Socket System Calls

Objectives

We would like to :

Take a first look at the system calls to work with sockets.

Notes

int socket(int domain, int type, int protocol)
- Requires sys/socket.h
- Domain : AF_INET, AF_INET6, AF_UNIX
- Type: SOCK_STREAM, SOCK_DGRAM
  - This might also be used to configure the socket
    - SOCK_NONBLOCK
    - SOCK_CLOEXEC
- Protocol: is always 0 for what we will do.
- Returns -1 on error or a file descriptor otherwise.
- But see man socket.
int bind(int sockfd, const struct sockaddr * addr, socklen_t addrlen)
- sockfd: the output of socket.
- addr: an address structure - depends on address family in use
- addrlen: the length of the address
- 0 on success, -1 on error.
At this point the mechanism is in place for communications.
On the "server" side
- int listen(int sockfd, int backlog)
  - This marks the socket as "passive" or willing to accept incoming connections.
  - sockfd is the output of socket.
  - backlog is the maximum queue length of pending connections
    - IE how many incoming connections can be queued up
  - 0 on success, -1 on error.
- int accept(int sockfd, struct sockaddr *_Nullable restrict addr, socklen_t *_Nullable restrict addrlen);
  - sockfd - the socket
  - addr - the address of the connecting program, or null
    - Allocated on this side.
  - addrlen - the length of the address, depending on protocol
    - Size of the structure that has been allocated.
  - This returns a file descriptor on success and -1 on error.
- int accept4(int sockfd, struct sockaddr *_Nullable restrict addr, socklen_t *_Nullable restrict addrlen, int flags);
  - As accept but
  - Flags: SOCK_NBLOCK, SOCK_CLOEXEC
On the client side
- int connect(int sockfd, int accept4(int sockfd, struct sockaddr *_Nullable restrict addr, socklen_t *_Nullable restrict addrlen, int flags);
- sockfd is the socket
- addr is the filled in address structure for the server.
- addrlen is the length of the address structure.
- 0 on success -1 on error.
For stream sockets both can read and write.
when finished close.

Sockaddr

struct sockaddr{
    sa_family_t sa_family;   // this is the socket type. (probably int)
    char sa_data[14];        // the address, but the 14 is probably wrong.
};

For datagrams the world is somewhat different.
- See the picture on 1160
- Server and client both call socket.
- Server calls bind.
- recvfrom and sendto
- ssize_t sendto(int socket, const void *message, size_t length, int flags, const struct sockaddr *dest_addr, socklen_t dest_len);
  - socket - the socket fd
  - message - the data to send
  - length - the length of the message
  - dest_addr - a completed destination address
  - dest_len - the size of the destination address
- ssize_t sendto(int socket, const void *message, size_t length, int flags, const struct sockaddr *dest_addr, socklen_t dest_len);
  - socket
  - message - a buffer where we will write the message:
  - length - size of the buffer
  - flags:
    - MSG_PEEK - look at the message but do not receive it.
    - MSG_OOB - requres out of bounds data.
    - MSG_WAITALL - block for full data.