Download Data types

Berkeley Sockets
Details for everyday use
Carsten Griwodz
Socket example
1
Data types
 Basic data types
 Have a certain number of bits
 Not very important in platform-dependent nonnetworked applications
 Not very important in platform-dependent
networked applications
 Very important in platform-independent networked
applications
Socket example
2
Data types
 Basic data types
 C data types
char
short
int
long
long long

unsigned char
unsigned short
unsigned int
unsigned long
unsigned long long
-
8 bits
16 bits
32 (or 64) bits
32 or 64 bits
64 (or 128) bits
Socket API portable types
size_t – 32 bits unsigned, 0 – 2^32-1
ssize_t – 32 bits signed, -2^31 – 2^31-1
Socket example
3
IP addresses and hostnames
 IPv4 host address
 When written on paper, it looks like 129.240.71.213
• ”dotted decimal notation”


Represents a 32 bit address
Binary in bits
• 10000001 11110000 01000111 11010101

Hexadecimal in bytes
• 0x81 0xf0 0x47 0xd5

One 4 byte int on x86, StrongARM, XScale, …
• 0xd547f081

One 4 byte int on PowerPC, POWER, Sparc, …
• 0x81f047d5

In network byte order
• 0x81f047d5
Socket example
4
IP addresses and hostnames
 On x86 etc.
 ntohl(0x81f047d5) == 0xd547f081
 On PowerPC etc.
 ntohl(0x81f047d5) == 0x81f047d5
Socket example
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IP addresses and hostnames
 IPv4 host address
 Corresponds to one network interface card (NIC)
 IPv4 network address
 Looks like 129.240.69.0/24
• Refers to add addresses that that the same first 24 bits
• 10000001 11110000 01000111 11010101
• 129.240.69.213 is in that network
Socket example
6
IP addresses and hostnames
 IPv4 networks

Institutes and companies own network address ranges
• e.g. 129.240.0.0/16 - UiO
• e.g. 9.0.0.0/8 – IBM

Institutes and companies assign addresses to their computers
• Fixed addresses
• Temporary addresses
Class A addresses
0 network
Class B addresses
10 network
Class C addresses
110 network
Class D or multicast
addresses
1110 multicast address
host
1.0.0.0/8 - 127.0.0.0/8
host
128.0.0.0/16 - 191.255.0.0/16
host
192.0.0.0/24 223.255.255.0/24
224.0.0.0 239.255.255.255
Socket example
7
IP addresses and hostnames
 IPv4 networks
 Institutes and companies own network address ranges
• e.g. 129.240.0.0/16 - UiO
• e.g. 9.0.0.0/8 – IBM

Institutes and companies assign addresses to their
computers
• Fixed addresses
• Temporary addresses

