伯克利Socket介绍

Berkeley sockets

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Berkeley sockets is an application programming interface (API) for Internet sockets and Unix domain sockets, used for inter-process communication (IPC). It is commonly implemented as a library of linkable modules. It originated with the 4.2BSD Unix released in 1983.

A socket is an abstract representation (handle) for the local endpoint of a network communication path. The Berkeley sockets API represents it as a file descriptor (file handle) in theUnix philosophy that provides a common interface for input and output to streams of data.

Berkeley sockets evolved with little modification from a de facto standard into a component of the POSIX specification. Therefore, the term POSIX sockets is essentially synonymous withBerkeley sockets. They are also known as BSD sockets, acknowledging the first implementation in theBerkeley Software Distribution.

Contents

• 1History and implementations
  • 1.1BSD and POSIX sockets
  • 1.2Alternatives
• 2Header files
• 3Socket API functions
  • 3.1socket
  • 3.2bind
  • 3.3listen
  • 3.4accept
  • 3.5connect
  • 3.6gethostbyname and gethostbyaddr
• 4Protocol and address families
  • 4.1Raw sockets
• 5Options for sockets
• 6Blocking and non-blocking mode
• 7Terminating sockets
• 8Client-server example using TCP
  • 8.1Server
  • 8.2Client
• 9Client-server example using UDP
  • 9.1Server
  • 9.2Client
• 10Caveats
• 11References
• 12External links

History and implementations

Berkeley sockets originated with the 4.2BSD Unix operating system (released in 1983) as a programming interface. Only in 1989, however, couldUC Berkeley release versions of its operating system and networking library free from the licensing constraints ofAT&T Corporation's proprietary Unix.

All modern operating systems implement a version of the Berkeley or POSIX socket interface. It became the standard interface for connecting to the Internet. Even theWinsock implementation for MS Windows, developed by unaffiliated developers, closely follows the standard.

The BSD sockets API is written in the C programming language. Most other programming languages provide similar interfaces, typically written as awrapper library based on the C API.^[1]

BSD and POSIX 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 bereentrant.

ActionBSDPOSIXConversion from text address to packed addressinet_atoninet_ptonConversion from packed address to text addressinet_ntoainet_ntopForward lookup for host name/servicegethostbyname, gethostbyaddr, getservbyname, getservbyportgetaddrinfoReverse lookup for host name/servicegethostbyaddr, getservbyportgetnameinfo

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:

FileDescriptionsys/socket.hCore socket functions and data structures.netinet/in.hAF_INET and AF_INET6 address families and their corresponding protocol families, PF_INET and PF_INET6. These include standard IP addresses and TCP and UDP port numbers.sys/un.hPF_UNIX and PF_LOCAL address family. Used for local communication between programs running on the same computer.arpa/inet.hFunctions for manipulating numeric IP addresses.netdb.hFunctions for translating protocol names and host names into numeric addresses. Searches local data as well as name services.

Socket API functions

Flow diagram of client-server transaction using sockets with the Transmission Control Protocol (TCP).

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

• socket() creates a new socket of a certain socket type, identified by an integer number, and allocates system resources to it.
• bind() is typically used on the server side, and associates a socket with a socket address structure, i.e. a specified local port number and IP address.
• listen() is used on the server side, and causes a bound TCP socket to enter listening state.
• connect() is used on the client side, and assigns a free local port number to a socket. In case of a TCP socket, it causes an attempt to establish a new TCP connection.
• accept() is used on the server side. It accepts a received incoming attempt to create a new TCP connection from the remote client, and creates a new socket associated with the socket address pair of this connection.
• send() and recv(), or write() andread(), or sendto() and recvfrom(), are used for sending and receiving data to/from a remote socket.
• close() causes the system to release resources allocated to a socket. In case of TCP, the connection is terminated.
• gethostbyname() and gethostbyaddr() are used to resolve host names and addresses. IPv4 only.
• select() is used to suspend, waiting for one or more of a provided list of sockets to be ready to read, ready to write, or that have errors.
• poll() is used to check on the state of a socket in a set of sockets. The set can be tested to see if any socket can be written to, read from or if an error occurred.
• getsockopt() is used to retrieve the current value of a particular socket option for the specified socket.
• setsockopt() is used to set a particular socket option for the specified socket.

socket

The function socket() creates an endpoint for communication and returns afile descriptor for the socket. socket() takes three arguments:

• domain, which specifies the protocol family of the created socket. For example:
  • AF_INET for network protocolIPv4 (IPv4-only)
  • AF_INET6 for IPv6 (and in some cases, backward compatible with IPv4)
  • AF_UNIX for local socket (using a file)
• type, one of:
  • SOCK_STREAM (reliable stream-oriented service orStream Sockets)
  • SOCK_DGRAM (datagram service orDatagram 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 areIPPROTO_TCP, IPPROTO_SCTP, IPPROTO_UDP, IPPROTO_DCCP. These protocols are specified in filenetinet/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

bind() assigns a socket to an address. When a socket is created usingsocket(), 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:

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

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

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:

• 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.

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:

• 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(). Whenaccept() returns, this location indicates how many bytes of the structure were actually used.

