Introduction Sockets to Programming in C using TCP/IP
2017-06-04 10:33
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Introduction
Computer Network: hosts, routers, communication channelsHosts run applications
Routers forward information
Packets: sequence of bytes contain control information
e.g. destination host
Protocolis an agreement: meaning of packets structure and size of packets
e.g. Hypertext Transfer Protocol (HTTP)
Protocol Families -TCP/IP
Several protocols for different problems: Protocol Suites(套件) or Protocol Families: TCP/IPTCP/IP provides end-to-end connectivity specifying how data should be formatted, addressed, transmitted, routed, and received at the destination
can be used in the internet and in stand-alone private networks(标准的私人网络)
it is organized into layers
![](https://images2015.cnblogs.com/blog/635602/201705/635602-20170507174415882-2130343243.png)
Internet Protocol (IP)
provides a datagram servicepackets are handled and delivered independently
best-effort protocol: may loose, reorder(重排序) or duplicate(复制) packets
each packet must contain an IP address of its destination
Addresses -IPv4
The 32bits of an IPv4 address are broken into 4 octets(字节), or 8 bit fields (0-255 value in decimal notation(10进制标记法)).For networks of different size, the first one (for large networks) to three (for small networks)octets can be used to identify the network, while the rest of the octets can be used to identify the node on the network.
![](https://images2015.cnblogs.com/blog/635602/201705/635602-20170507174454382-1253195215.png)
TCP vs UDP
Both use port numbersapplication-specific construct serving as a communication endpoint
16-bit unsigned integer, thus ranging from 0 to 65535
to provide end-to-end transport
UDP: User Datagram Protocol
no acknowledgements(不确认的)
no retransmissions(不重传)
out of order, duplicates possible(无序的,可能存在复制)
connectionless(无连接的), i.e., app indicates destination for each packet(在每个包中显示目的地)
TCP: Transmission Control Protocol
reliable byte-stream channel(in order, all arrive, no duplicates) similar to file I/O
flow control
connection-oriented
bidirectional(双向的)
TCP is used for services with a large data capacity(容量), and a persistent(持续的) connection.
UDP is more commonly used for quick lookups, and single use query-reply actions.
Some common examples of TCP and UDP with their default ports:
Protocol | TCP/UDP |
---|---|
DNS lookup | UDP 53 |
FTP | TCP 21 |
HTTP | TCP 80 |
POP3 | TCP 110 |
Telnet | TCP 23 |
Berkley Sockets
Universally known as Sockets(众所周知): It is an abstraction through which an application may send and receive data. Provide generic accessto interprocesscommunication(进程间通信) services. e.g. IPX/SPX, Appletalk, TCP/IPStandard API for networking
![](https://images2015.cnblogs.com/blog/635602/201705/635602-20170507174605398-541558539.png)
Sockets
Uniquely identified byan internet address
an end-to-end protocol (e.g. TCP or UDP)
a port number
Two types of (TCP/IP) sockets
Stream sockets (e.g. uses TCP) provide reliable byte-stream service
Datagram sockets (e.g. uses UDP) provide best-effort datagram service messages up to 65.500 bytes
Socket extend the convectional UNIX I/O facilities
file descriptors for network communication
extended the read and write system calls
![](https://images2015.cnblogs.com/blog/635602/201705/635602-20170507174657211-44939773.png)
Socket Programming
Client-Server communication
Serverpassively waits for and responds to clients
passivesocket
Client
initiates the communication
must know the address and the port of the server
activesocket
Sockets Procesures
Primitive(原语) | Meaning |
---|---|
Socket | Create a new communication endpoint |
Bind | Attach a local address to a socket |
Listen | Announce willingness to accept connections |
Accept | Block caller until a connection request arrive |
Connect | Actvely attempt to establish(建立) a connection |
Send | Send some date over the connection |
Receive | Receive some date over the connection |
Close | Release the connection |
Client-Server Communication - Unix
![](https://images2015.cnblogs.com/blog/635602/201705/635602-20170507174731679-1742576492.png)
Socket creation in C: socket()
int sockid= socket(family, type, protocol);
sockid: socket descriptor, an integer (like a file-handle)
family: integer, communication domain, e.g.,PF_INET, IPv4 protocols, Internet addresses (typically used); PF_UNIX, Local communication, File addresses
type: communication type
SOCK_STREAM -reliable, 2-way, connection-based service
SOCK_DGRAM -unreliable, connectionless, messages of maximum length
protocol: specifies protocol
IPPROTO_TCP IPPROTO_UDP
usually set to 0 (i.e., use default protocol)
upon failure returns -1
NOTE: socket call does not specify where data will be coming from, nor where it will be going to –it just creates the interface!
