Address Information

Overview

Typically C network code begins with address resolution.

Each addrinfo struct, one or more of which is returned by getaddrinfo, contains an Internet address that can be specified in a call to bind or connect. It contains the following fields:

struct addrinfo {
  int              ai_flags;
  int              ai_family;
  int              ai_socktype;
  int              ai_protocol;
  socklen_t        ai_addrlen;
  struct sockaddr *ai_addr;
  char            *ai_canonname;
  struct addrinfo *ai_next;
};

For Internet-facing code:

• The ai_family field indicates which address family is being used.
  • AF_INET or AF_INET6 are used for IPv4 and IPv6 respectively.
  • AF_UNSPEC can be provided instead if IPv4 or IPv6 can be used.
• The ai_socktype field indicates which socket type is being used.
  • SOCK_STREAM or SOCK_DGRAM are used for TCP and UDP respectively.
• The ai_addr field contains data related to the actual address.
  • Tthis value can be cast to a struct sockaddr_in or struct sockaddr_in6, depending on the value of the ai_family.
  • This also specifies a 16-bit port number used to distinguish per-application traffic on the same host.

API

Socket

The socket function is used to get a socket descriptor. Typically this function is supplied values from a struct addrinfo populated by the getaddrinfo function.

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

Bind

The bind function is used to associate a socket descriptor to a given port on the host machine.

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

The AI_PASSIVE constant can be passed to a struct addrinfo instance beforehand to indicate on what addresses a socket can accept on. This resolves to either one of the following:

• INADDR_ANY. Usually corresponds to address 0.0.0.0.
• IN6ADDR_ANY_INIT. Usually corresponds to address ::.
• INADDR_LOOPBACK. Usually corresponds to address 127.0.0.1.
• IN6ADDR_LOOPBACK_INIT. Usually corresponds to address ::1.

Connect

The connect function is used to connect to some host interface listening for incoming connections.

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

Listen

The listen function is used to prepare a socket for incoming connections.

int listen(int sockfd, int backlog);

Accept

The accept function returns a new socket file descriptor for any pending connection found.

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

Close

The close syscall can be used on socket to close the underlying connection.

For more control, the shutdown function can be used instead. This allows cutting communication off only in one direction. Note this function doesn't close the file descriptor though - close still needs to be called.

int shutdown(int sockfd, int how);

Send/Recv

To send and receive data on a connection-oriented socket, send and recv are used respectively.

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

To send and receive data on a connectionless socket, sendto and recvfrom are used respectively.

int sendto(int sockfd, const void *msg,
           int len, unsigned int flags,
           const struct sockaddr *to, socklen_t tolen);
int recvfrom(int sockfd, void *buf,
             int len, unsigned int flags,
             struct sockaddr *from, int *fromlen);)