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.

connect

The connect() system call 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 assend() 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 the 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.^[3]

gethostbyname and gethostbyaddr

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

• name specifies the name of the host. For example: www.wikipedia.org
• addr specifies a pointer to astruct in_addr containing the address of the host.
• len specifies the length, in bytes, ofaddr.
• 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 integerh_errno may be checked to see whether this is a temporary failure or an invalid or unknown host. Otherwise a validstruct 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.

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 modernLinux orBSD implementation:

IdentifierFunction or usePF_LOCAL, PF_UNIX, PF_FILELocal to host (pipes and file-domain)PF_INETInternet Protocol version 4PF_AX25Amateur Radio AX.25PF_IPXNovell's Internetwork Packet ExchangePF_APPLETALKAppletalkPF_NETROMAmateur radio NetROM (related to AX.25)PF_BRIDGEMultiprotocol bridgePF_ATMPVCAsynchronous Transfer Mode Permanent Virtual CircuitsPF_ATMSVCAsynchronous Transfer Mode Switched Virtual CircuitsPF_INET6Internet Protocol version 6PF_DECnetReserved for DECnet projectPF_NETBEUIReserved for 802.2LLC projectPF_SECURITYSecurity callback pseudo AFPF_KEYPF_KEY key management APIPF_NETLINK, PF_ROUTErouting APIPF_PACKETPacket capture socketsPF_ECONETAcorn EconetPF_SNALinux IBM Systems Network Architecture (SNA) ProjectPF_IRDAIrDA socketsPF_PPPOXPPP over X socketsPF_WANPIPESangoma Wanpipe API socketsPF_BLUETOOTHBluetooth sockets

A socket for communications using any family is created with the socket() function, 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. TheAF-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 defined by the corresponding protocol identifier, leaving the distinction betweenAF and PF constants as a technical argument of no practical consequence. Indeed, much confusion exists in the proper usage of both forms.^[4]

The POSIX.1—2008 specification doesn't specify any PF-constants, but only AF-constants^[5]

Raw sockets

Raw sockets provide a simple interface that bypasses the processing by the host's TCP/IP stack. They permit implementation of networking protocols inuser space and aid in debugging of the protocol stack.^[6] Raw sockets are used by some services, such as ICMP, that operate at the Internet Layer of the TCP/IP model.

Options for sockets

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

• TCP_NODELAY disables theNagle algorithm.
• SO_KEEPALIVE enables periodic 'liveness' pings, if supported by the OS.

Blocking and non-blocking mode

Berkeley sockets can operate in one of two modes: blocking or non-blocking.

A blocking socket does 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.^[7] 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.

On the other hand, a non-blocking socket returns whatever is in the receive buffer and immediately continues. If not written correctly, programs using non-blocking sockets are particularly susceptible torace conditions due to variances in network link speed.

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

Terminating sockets

The operating system does not release the resources allocated to a socket until the socket is closed. This is especially important if theconnect call fails and will be retried.

When an application closes a socket, only the interface to the socket is destroyed. It is the kernel's responsibility to destroy the socket internally. Sometimes, a socket may enter aTIME_WAIT state, on the server side, for up to 4 minutes.^[8]

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.^[9]

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 thesocket() 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:

1. Creating a TCP socket, with a call to socket().
2. Binding the socket to the listen port, with a call to bind(). Before calling bind(), a programmer must declare a sockaddr_in structure, clear it (with memset()), and the sin_family (AF_INET), and fill itssin_port (the listening port, innetwork byte order) fields. Converting a short int to network byte order can be done by calling the functionhtons() (host to network short).
3. Preparing the socket to listen for connections (making it a listening socket), with a call tolisten().
4. Accepting incoming connections, via a call to accept(). This blocks until an incoming connection is received, and then returns a socket descriptor for the accepted connection. The initial descriptor remains a listening descriptor, andaccept() can be called again at any time with this socket, until it is closed.
5. Communicating with the remote host, which can be done through send() and recv() or write() and read().
6. Eventually closing each socket that was opened, once it is no longer needed, usingclose().

The following program creates a TCP server 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 (0 > ConnectFD) {        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;  }

Client

Programming a TCP client application involves the following steps:

1. Creating a TCP socket, with a call to socket().
2. Connecting to the server with the use of connect(), 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), andsin_addr set to the IP address of the listening server (also in network byte order.)
3. Communicating with the server by using send() and recv() or write() and read().
4. Terminating the connection and cleaning up with a call to 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 ... */      shutdown(SocketFD, SHUT_RDWR);      close(SocketFD);    return EXIT_SUCCESS;  }

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

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

#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);  }}

Client

A simple client program for sending a UDP packet containing the string "Hello World!" to address 127.0.0.1 and port number 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(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 adresses 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.

Caveats

For TCP connections, the operating system may have to retransmit the data given to it with awrite() call. However, the user space program is free to delete the data buffer passed towrite() afterwrite() returns. This implies that the operating system must make a copy of the data which can lead to a considerable CPU load in high throughput/performance applications. Other APIs, such as those supporting RDMA require that the data buffer is not released until the acknowledgement from the remote end has been received and thus make it possible to havezero memory copy operations.