Socket close in C: close()
When finished using a socket, the socket should be closedstatus= close(sockid);
sockid: the file descriptor (socket being closed)
status: 0 if successful, -1 if error
Closing a socket. closes a connection (for stream socket). frees up the port used by the socket
Specifying Addresses
Socket API defines a genericdata type for addresses:struct sockaddr{ unsigned short sa_family; /* Address family (e.g. AF_INET) */ char sa_data[14]; /* Family-specific address information */ }
Particular form of the sockaddr used for TCP/IPaddresses:
struct in_addr { unsigned long s_addr; /* Internet address (32 bits) */ } struct sockaddr_in{ unsigned short sin_family; /* Internet protocol (AF_INET) */ unsigned short sin_port; /* Address port (16 bits) */ struct in_addrsin_addr; /* Internet address (32 bits) */ char sin_zero[8]; /* Not used */ }
Important: sockaddr_in can be casted to a sockaddr
Assign address to socket: bind()
associates and reserves a port for use by the socketint status = bind(sockid, &addrport, size);
sockid: integer, socket descriptor
addrport: struct sockaddr, the (IP) address and port of the machine: for TCP/IP server, internet address is usually set to INADDR_ANY, i.e., chooses any incoming interface
size: the size (in bytes) of the addrport structure
status: upon failure -1 is returned
int sockid; struct sockaddr_in addrport; sockid= socket(PF_INET, SOCK_STREAM, 0); addrport.sin_family= AF_INET; addrport.sin_port= htons(5100); addrport.sin_addr.s_addr = htonl(INADDR_ANY); if(bind(sockid, (struct sockaddr *) &addrport, sizeof(addrport))!= -1){ //… }
Skipping the bind()
bind can be skipped for both types of sockets:
Datagram socket:
if only sending, no need to bind. The OS finds a port each time the socket sends a packet
if receiving, need to bind
Stream socket:
destination determined during connection setup
don’t need to know port sending from (during connection setup, receiving end is informed of port)
Assign address to socket: bind()
Instructs(指示) TCP protocol implementation to listen for connectionsint status = listen(sockid, queueLimit);
sockid: integer, socket descriptor
queuelen: integer, # of active participants that can “wait”for a connection
status: 0 if listening, -1 if error
listen()is non-blocking: returns immediately
The listening socket (sockid)
1. is never used for sending and receiving
2. is used by the server only as a way to get new sockets
Establish Connection: connect()
The client establishes a connection with the server by calling connect()int status = connect(sockid, &foreignAddr, addrlen);
sockid: integer, socket to be used in connection
foreignAddr: struct sockaddr: address of the passive participant
addrlen: integer, sizeof(name)
status: 0 if successful connect, -1 otherwise
connect()is blocking
Incoming Connection: accept()
The server gets a socket for an incoming client connection by calling accept()int s= accept(sockid, &clientAddr, &addrLen);
s: integer, the new socket (used for data-transfer)
sockid: integer, the orig. socket (being listened on)
clientAddr: struct sockaddr, address of the active participant;filled in upon return
addrLen: sizeof(clientAddr): value/result parameter;must be set appropriately(适当地) before call;adjusted upon return
