Error Creating Socket");
      exit(EXIT_FAILURE);
  }
 
  /* Zero out socket address */
  memset(&sa, 0, sizeof sa);
  
  /* The address is IPv4 */
  sa.sin_family = AF_INET;
 
   /* IPv4 addresses is a uint32_t, convert a string representation of the octets to the appropriate value */
  sa.sin_addr.s_addr = inet_addr("127.0.0.1");
  
  /* sockets are unsigned shorts, htons(x) ensures x is in network byte order, set the port to 7654 */
  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.