Unix Socket - Core Functions


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This chapter describes the core socket functions required to write a complete TCP client and server.

The following diagram shows the complete Client and Server interaction:

Socket Client Server

The socket Function

To perform network I/O, the first thing a process must do is, call the socket function, specifying the type of communication protocol desired and protocol family, etc.

#include <sys/types.h>
#include <sys/socket.h>

int socket (int family, int type, int protocol);

This call returns a socket descriptor that you can use in later system calls or 1 on error.

Parameters

family: It specifies the protocol family and is one of the constants shown below:

Family Description
AF_INET IPv4 protocols
AF_INET6 IPv6 protocols
AF_LOCAL Unix domain protocols
AF_ROUTE Routing Sockets
AF_KEY Ket socket

This chapter does not cover other protocols except IPv4.

type: It specifies the kind of socket you want. It can take one of the following values:

Type Description
SOCK_STREAM Stream socket
SOCK_DGRAM Datagram socket
SOCK_SEQPACKET Sequenced packet socket
SOCK_RAW Raw socket

protocol: The argument should be set to the specific protocol type given below, or 0 to select the system's default for the given combination of family and type:

Protocol Description
IPPROTO_TCP TCP transport protocol
IPPROTO_UDP UDP transport protocol
IPPROTO_SCTP SCTP transport protocol

The connect Function

The connect function is used by a TCP client to establish a connection with a TCP server.

#include <sys/types.h>
#include <sys/socket.h>

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

This call returns 0 if it successfully connects to the server, otherwise it returns -1 on error.

Parameters

  • sockfd: It is a socket descriptor returned by the socket function.

  • serv_addr: It is a pointer to struct sockaddr that contains destination IP address and port.

  • addrlen: Set it to sizeof(struct sockaddr).

The bind Function

The bind function assigns a local protocol address to a socket. With the Internet protocols, the protocol address is the combination of either a 32-bit IPv4 address or a 128-bit IPv6 address, along with a 16-bit TCP or UDP port number. This function is called by TCP server only.

#include <sys/types.h>
#include <sys/socket.h>

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

This call returns 0 if it successfully binds to the address, otherwise it returns -1 on error.

Parameters

  • sockfd: It is a socket descriptor returned by the socket function.

  • my_addr: It is a pointer to struct sockaddr that contains the local IP address and port.

  • addrlen: Set it to sizeof(struct sockaddr).

You can put your IP address and your port automatically

A 0 value for port number means that the system will choose a random port, and INADDR_ANY value for IP address means the server's IP address will be assigned automatically.

server.sin_port = 0;  		     
server.sin_addr.s_addr = INADDR_ANY;

NOTE: All ports below 1024 are reserved. You can set a port above 1024 and below 65535 unless they are the ones being used by other programs.

The listen Function

The listen function is called only by a TCP server and it performs two actions:

  • The listen function converts an unconnected socket into a passive socket, indicating that the kernel should accept incoming connection requests directed to this socket.

  • The second argument to this function specifies the maximum number of connections the kernel should queue for this socket.

#include <sys/types.h>
#include <sys/socket.h>

int listen(int sockfd,int backlog);

This call returns 0 on success, otherwise it returns -1 on error.

Parameters

  • sockfd: It is a socket descriptor returned by the socket function.

  • backlog: It is the number of allowed connections.

The accept Function

The accept function is called by a TCP server to return the next completed connection from the front of the completed connection queue. The signature of the call is as follows:

#include <sys/types.h>
#include <sys/socket.h>

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

This call returns a non-negative descriptor on success, otherwise it returns -1 on error. The returned descriptor is assumed to be a client socket descriptor and all read-write operations will be done on this descriptor to communicate with the client.

Parameters

  • sockfd: It is a socket descriptor returned by the socket function.

  • cliaddr: It is a pointer to struct sockaddr that contains client IP address and port.

  • addrlen : Set it to sizeof(struct sockaddr).

The send Function

The send function is used to send data over stream sockets or CONNECTED datagram sockets. If you want to send data over UNCONNECTED datagram sockets, you must use sendto() function.

You can use write() system call to send data. Its signature is as follows:

int send(int sockfd, const void *msg, int len, int flags);

This call returns the number of bytes sent out, otherwise it will return -1 on error.

Parameters

  • sockfd: It is a socket descriptor returned by the socket function.

  • msg: It is a pointer to the data you want to send.

  • len: It is the length of the data you want to send (in bytes).

  • flags: It is set to 0.

The recv Function

The recv function is used to receive data over stream sockets or CONNECTED datagram sockets. If you want to receive data over UNCONNECTED datagram sockets you must use recvfrom().

You can use read() system call to read the data. This call is explained in helper functions chapter.

int recv(int sockfd, void *buf, int len, unsigned int flags);

This call returns the number of bytes read into the buffer, otherwise it will return -1 on error.

Parameters

  • sockfd: It is a socket descriptor returned by the socket function.

  • buf: It is the buffer to read the information into.

  • len: It is the maximum length of the buffer.

  • flags: It is set to 0.

The sendto Function

The sendto function is used to send data over UNCONNECTED datagram sockets. Its signature is as follows:

int sendto(int sockfd, const void *msg, int len, unsigned int flags, const struct sockaddr *to, int tolen);

This call returns the number of bytes sent, otherwise it returns -1 on error.