accept():is blocking:
1. waits for connection before returning;
2. dequeues the next connection on the queue for socket (sockid);
Exchanging data with stream socket
int count = send(sockid, msg, msgLen, flags);
msg: const void[], message to be transmitted
msgLen: integer, length of message (in bytes) to transmit
flags: integer, special options, usually just 0
count: # bytes transmitted (-1 if error)
int count = recv(sockid, recvBuf, bufLen, flags);
recvBuf: void[], stores received bytes
bufLen: # bytes received
flags: integer, special options, usually just 0
count: # bytes received (-1 if error)
Calls are blocking: returns only after data is sent / received
Exchanging data with datagram socket
int count = sendto(sockid, msg, msgLen, flags, &foreignAddr, addrlen);
msg, msgLen, flags, count: same with send()
foreignAddr: struct sockaddr, address of the destination
addrLen: sizeof(foreignAddr)
- int count = recvfrom(sockid, recvBuf, bufLen, flags,&clientAddr, addrlen);
recvBuf, bufLen, flags, count: same with recv()
clientAddr: struct sockaddr, address of the client
addrLen: sizeof(clientAddr)
Calls are blocking:returns only after data is sent / received
Example -Echo
A client communicates with an “echo”serverThe server simply echoes whatever it receives back to the client
Example -Echo using stream socket
Client | Server |
---|---|
Create a TCP socket | Create a TCP socket |
Establish connection | Assign a port to socket |
Communicate | Set socket to listen |
Close the connection | Repeatedly:1. Accept new connection; 2. Communicate; 3. Close the connection |
Server Create a TCP socket
/* Create socket for incoming connections */ if ((servSock= socket(PF_INET, SOCK_STREAM, IPPROTO_TCP)) < 0) DieWithError("socket() failed");
Server Assign a port to socket
echoServAddr.sin_family= AF_INET; /* Internet address family */ echoServAddr.sin_addr.s_addr= htonl(INADDR_ANY); /* Any incoming interface */ echoServAddr.sin_port= htons(echoServPort); /* Local port */ if (bind(servSock, (struct sockaddr *) &echoServAddr, sizeof(echoServAddr)) < 0) DieWithError("bind() failed");
Server Set socket to listen
/* Mark the socket so it will listen for incoming connections */ if (listen(servSock, MAXPENDING) < 0) DieWithError("listen() failed");
Server Accept new connection
for (;;) /* Run forever */{ clntLen= sizeof(echoClntAddr); if ((clientSock=accept(servSock,(structsockaddr *)&echoClntAddr,&clntLen))<0) DieWithError("accept() failed"); ...
A client decides to talk to the server
Client Create a TCP socket
/* Create a reliable, stream socket using TCP */ if ((clientSock= socket(PF_INET, SOCK_STREAM, IPPROTO_TCP)) < 0) DieWithError("socket() failed");
Client Establish connection
echoServAddr.sin_family= AF_INET; /* Internet address family */ echoServAddr.sin_addr.s_addr= inet_addr(echoservIP); /* Server IP address*/ echoServAddr.sin_port= htons(echoServPort); /* Server port */ if (connect(clientSock, (struct sockaddr *) &echoServAddr, sizeof(echoServAddr)) < 0) DieWithError("connect() failed");
Server’s accept procedure in now unblocked and returns client’s socket
for (;;) /* Run forever */{ clntLen= sizeof(echoClntAddr); if ((clientSock=accept(servSock,(structsockaddr *)&echoClntAddr,&clntLen))<0) DieWithError("accept() failed"); ...
