open(),read(),write(),close()可以应用于管道，FIFO，socket，或者终端等所有文件类型执行IO操作。
lseek()并不适用于所有类型的文件。不允许将lseek()应用于管道，FIFO，socket或者终端，只要合情合理，也可以将lseek应用于设备。例如在磁盘或磁带上查找一处具体位置。
cp test test.old
cp test /dev/pts/0
cp /dev/pts/0 test
cp /dev/pts/0 /dev/pts/1
[[email protected] pts]# echo "wo shi " > /dev/pts/0
[[email protected] ~]# cp /dev/pts/1 b.txt
fjei
sfoei
rifeo
wfowef
[[email protected] ~]# cat b.txt
fjei
sfoei
rifeo
wfowef
/usr/include/netinet/in.h
/* Structure describing an Internet socket address. */
struct sockaddr_in
{
__SOCKADDR_COMMON (sin_);
in_port_t sin_port; /* Port number. */
struct in_addr sin_addr; /* Internet address. */
/* Pad to size of `struct sockaddr‘. */
unsigned char sin_zero[sizeof (struct sockaddr) -
__SOCKADDR_COMMON_SIZE -
sizeof (in_port_t) -
sizeof (struct in_addr)];
};
/* Ditto, for IPv6. */
struct sockaddr_in6
{
__SOCKADDR_COMMON (sin6_);
in_port_t sin6_port; /* Transport layer port # */
uint32_t sin6_flowinfo; /* IPv6 flow information */
struct in6_addr sin6_addr; /* IPv6 address */
uint32_t sin6_scope_id; /* IPv6 scope-id */
};
/usr/include/sys/socket.h
/* Create a new socket of type TYPE in domain DOMAIN, using
protocol PROTOCOL. If PROTOCOL is zero, one is chosen automatically.
Returns a file descriptor for the new socket, or -1 for errors. */
extern int socket (int __domain, int __type, int __protocol) __THROW;
socketpair()系统调用只能用在unix domain中，
/* Create two new sockets, of type TYPE in domain DOMAIN and using
protocol PROTOCOL, which are connected to each other, and put file
descriptors for them in FDS[0] and FDS[1]. If PROTOCOL is zero,
one will be chosen automatically. Returns 0 on success, -1 for errors. */
extern int socketpair (int __domain, int __type, int __protocol,
int __fds[2]) __THROW;