Parameters

  • sockfd: It is a socket descriptor returned by the socket function.

  • msg: It is a pointer to the data you want to send.

  • len: It is the length of the data you want to send (in bytes).

  • flags: It is set to 0.

  • to: It is a pointer to struct sockaddr for the host where data has to be sent.

  • tolen: It is set it to sizeof(struct sockaddr).

The recvfrom Function

The recvfrom function is used to receive data from UNCONNECTED datagram sockets.

int recvfrom(int sockfd, void *buf, int len, unsigned int flags struct sockaddr *from, int *fromlen);

This call returns the number of bytes read into the buffer, otherwise it returns -1 on error.

Parameters

  • sockfd: It is a socket descriptor returned by the socket function.

  • buf: It is the buffer to read the information into.

  • len: It is the maximum length of the buffer.

  • flags: It is set to 0.

  • from: It is a pointer to struct sockaddr for the host where data has to be read.

  • fromlen: It is set it to sizeof(struct sockaddr).

The close Function

The close function is used to close the communication between the client and the server. Its syntax is as follows:

int close( int sockfd );

This call returns 0 on success, otherwise it returns -1 on error.

Parameters

  • sockfd: It is a socket descriptor returned by the socket function.

The shutdown Function

The shutdown function is used to gracefully close the communication between the client and the server. This function gives more control in comparison to the close function. Given below is the syntax of shutdown:

int shutdown(int sockfd, int how);

This call returns 0 on success, otherwise it returns -1 on error.

Parameters

  • sockfd: It is a socket descriptor returned by the socket function.

  • how: Put one of the numbers:

    • 0: indicates that receiving is not allowed,

    • 1: indicates that sending is not allowed, and

    • 2: indicates that both sending and receiving are not allowed. When how is set to 2, it's the same thing as close().

The select Function

The select function indicates which of the specified file descriptors is ready for reading, ready for writing, or has an error condition pending.

When an application calls recv or recvfrom, it is blocked until data arrives for that socket. An application could be doing other useful processing while the incoming data stream is empty. Another situation is when an application receives data from multiple sockets.

Calling recv or recvfrom on a socket that has no data in its input queue prevents immediate reception of data from other sockets. The select function call solves this problem by allowing the program to poll all the socket handles to see if they are available for non-blocking reading and writing operations.

Given below is the syntax of select:

 int select(int  nfds, fd_set  *readfds, fd_set  *writefds, fd_set *errorfds, struct timeval *timeout);

This call returns 0 on success, otherwise it returns -1 on error.

Parameters

  • nfds: It specifies the range of file descriptors to be tested. The select() function tests file descriptors in the range of 0 to nfds-1

  • readfds: It points to an object of type fd_set that on input, specifies the file descriptors to be checked for being ready to read, and on output, indicates which file descriptors are ready to read. It can be NULL to indicate an empty set.

  • writefds: It points to an object of type fd_set that on input, specifies the file descriptors to be checked for being ready to write, and on output, indicates which file descriptors are ready to write. It can be NULL to indicate an empty set.

  • exceptfds: It points to an object of type fd_set that on input, specifies the file descriptors to be checked for error conditions pending, and on output indicates, which file descriptors have error conditions pending. It can be NULL to indicate an empty set.

  • timeout: It points to a timeval struct that specifies how long the select call should poll the descriptors for an available I/O operation. If the timeout value is 0, then select will return immediately. If the timeout argument is NULL, then select will block until at least one file/socket handle is ready for an available I/O operation. Otherwise select will return after the amount of time in the timeout has elapsed OR when at least one file/socket descriptor is ready for an I/O operation.

The return value from select is the number of handles specified in the file descriptor sets that are ready for I/O. If the time limit specified by the timeout field is reached, select return 0. The following macros exist for manipulating a file descriptor set:

  • FD_CLR(fd, &fdset): Clears the bit for the file descriptor fd in the file descriptor set fdset.

  • FD_ISSET(fd, &fdset): Returns a non-zero value if the bit for the file descriptor fd is set in the file descriptor set pointed to by fdset, and 0 otherwise.

  • FD_SET(fd, &fdset): Sets the bit for the file descriptor fd in the file descriptor set fdset.

  • FD_ZERO(&fdset): Initializes the file descriptor set fdset to have zero bits for all file descriptors.

The behavior of these macros is undefined if the fd argument is less than 0 or greater than or equal to FD_SETSIZE.

Example

fd_set fds;

struct timeval tv;

/* do socket initialization etc.
tv.tv_sec = 1;
tv.tv_usec = 500000;

/* tv now represents 1.5 seconds */
FD_ZERO(&fds);

/* adds sock to the file descriptor set */
FD_SET(sock, &fds); 

/* wait 1.5 seconds for any data to be read from any single socket */
select(sock+1, &fds, NULL, NULL, &tv);

if (FD_ISSET(sock, &fds))
   {
   recvfrom(s, buffer, buffer_len, 0, &sa, &sa_len);
   
         /* do something */
   
   }
else
   {
   
         /* do something else */
   
   }


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