Client Communicate
echoStringLen= strlen(echoString); /* Determine input length */ /* Send the string to the server */ if (send(clientSock, echoString, echoStringLen, 0) != echoStringLen) DieWithError("send() sent a different number of bytes than expected");
Server using stream socket to Communicate
/* Receive message from client */ if ((recvMsgSize= recv(clntSocket, echoBuffer, RCVBUFSIZE, 0)) < 0) DieWithError("recv() failed"); /* Send received string and receive again until end of transmission */ while (recvMsgSize> 0) { /* zero indicates end of transmission */ if (send(clientSocket, echobuffer, recvMsgSize, 0) != recvMsgSize) DieWithError(“send() failed”); if ((recvMsgSize= recv(clientSocket, echoBuffer, RECVBUFSIZE, 0)) < 0) DieWithError(“recv() failed”); }
Similarly, the client receives the data from the server
Close communicate
close(clientSocket); /* Client close socket */ close(ClientSocket); /* Server close clientSocket */
Server is now blocked waiting for connection from a client …
Echo using datagram socket
Client | Server |
---|---|
Create a UDP socket | Create a UDP socket |
Assign a port to socket | Assign a port to socket |
Communicate | Repeatedly |
Close the socket | Communicate |
/* Create socket for sending/receiving datagrams*/ if ((servSock= socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP)) < 0) DieWithError("socket() failed");
/* Create a datagram/UDP socket */ if ((clientSock= socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP)) < 0) DieWithError("socket() failed");
Assign a port to socket
echoServAddr.sin_family= AF_INET; /* Internet address family */ echoServAddr.sin_addr.s_addr= htonl(INADDR_ANY); /* Any incoming interface */ echoServAddr.sin_port= htons(echoServPort); /* Local port */ if (bind(servSock, (struct sockaddr *) &echoServAddr, sizeof(echoServAddr)) < 0) DieWithError("bind() failed");
echoClientAddr.sin_family= AF_INET; /* Internet address family */ echoClientAddr.sin_addr.s_addr= htonl(INADDR_ANY); /* Any incoming interface */ echoClientAddr.sin_port= htons(echoClientPort); /* Local port */ if(bind(clientSock,(structsockaddr *)&echoClientAddr,sizeof(echoClientAddr))<0) DieWithError("connect() failed");
Client Sendto Server
echoServAddr.sin_family= AF_INET; /* Internet address family */ echoServAddr.sin_addr.s_addr= inet_addr(echoservIP); /* Server IP address*/ echoServAddr.sin_port= htons(echoServPort); /* Server port */ echoStringLen= strlen(echoString); /* Determine input length */ /* Send the string to the server */ if (sendto( clientSock, echoString, echoStringLen, 0, (struct sockaddr *) &echoServAddr, sizeof(echoServAddr)) != echoStringLen) DieWithError("send() sent a different number of bytes than expected");
Server recive and send to Client
for (;;) /* Run forever */{ clientAddrLen= sizeof(echoClientAddr) /* Set the size of the in-out parameter */ /*Block until receive message from client*/ if ((recvMsgSize= recvfrom(servSock, echoBuffer, ECHOMAX, 0),(struct sockaddr *) &echoClientAddr, sizeof(echoClientAddr))) < 0) DieWithError("ecvfrom() failed"); if (sendto(servSock, echobuffer, recvMsgSize, 0,(struct sockaddr *) &echoClientAddr, sizeof(echoClientAddr)) != recvMsgSize) DieWithError(“send() failed”); }
Similarly, the client receives the data from the server
Client close socket
close(clientsocket);
Constructing Messages -Encoding Data
Client wants to send two integers x and y to server
Solution: Character Encodinge.g. ASCII
the same representation is used to print or display them to screen
allows sending arbitrarily(任意的) large numbers (at least in principle)
e.g. x = 17,998,720 and y = 47,034,615
49 55 57 57 56 55 50 48 32 52 55 48 51 52 54 49 53 32
1 7 9 9 8 7 2 0 _ 4 7 0 3 4 6 1 5 _
sprintf(msgBuffer, “%d %d ”, x, y); send(clientSocket, strlen(msgBuffer), 0);
Pitfalls(陷阱)
the second delimiter is required
otherwise the server will not be able to separate it from whatever it follows
msgBuffermust be large enough
strlencounts only the bytes of the message, not the null at the end of the string
This solution is not efficient
each digit can be represented using 4 bits, instead of one byte
it is inconvenient to manipulate numbers
Solution: Sending the valuesof x and y
pitfall: native integer format,a protocolis used
how many bits are used for each integer
what type of encoding is used (e.g. two’s complement(补码), sign(标记)/magnitude(大小), unsigned)
typedef struct { int x,y; } msgStruct; //…1 implementation msgStruct.x = x; msgStruct.y = y; send(clientSock, &msgStruct, sizeof(msgStruct), 0); // or 2 implementation send(clientSock, &x, sizeof(x)), 0); send(clientSock, &y, sizeof(y)), 0);
Address and port are stored as integers