They can also create subnets
• IFI has subnets of UiO’s address space
• E.g. 129.250.69.0/24
• 129.250.69.0 can’t be used for a computer, it’s the network’s
address
• 129.250.69.255 can’t be used for a computer, it’s an address
for all computers in the network, the broadcast address
Socket example
8
IP addresses and hostnames
 These are many addresses
 Why do we need IPv6?
 Most IPv4 addresses have owners
•
•
•
•
No matter whether the addresses are needed
Most in the US, using 1%
Several in Europe
Really tight in Asia, only assigned when need is proven
 IPv6 addresses
 128 bits
 In text
• 2FFF:80:0:0:0:0:94:1 – 8 times 16 bits
• Hard to remember own address
001 13 bit top level
32 bit next level
16 bit site level
64 bit interface id
Socket example
9
IP addresses and hostnames
 Hostnames
 More exactly fully qualified host names
 Look like niu.ifi.uio.no
•
•
•
•
Host niu
In subdomain ifi, Institutt for Informatik
In domain uio, Universitet i Oslo
In top level domain no, Norway
 Who decided this?
 .no
- IANA gave it to Uninett
 .uio
- Uninett gave it to UiO
 .ifi
- USIT, UiO’s drift, gave it to IFI
 niu
- IFI drift gave it to the machine
Socket example
10
Name resolution
 Gethostbyname
 Takes a hostname
 Returns information
about that hostname
 Including its IP address
 How?
struct hostent *hostp;
struct sockaddr_in serveraddr;
int sock;
/* Look in DNS for the IP address of the name */
if ((hostp = gethostbyname(machine)) == 0)
{
fprintf(stderr,“Unknown machine %s\n",machine);
exit(1);
}
bzero((void *) &serveraddr, sizeof(serveraddr));
serveraddr.sin_family = AF_INET;
memcpy(&serveraddr.sin_addr,
hostp->h_addr, hostp->h_length);
serveraddr.sin_port = htons(port_number);
Socket example
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Name resolution
 Gethostbyname
 Takes a hostname
 Returns information
about that hostname
 Including its IP address
#
# list of statically known hosts
#
127.0.0.1
localhost
129.27.2.1 argul.ifi.uio.no argul
129.27.2.21 fileserver.ifi.uio.no fileserver
 How?
 First:
 Look into /etc/hosts
 But only for few, special,
well-known hosts
Socket example
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Name resolution
Root name server
 Gethostbyname
 Takes a hostname
 Returns information
about that hostname
 Including its IP address
2
5
3
4
 How?
 Then:
 Using DNS
 the Domain Name
System
ifi.uio.no
1
dns.umass.edu
6
a121.ifi.uio.no
gaia.cs.umass.edu
Socket example
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Name resolution
 Gethostbyname
 Takes a hostname
 Returns information
about that hostname
 Including its IP address
 Return value
 Pointer to
struct hostent

Contains more than just
an IP address
#define h_addr h_addr_list[0]
struct hostent
{
/* official hostname */
char* h_name;
/* alias names of the host
* entry NULL indicates end of the list */
char **h_aliases;
/* host address type, e.g. AF_INET */
int h_addrtype;
/* length of each address */
int h_length;
/* list of addresses, primary is 0th entry
* entry 0 indicates end of the list */
char** h_addr_list;
};
• Other names
• All addresses
Socket example
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Name resolution
 Finding the
own hostname
 Command line




ifconfig –a (Unix)
ipconfig /a (Windows)
Gives you all IP addresses
Typically 2:
• 127.0.0.1 localhost
• and the actual one
 Inside a program


Difficult without a
connected socket
With a connected socket:
eth0 Link encap:Ethernet HWaddr 00:C0:4F:A3:0C:D3
inet addr:129.240.70.96 Bcast:129.240.71.255 Mask:255.255.248.0
UP BROADCAST NOTRAILERS RUNNING MULTICAST MTU:1500 Metric:1
RX packets:40543011 errors:65 dropped:0 overruns:203 frame:65
TX packets:9222810 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:100
RX bytes:3360933562 (3205.2 Mb) TX bytes:2321704496 (2214.1 Mb)
Interrupt:19 Base address:0xfcc0
lo
Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
UP LOOPBACK RUNNING MTU:16436 Metric:1
RX packets:26429 errors:0 dropped:0 overruns:0 frame:0
TX packets:26429 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:5877172 (5.6 Mb) TX bytes:5877172 (5.6 Mb)
• struct hostent*
getsockname(int
sock);
• Similar to gethostbyname
Socket example
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Netmasks
 Help your computer find the way

All computers with addresses in the same network can be reached
directly
• If you know your own computer’s address, e.g. 129.240.70.96
• … and the relevant bits of your network, e.g. /24
• … you know that all computers with addresses 129.240.70.? are in
the same network

For each computer with another address, a member of the own
network (computer or router) must be found that can send the
data into the right direction
• This is sometimes wrong
• Understanding of network addresses becomes important
 Tell your computer its network address


For computers with a fixed IP address: set up once
For computers with a dynamic address: set up together with
address
Socket example
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Netmasks
 The computer
 Is not told directly that it’s on the network 129.240.70.0/24
 Instead it is told that it’s address is 129.240.70.97
 And that its netmask is 255.255.255.0 – these are 24 bits
 It figures the network address out from this
 In ifconfig or ipconfig
 Lists hostname
• E.g. 129.240.70.97

