CS 311 – Lecture 19 Outline

• Internet Sockets– gethostname utility– struct hostent– inet_addr– Machine byte to Network byte order translation

and vice versa

Lecture 19 1CS311 - Operating Systems I

Internet Sockets

• Processes communicate via network.• Two identify a process on the network we

need– IP address of the process (IPv4 or IPv6)– Port number where the process listens

Lecture 19 2CS311 - Operating Systems I

Getting the IP Address

• To get the IP address in network byte order for the IP that is known use inet_addr().

• Prototype: in_addr_t inet_addr(const char *string)– Returns the 32-bit IP of network byte order

corresponding to the A.B.C.D string.

• If IP not known then we can get that using gethostname() and gethostbyname()

Lecture 19 3CS311 - Operating Systems I

gethostname() and gethostbyname()

• gethostname() returns the host’s name where the process is executing.

• Prototype: int gethostname(char *name, int namelen)

• gethostbyname() returns a pointer to struct hostent

• Protoype: struct hostent * gethostbyname(const char *name)– The only parameter is the hostname.– Searches the /etc/host or the DNS database to get the

structure information.

Lecture 19 4CS311 - Operating Systems I

struct hostent

• The structure has these members– char *h_name - “official” name of the host. – char **h_aliases - alternative names for the host– int h_addrtype - either AF_INET or AF_INET6– int h_length - length, in bytes, of each address.– char **h_addr_list – array of addresses for the

host. – char *h_addr - first host address (or

*h_addr_list[0]).

Lecture 19 5CS311 - Operating Systems I

inet_ntoa() and inet_aton()

• inet_ntoa() - Takes IP address in network-byte order and converts to string format.

• Prototype: char *inet_ntoa(struct in_addr addr)

• inet_aton() – same as inet_addr() but better• Prototype: int inet_aton(const char *addr,

struct in_addr *addr)– Stores the char* addr into struct in_addr

Lecture 19 6CS311 - Operating Systems I

memset() and others• memset() sets a block of memory to the desired

value.• Prototype: void * memset (void * ptr, int value,

size_t num);• memcpy() copies a block of memory to another

block.• Prototype: void * memcpy ( void * destination,

const void * source, size_t num );• memcmp() compares two blocks of memory• Prototype: int memcmp ( const void * ptr1,

const void * ptr2, size_t num );

Lecture 19 7CS311 - Operating Systems I

bzero() and bcopy()

• bzero() same as memset() but initializes a block of memory to zero.

• Prototype: void bzero (void* buffer, size_t length)• bcopy() is the same as memcpy()

Lecture 19 8CS311 - Operating Systems I

Machine-byte to network-byte

• Converting machine-byte order to network order

• Protoype:– in_port_t htons (in_port_t hostShort)– in_addr_t ntohl (in_addr_t networkLong)

• Converting network-byte to machine-byte• Prototype:– in_port_t ntohs (in_port_t networkShort)– in_addr_t htonl (in_addr_t hostLong)

Lecture 19 9CS311 - Operating Systems I