u_short sin_port; (16 bit) in_addr sin_addr; (32 bit)
Problem
different machines / OS’s use different word orderingslittle-endian: lower bytes first
big-endian: higher bytes first
these machines may communicate with one another over the network
Big-Endian machine | Little_Endian machine |
---|---|
128.119.40.12 | 12.40.119.128 |
128 119 40 12 | 128 119 40 12 |
![](https://images2015.cnblogs.com/blog/635602/201705/635602-20170507174843914-751405354.png)
Little-Endian
![](https://images2015.cnblogs.com/blog/635602/201705/635602-20170507174853476-2110395086.png)
Byte Ordering -Solution: Network Byte Ordering
Host Byte-Ordering: the byte ordering used by a host (big or little)
Network Byte-Ordering: the byte ordering used by the network –always big-endian
u_long htonl(u_longx); u_short htons(u_shortx); u_long ntohl(u_longx); u_short ntohs(u_shortx);
On big-endian machines, these routines do nothing;
On little-endian machines, they reverse the byte order;
![](https://images2015.cnblogs.com/blog/635602/201705/635602-20170507174920179-239688187.png)
Example
Client
unsigned short clientPort, message; unsigned int messageLenth; servPort= 1111; message = htons(clientPort); messageLength= sizeof(message); if (sendto( clientSock, message, messageLength, 0, (struct sockaddr *) &echoServAddr, sizeof(echoServAddr)) != messageLength) DieWithError("send() sent a different number of bytes than expected");
Server
unsigned short clientPort, rcvBuffer; unsigned int recvMsgSize; if(recvfrom(servSock, &rcvBuffer, sizeof(unsignedint), 0),(struct sockaddr*) &echoClientAddr, sizeof(echoClientAddr)) < 0) DieWithError("ecvfrom() failed"); clientPort= ntohs(rcvBuffer); printf (“Client’s port: %d”, clientPort);
Constructing Messages -Alignment and Padding
consider the following 12 bytestructuretypedef struct { int x; short x2; int y; short y2; } msgStruct;
After compilation it will be a 14 bytestructure!
Why? Alignment!
Remember the following rules:data structures are maximally aligned, according to the size of the largest native integer;other multibytefields are aligned to their size, e.g., a four-byte integer’s address will be divisible by four
x x2 y y2 ==> x x2 [pad] y y2
4 bytes 2 bytes 4 bytes 2 bytes ==> 4 bytes 2 bytes 2bytes 4 bytes 2 bytes
This can be avoided
1. include padding to data structure
2. reorder fields
typedef struct { int x; short x2; char pad[2]; int y; short y2; } msgStruct;
or
typedef struct { int x; int y; short x2; short y2; } msgStruct;
Constructing Messages -Framing and Parsing
Framingis the problem of formatting the information so that the receiver can parse messages.Parse means to locate the beginning and the end of message.
This is easy if the fields have fixed sizes, e.g., msgStruct
For text-string representations is harder
Solution: use of appropriate delimiters(使用适当的分隔符)
caution(慎重) is needed since a call of recvmay return the messages sent by multiple calls of send
Socket Options
getsockoptand
setsockoptallow socket options values to be queried and set, respectively(分别的).
int getsockopt (sockid, level, optName, optVal, optLen);
sockid: integer, socket descriptor
level: integer, the layers of the protocol stack (socket, TCP, IP)
optName: integer, option
optVal: pointer to a buffer; upon return it contains the value of the specified option
optLen: integer, in-out parameter
it returns -1 if an error occured
int setsockopt(sockid, level, optName, optVal, optLen);
optLen is now only an input parameter
Socket Options Table
![](https://images2015.cnblogs.com/blog/635602/201705/635602-20170507174948476-1694711820.png)
Example
Fetch and then double the current number of bytes in the socket’s receive bufferint rcvBufferSize; int sockOptSize; //… /* Retrieve and print the default buffer size */ sockOptSize= sizeof(recvBuffSize); if (getsockopt(sock, SOL_SOCKET, SO_RCVBUF, &rcvBufferSize, &sockOptSize) < 0) DieWithError(“getsockopt() failed”); printf(“InitialReceive Buffer Size: %d\n”, rcvBufferSize); /* Double the buffer size */ recvBufferSize*= 2; /* Set the buffer size to new value */ if (setsockopt(sock, SOL_SOCKET, SO_RCVBUF, &rcvBufferSize, sizeof(rcvBufferSize)) < 0) DieWithError(“getsockopt() failed”);
Dealing with blocking calls
Many of the functions we saw block (by default) until a certain eventaccept: until a connection comes in
connect: until the connection is established
recv, recvfrom: until a packet (of data) is received
what if a packet is lost (in datagram socket)?
send: until data are pushed into socket’s buffer
sendto: until data are given to the network subsystem
For simple programs, blocking is convenient; What about more complex programs?