Lists netmask
• e.g. 255.255.255.0
• Or 0xffffff00
• Or the highest 24 bits set

Implies that
• 129.240.70.97
• is part of the subnet 129.240.70.97/24
• which is the same as 129.240.70.0/24

This answer from ifconfig implies also that
• All computers in the net 129.240.70.0/24 are reachable directly
• Without router
Socket example
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Binding TCP client sockets to ports
 Main reasons to do this
 Use a specific network
card
 Other potential reasons
 Firewalls may restrict
open ports
 Server demands
connection from a
priviledged port
int boundconnect( char* servername, int port, int ownport )
{
…
if ((sock = socket(AF_INET, SOCK_STREAM,
IPPROTO_TCP)) < 0) { … }
bzero(&ownaddr, sizeof(ownaddr));
ownaddr.sin_family = AF_INET;
ownaddr.sin_addr.s_addr = INADDR_ANY;
ownaddr.sin_port = htons(ownport);
if (bind(sock, (struct sockaddr *)&ownaddr,
sizeof(ownaddr)) < 0) { … }
if ((hostp = gethostbyname(machine)) == 0) { … }
bzero(&serveraddr, sizeof(serveraddr));
serveraddr.sin_family = AF_INET;
memcpy(&serveraddr.sin_addr,
hostp->h_addr, hostp->h_length);
serveraddr.sin_port = htons(port_number);
if (connect(sock, (struct sockaddr *)&serveraddr,
sizeof serveraddr) < 0) { … }
return sock;
• 0 – 1023

Associate virtual circuit
tags with source ports
}
Socket example
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Connecting TCP socket
Client
Server
socket
bind
socket
connect
CLOSED
Wait
30 sec
Send SYN
TIME_WAIT
SYN_SENT
Recv FIN,
Send ACK
read
write
FIN_WAIT_2
Rcvd SYN, ACK
Send ACK
ESTABLISHED
CLOSED
Recv ACK
LAST_ACK
Send FIN
FIN_WAIT_1
LISTEN
Recv SYN,
Send ACK
and SYN
Send FIN
CLOSE_WAIT
close
Recv ACK
listen
Recv FIN,
Send ACK
SYN_REVD
read
write
Recv ACK
ESTABLISHED
Socket example
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TCP Handshake
Client
Server
CLOSED
connect
Send SYN
SYN_SENT
LISTEN
Recv SYN,
Send ACK
and SYN
Rcvd SYN, ACK
Send ACK
ESTABLISHED
SYN_REVD
Recv ACK
ESTABLISHED
Socket example
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Connecting UDP sockets
 Datagram service UDP
 Usually unconnected
 connect can be used


Has only local meaning
Local UDP ”remembers” a target socket
 Advantage
 Reading and writing is simpler
 Possible to figure best packet size
 Disadvantage
 Can use unconnected sockets to send to several receivers
 Connected sockets only to one
Socket example
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Reading and writing function calls
 int read( int sock, void* buffer, int n)

sock must be connected
 int recv( int sock, void* buffer, int n,
int options )


sock must be connected
Sometimes useful option MSG_PEEK: read packet but leave it in the
queue
 int recvfrom( int sock, void* buffer, int n,
int options,
struct sockaddr* src,
int* srclen )


Meant for unconnected sockets
If sock is connected, src is identical to sockaddr from accept
Socket example
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Reading and writing function calls
 int write( int sock, void* buffer, int n)

sock must be connected
 int send( int sock, void* buffer, int n,
int options )

sock must be connected
 int sendto( int sock, void* buffer, int n,
int options,
struct sockaddr* dest,
int destlen )


Meant for unconnected sockets
If sock is connected, dest address must refer to the other side
Socket example
23
Select
 Complicated at first
 But very useful
int select( int max_fd,
fd_set* read_set,
fd_set* write_set,
fd_set* except_set,
struct timeval* timeout );
 Can wait for activity on many
sockets




