C Socket Programming

Needed Includes

<stdio.h>: Contains declarations for most I/O and printf() and scanf() are a part of this.
<sys/types.h>: Contains definitions of a number of data types.
<sys/socket.h>: Contains socket-related definitions, e.g. sockaddr, bind(), listen(), accept()
<netin/in.h>: Contains constants and structures needed for internet-related domain addresses, e.g. sockaddr_in
<stdlib.h>: Defines variables and macros for performing general functions.
- Example: int atoi(const char* str) which converts a string (ascii const char *) to int.

Socket Client-Server Model

From the above link:

The BSD server creates a socket, uses bind to attach that socket to a port, and configures it as a listening socket. This allows the server to receive incoming connection requests. Afterwards, accept is called, which will block the socket, until an incoming connection request is received. When accept returns, the SOCKADDR structure will have been filled out with the originating IP Address and port of the incoming connection. Then, accept creates a new socket, which is then used to receive data until the connection is closed by the other side.

Server Setup

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

socket() creates an endpoint for communication and returns a file descriptor that refers to that endpoint. The file descriptor returned by a successful call will be the lowest-numbered file descriptor not currently open for the process.

domain: The domain argument specifies a communication domain; this selects the protocol family which will be used for communication.
- Here we’ll be using AF_INET, which is the IPv4 Internet protocol.
type: Specifies the communication semantics.
- Here we’ll be using SOCK_STREAM for a TCP stream.
protocol: specifies a particular protocol to be used with the socket. Normally only a single protocol exists to support a particular socket type within a given protocol family, in which case protocol can be specified as 0.
- Here we’ll be using 0.

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

When a socket is created with socket(), it exists in a name space (address family) but has no address assigned to it. bind() assigns the address specified by addr to the socket referred to by the file descriptor sockfd. addrlen specifies the size, in bytes, of the address structure pointed to by addr. Traditionally, this operation is called "assigning a name to a socket".

sockaddr_in

Describes an IPv4 Internet domain socket address. The sin_port and sin_addr members are stored in network byte order.

Definition

struct sockaddr_in {
           sa_family_t     sin_family;     /* AF_INET */
           in_port_t       sin_port;       /* Port number */
           struct in_addr  sin_addr;       /* IPv4 address */
       };

See more information about sockaddr_in and in_addr here: https://www.gta.ufrj.br/ensino/eel878/sockets/sockaddr_inman.html

int listen(int sockfd, int backlog);

listen() marks the socket referred to by sockfd as a passive socket, that is, as a socket that will be used to accept incoming connection requests using accept(2).

int accept(int sockfd, struct sockaddr *_Nullable restrict addr, socklen_t *_Nullable restrict addrlen);

The accept() system call is used with connection-based socket types (SOCK_STREAM, SOCK_SEQPACKET). It extracts the first connection request on the queue of pending connections for the listening socket, sockfd, creates a new connected socket, and returns a new file descriptor referring to that socket. The newly created socket is not in the listening state. The original socket sockfd is unaffected by this call.

ssize_t read(int fd, void buf\[.count], size_t count);

read() attempts to read up to count bytes from file descriptor fd into the buffer starting at buf.

Server Teardown

Finally, we’ll need to clean up any file descriptors we opened for sockets.

int close(int fd);

close() closes a file descriptor, so that it no longer refers to any file and may be reused. Any record locks (see fcntl(2)) held on the file it was associated with, and owned by the process, are removed (regardless of the file descriptor that was used to obtain the lock).

If fd is the last file descriptor referring to the underlying open file description (see open(2)), the resources associated with the open file description are freed; if the file descriptor was the last reference to a file which has been removed using unlink(2), the file is deleted.

Client Setup

int getaddrinfo()

Given node and service, which identify an Internet host and a service, getaddrinfo() returns one or more addrinfo structures, each of which contains an Internet address that can be specified in a call to bind(2) or connect(2). The getaddrinfo() function combines the functionality provided by the gethostbyname(3) and getservbyname(3) functions into a single interface, but unlike the latter functions, getaddrinfo() is reentrant and allows programs to eliminate IPv4-versus-IPv6 dependencies.

Definition

int getaddrinfo(const char *restrict node,
                const char *restrict service,
                const struct addrinfo *restrict hints,
                struct addrinfo **restrict res);

const char *inet_ntop(int af, const void *restrict src, char dst[restrict .size, socklen_t size);]

This function converts the network address structure src in the af address family into a character string. The resulting string is copied to the buffer pointed to by dst, which must be a non- null pointer. The caller specifies the number of bytes available in this buffer in the argument size.