- multiple connections
- simultaneous(同时的) sends and receives
- simultaneously doing non-networking processing
Non-blocking Sockets
If an operation can be completed immediately, success is returned; otherwise, a failure is returned (usually -1)errnois properly set, to distinguish this (blocking) failure from other -(EINPROGRESSfor connect, EWOULDBLOCKfor the other)
Solution:
int fcntl (sockid, command, argument);
sockid: integer, socket descriptor
command: integer, the operation to be performed (F_GETFL, F_SETFL)
argument: long, e.g. O_NONBLOCK
fcntl (sockid, F_SETFL, O_NONBLOCK);
Solution: flags parameter of send, recv, sendto, recvfrom
MSG_DONTWAIT
not supported by all implementations
Signals
Provide a mechanism for operating system to notify processes that certain events occur e.g., the user typed the “interrupt”character, or a timer expired;signals are delivered asynchronously;
upon signal delivery to program
- it may be ignored, the process is never aware of it
- the program is forcefully terminatedby the OS
- a signal-handling routine, specified by the program, is executed
this happens in a different thread
- the signal is blocked, until the program takes action to allow its delivery
each process (or thread) has a corresponding mask
Each signal has a default behavior
e.g. SIGINT (i.e., Ctrl+C) causes termination, it can be changed using sigaction(); Signals can be nested(嵌套)(i.e., while one is being handled another is delivered)
int sigaction(whichSignal, &newAction, &oldAction);
whichSignal: integer
newAction: struct sigaction, defines the new behavior
oldAction: struct sigaction, if not NULL, then previous behavior is copied
it returns 0 on success, -1 otherwise
struct sigaction { void (*sa_handler)(int); /* Signal handler */ sigset_tsa_mask; /* Signals to be blocked during handler execution */ int sa_flags; /* Flags to modify default behavior */ };
sa_handler determines which of the first three possibilities occurs when signal is delivered, i.e., it is not masked
SIG_IGN, SIG_DFL, address of a function
sa_masks pecifies the signals to be blocked while handling whichSignal
whichSignal is always blocked
it is implemented as a set of boolean flags
int sigemptyset(sigset_t*set); /* unset all the flags */ int sigfullset(sigset_t*set); /* set all the flags */ int sigaddset(sigset_t*set, int whichSignal); /* set individual flag */ int sigdelset(sigset_t*set, int whichSignal); /* unset individual flag */
Signal Example
#include <stdio.h> #include <signal.h> #include <unistd.h> void DieWithError(char *errorMessage); voidInterruptSignalHandler(int signalType); int main (int argc, char *argv[]) { struct sigaction handler;/* Signal handler specification structure */ handler.sa_handler =InterruptSignalHandler; /* Set handler function */ if (sigfillset(&handler.sa_mask) < 0)/* Create mask that masks all signals */ DieWithError (“sigfillset() failed”); handler.sa_flags = 0; if (sigaction(SIGINT,&handler, 0) < 0)/* Set signal handling for interrupt signals */ DieWithError (“sigaction() failed”); for(;;) pause();/* Suspend program until signal received */ exit(0); } voidInterruptHandler(intsignalType) { printf (“Interrupt received. Exiting program.\n); exit(1); }
Asynchronous I/O
Non-blocking sockets require “polling(查询)”;With asynchronous I/O the operating system informs the program when a socket call is completed.the SIGIO signal is delivered to the process, when some I/O-related event occurs on the socket;
Three steps:
/* i. inform the system of the desired dispositionof(所需配置) the signal */ struct sigaction handler; handler.sa_handler = SIGIOHandler; if (sigfillset(&handler.sa_mask) < 0) DiewithError(“…”); handler.sa_flags = 0; if (sigaction(SIGIO, &handler, 0) < 0) DieWithError(“…”); /* ii. ensure that signals related to the socket will be delivered to this process*/ if (fcntl(sock, F_SETOWN, getpid()) < 0) DieWithError(); /* iii. mark the socket as being primed for asynchronous I/O*/ if (fcntl(sock, F_SETFL, O_NONBLOCK | FASYNC) < 0) DieWithError();
Asynchronous I/O Example
/* Inform the system of the desired dispositionof the signal */ struct sigaction myAction; myAction.sa_handler= CatchAlarm; if (sigfillset(&myAction.sa_mask) < 0) DiewithError(“…”); myAction.sa_flags= 0; if (sigaction(SIGALARM, &handler, 0) < 0) DieWithError(“…”); /* Set alarm */ alarm(TIMEOUT_SECS); /* Call blocking receive */ if (recvfrom(sock, echoBuffer, ECHOMAX, 0, …) < 0) { if (errno = EINTR) …/*Alarm went off */ else DieWithError(“recvfrom() failed”); }
Timeouts
Using asynchronous I/O the operating system informs the program for the occurrence of an I/O related eventwhat happens if a UPD packet is lost?