New connections on request
sockets
New data on connected
sockets
Closing of a connected
socket
Ready-to-send on a socket
 Can wait for user input
 Can wait for timeouts
Socket example
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Select
int select( int max_fd,
fd_set* read_set,
fd_set* write_set,
fd_set* except_set,
struct timeval* timeout );
 For servers
 Serve many clients at
once
 Handle clients that close
connections, clients that
crash, …
 For the chat example
 Wait for data from chat
partner
 Wait for typing of the
user
Socket example
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Select
int select( int max_fd,
fd_set* read_set,
fd_set* write_set,
fd_set* except_set,
struct timeval* timeout );
 read_set
 Arriving connect requests
 Arriving data
 Closing sockets
 write_set
 Non often used
 Non-blocking send is
finished
 except_set
 Hardly ever used

sendto(.,.,.,MSG_OOB)
Socket example
26
Select
void wait_for_all(int clientsock[], int clients)
{
fd_set read_set;
int i,act,top=0;
 Using only the read_set is
typical
 Clear the read_set

FD_ZERO(&read_set);
for( i=0; i<clients; i++ )
{
FD_SET(clientsock[i],&read_set);
top = MAX(top,clientsock[i]);
}
act = select( top+1,
&read_set, NULL, NULL,
NULL);
…
}
Must be done every time
 Put all sockets into the read
set
 Find the highest socket
number, add 1
 NULL timeout

Means forever
 Call select

waits for arriving data
Socket example
27
Select
void wait_some_time(int sec, int usec)
{
struct timeval timeout;
timeout.tv_sec = sec;
timeout.tv_usec = usec;
act = select( 0,
NULL, NULL, NULL,
&timeout );
…
}
 select can also wait only
for a timeout

Without sockets
 Timeout parameter



NULL means wait forever
Timeval {5,0} means wait 5
seconds
Timeval {0,0} means don’t
wait at all
Socket example
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Safe writing
 You never know when
something strange happens
on your TCP connection
 Safe writing is necessary
 Code from last lecture

int safewrite(int so, char buf[], int l)
{
int i;
if (i=write(so, buf, l)==0)
{
printf("Can't write to socket, connection is closed" );
exit(1);
}
return i;
}
Considers closing of the
socket
Socket example
29
Safer writing
 Not all data may be sent at once
 TCP takes as much as it wants to
 Try to send again
• If there was no error
Socket example
30
Safer writing
int safewrite( int sock, char buffer[], int len )
{
int ij=0;
do
{
i = write( sock, &buffer[j], len );
if( i == -1 )
{
perror( “Error writing to socket” );
exit( 1 );
}
else if( i == 0 )
{
printf( “The socket has been closed, quitting\n” );
exit( 1 );
}
len = len – i;
j = j + i;
}
while( len > 0 );
return 1;
}
Socket example
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Safer writing
 Not all data may be sent at
once


TCP takes as much as it
wants to
Try to send again
• If there was no error
• If there was no important
error
Socket example
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Safer writing
int safewrite( int sock, char buffer[], int len )
{
int I,j=0;
do
{
i = write( sock, &buffer[j], len );
if( i == -1 )
{
switch( errno )
{
case EINTR : /* do nothing, no problem */
break;
default:
perror( “Error writing to socket” );
exit( 1 );
}
}
else if( i == 0 )
{
printf( “The socket has been closed, quitting\n” );
exit( 1 );
}
else if( i > 0 )
{
len = len – i;
j = j + i;
}
}
while( len > 0 );
return 1;
}
Socket example
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Safer writing
 Not all data may be sent at
once


TCP takes as much as it wants
to
Try to send again
• If there was no error
• If there was no important error
 You may receive a signal
 These are alarm signals in case
of emergencies
 Pressing CTRL-C raises a signal
• SIGKILL


