Berkeley sockets

History and implementations

Berkeley sockets originated with the 4.2BSD Unix operating system (released in 1983) as a programming interface. Only in 1989, however, could UC Berkeley release versions of its operating system and networking library free from the licensing constraints of AT&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 the Winsock 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 a wrapper library based on the C API.

BSD and POSIX sockets

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

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:

Socket API functions

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

socket

The function 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.

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.

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.

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.

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 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 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.

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.

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:

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. 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 defined by the corresponding protocol identifier, leaving the distinction between AF and PF constants as a technical argument of no practical consequence. Indeed, much confusion exists in the proper usage of both forms.

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

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 in user space and aid in debugging of the protocol stack. 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:

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. 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 to race conditions due to variances in network link speed.

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 the socket is closed. This is especially important if the connect 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 a TIME_WAIT state, on the server side, for up to 4 minutes.

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.

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:

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 its sin_port (the listening port, in network byte order) fields. Converting a short int to network byte order can be done by calling the function htons() (host to network short).
3. Preparing the socket to listen for connections (making it a listening socket), with a call to listen().
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, and accept() 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, using close().

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

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), and sin_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().

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 using recvfrom().

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:

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 a write() call. However, the user space program is free to delete the data buffer passed to write() after write() 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 have zero memory copy operations.