We may need to know if something doesn’t happen after some time
unsigned int alarm (unsigned int secs);
starts a timer that expires after the specified number of seconds (secs)
returns the number of seconds remaining until any previously scheduled alarm was due to be delivered, or zero if there was no previously scheduled alarm
process receives SIGALARM signal when timer expires and errnois set to EINTR;
Iterative Stream Socket Server
Handles one client at a timeAdditional clients can connect while one is being served
connections are established
they are able to send requests
but, the server will respond after it finishes with the first client
Works well if each client required a small, bounded amount of work by the server
otherwise, the clients experience long delays
Iterative Server-Example: echo using stream socket
#include <stdio.h> /* for printf() and fprintf() */
#include <sys/socket.h> /* for socket(), bind(), connect(), recv() and send() */
#include <arpa/inet.h> /* for sockaddr_inand inet_ntoa() */
#include <stdlib.h> /* for atoi() and exit() */
#include <string.h> /* for memset() */
#include <unistd.h> /* for close() */
#define MAXPENDING 5 /* Maximum outstanding connection requests */
void DieWithError(char *errorMessage); /* Error handling function */
void HandleTCPClient(intclntSocket); /* TCP client handling function */
int main(intargc, char *argv[])
{
int servSock; /* Socket descriptor for server */
int clntSock; /* Socket descriptor for client */
struct sockaddr_inechoServAddr; /* Local address */
struct sockaddr_inechoClntAddr; /* Client address */
unsigned short echoServPort; /* Server port */
unsigned int clntLen; /* Length of client address data structure */
if (argc != 2) { /* Test for correct number of arguments*/
fprintf(stderr, "Usage: %s <Server Port>\n", argv[0]);
exit(1);
}
echoServPort = atoi(argv[1]); /* First arg: local port */
/* Create socket for incoming connections */
if ((servSock = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP)) < 0)
DieWithError("socket() failed");
/* Construct local address structure */
memset(&echoServAddr, 0, sizeof(echoServAddr)); /* Zero out structure */
echoServAddr.sin_family= AF_INET; /* Internet address family */
echoServAddr.sin_addr.s_addr= htonl(INADDR_ANY); /* Any incoming interface */
echoServAddr.sin_port= htons(echoServPort); /* Local port */
/* Bind to the local address */
if (bind(servSock, (struct sockaddr*) &echoServAddr, sizeof(echoServAddr)) < 0)
DieWithError("bind() failed");
/* Mark the socket so it will listen for incoming connections */ if (listen(servSock, MAXPENDING) < 0) DieWithError("listen() failed");
for (;;) /* Run forever */
{
/* Set the size of the in-out parameter */
clntLen= sizeof(echoClntAddr);
/* Wait for a client to connect */
if ((clntSock= accept(servSock, (struct sockaddr*) &echoClntAddr, &clntLen)) < 0)
DieWithError("accept() failed");
/* clntSock is connected to a client! */
printf("Handlingclient %s\n", inet_ntoa(echoClntAddr.sin_addr));
HandleTCPClient(clntSock);
}
/* NOT REACHED */
}
#define RCVBUFSIZE 32 /* Size of receive buffer */
void HandleTCPClient(intclntSocket)
{
char echoBuffer[RCVBUFSIZE]; /* Buffer for echo string */
int recvMsgSize; /* Size of received message */
/* Receive message from client */
if ((recvMsgSize= recv(clntSocket, echoBuffer, RCVBUFSIZE, 0)) < 0)
DieWithError("recv() failed");
/* Send received string and receive again until end of transmission */