An interrupt that ends your
program if you don’t handle it
Server that use TCP
• Get SIGPIPE often
• When the receiver crashes in
the middle of a write
Socket example
34
Safer writing
int main( )
{
…
signal( SIGPIPE, SIG_IGN ); /* I don’t want to do much */
…
}
int safewrite( int sock, char buffer[], int len )
{
int I, j = 0;
do
{
i = write( sock, &buffer[j], len );
if( i == -1 )
{
switch( errno )
{
case EINTR : /* do nothing, no problem */
break;
case EPIPE : /* do nothing, will write 0 next round */
break;
default:
perror( “Error writing to socket” );
exit( 1 );
}
}
else if( i == 0 )
{
printf( “The socket has been closed, quitting\n” );
exit( 1 );
}
else if( i > 0 )
{
len = len – i;
j = j + i;
}
}
while( len > 0 );
return 1;
}
Socket example
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Safer writing
 Not all data may be sent at
once


TCP takes as much as it wants
to
Try to send again
 Slow clients block the server
 Important if your server has
other things to do
 For example sending data to
many clients at once
• Send only the data that you
can send immediately
• Send more data when the
connection is ready to take
more
• If there was no error
• If there was no important error
 You may receive a signal
 These are alarm signals in case
of emergencies
 Pressing CTRL-C raises a signal
• SIGKILL




Set the socket in nonblocking mode …
… and use select
An interrupt that ends your
program if you don’t handle it
Server that use TCP
• Get SIGPIPE often
• When the receiver crashes in
the middle of a write
Socket example
36
Safer writing
int main( )
{
…
signal( SIGPIPE, SIG_IGN ); /* I don’t want to do much */
…
}
int safewrite( int sock, char buffer[], int len )
{
int I, j = 0;
i = write( sock, &buffer[j], len );
if( i == -1 )
{
switch( errno )
{
case EINTR : /* do nothing, no problem */
break;
case EPIPE : /* do nothing, will write 0 next round */
break;
default:
perror( “Error writing to socket” );
return -1;
}
}
else if( i == 0 )
{
printf( “The socket has been closed, quitting\n” );
return -1;
}
else if( i > 0 )
{
len = len – i;
j = j + i;
}
return i;
}
Socket example
37
Safer writing
int createASocket( char machine[], int port_number )
{
…
sock[i] = TCPClientSocket(machine, port_number);
int retval = fcntl( sock[i], F_SETFL, O_NONBLOCK);
if( retval == -1 )
{
perror( “Could not set socket into non-blocking mode” );
}
…
}
void safeWriteToMany( int sock[], int nsock, char buf [], int l[])
{
int i, retval, again=1, max=0;
int* w;
fd_set writable;
w = (int*)malloc( nsock );
memset( w, 0, sizeof(int)*nsock );
while( again == 1 )
{
again = 0;
FD_ZERO( &writable );
for( i=0; i<nsock; i++ ) {
FD_SET( sock[i], &writable ); max=MAX(sock[i],max);
}
retval = select( max+1, NULL, &writable, NULL, NULL );
if( retval > 0 ) {
for( i=0; i<nsock; i++ ) {
if( FD_ISSET( sock[i], &writable ) ) {
retval = safewrite( sock[i], &buf[w[i]], l[i] );
…
l[i] = l[i] – retval; /* if no error but not finished */
w[i] = w[i] + retval;
if( l[i] > 0 ) again = 1;
}
}
}
}
free( w );
}
Socket example
38
Socket options
 Two ways to set socket options

setsockopt( int sock, int level,
int optname, void* optvalue,
int optlen )
• Options for reading and writing all types of sockets
• Often used in
setsockopt( request_sock, SOL_SOCKET,
SO_REUSEADDR, &i, sizeof(i));

fcntl( int sock, int optname, long optvalue );
• Options for reading and writing all types of sockets, files,
pipes, user input, graphical output
• Often used in fcntl( sock[i], F_SETFL, O_NONBLOCK);
Socket example
39
Summary
 Names and addresses
 IP addresses
 Fully qualified hostnames
 Domain Name System
• How to use it
 Connections in TCP and UDP
 Functions
 The select function call
 Several functions for reading and writing
 Writing really safe
 To one
 To many
Socket example
40