while (recvMsgSize> 0) /* zero indicates end of transmission */
{
/* Echo message back to client */
if (send(clntSocket, echoBuffer, recvMsgSize, 0) != recvMsgSize)
DieWithError("send() failed");
/* See if there is more data to receive */
if ((recvMsgSize= recv(clntSocket, echoBuffer, RCVBUFSIZE, 0)) < 0)
DieWithError("recv() failed");
}
close(clntSocket); /* Close client socket */
}
Multitasking -Per-Client Process
For each client connection request, a new process is created to handle the communication.int fork();
a new process is created, identical to the calling process, except for its process ID and the return value it receives from fork()
returns 0 to childprocess, and the process ID of the new child to parent
aution:(慎重)
when a child process terminates, it does not automatically disappears
use waitpid()to parent in order to “harvest” zombies
Multitasking-Per-Client Process -Example: echo using stream socket
#include <sys/wait.h>/* for waitpid() */ int main(intargc, char *argv[]) { int servSock; /* Socket descriptor for server */ int clntSock; /* Socket descriptor for client */ unsigned short echoServPort; /* Server port */ pid_t processID;/* Process ID from fork()*/ unsigned int childProcCount= 0; /* Number of child processes */ if (argc != 2) { /* Test for correct number of arguments */ fprintf(stderr, "Usage: %s <Server Port>\n", argv[0]); exit(1); } echoServPort = atoi(argv[1]); /* First arg: local port */ servSock= CreateTCPServerSocket(echoServPort); for (;;) { /* Run forever */ clntSock= AcceptTCPConnection(servSock); if ((processID= fork()) < 0) DieWithError (“fork() failed”); /* Fork child process */ else if (processID= 0) { /* This is the child process */ close(servSock); /* child closes listening socket */ HandleTCPClient(clntSock); exit(0); /* child process terminates */ } close(clntSock);/* parent closes child socket */ childProcCount++;/* Increment number of outstanding child processes */ while (childProcCount) {/* Clean up all zombies */ processID= waitpid((pid_t) -1, NULL, WHOANG); /* Non-blocking wait */ if (processID< 0) DieWithError (“...”); else if (processID== 0) break;/* No zombie to wait */ else childProcCount--;/* Cleaned up after a child */ } } /* NOT REACHED */ }
Multitasking -Per-Client Thread
Forking a new process is expensiveduplicate the entire state (memory, stack, file/socket descriptors, …)
Threads decrease this cost by allowing multitasking within the same process
threads share the same address space (code and data)
Multitasking -Per-Client Thread -Example: echo using stream socket
#include <pthread.h>/* for POSIX threads */ void *ThreadMain(void*arg)/* Main program of a thread */ struct ThreadArgs{/* Structure of arguments to pass to client thread */ int clntSock;/* socket descriptor for client */ }; int main(intargc, char *argv[]) { int servSock; /* Socket descriptor for server */ int clntSock; /* Socket descriptor for client */ unsigned short echoServPort; /* Server port */ pthread_t threadID;/* Thread ID from pthread_create()*/ struct ThreadArgs*threadArgs; /* Pointer to argument structure for thread */ if (argc != 2) { /* Test for correct number of arguments */ fprintf(stderr, "Usage: %s <Server Port>\n", argv[0]); exit(1); } echoServPort = atoi(argv[1]); /* First arg: local port */ servSock= CreateTCPServerSocket(echoServPort); for (;;) { /* Run forever */ clntSock= AcceptTCPConnection(servSock); /* Create separate memory for client argument */ if ((threadArgs= (struct ThreadArgs*) malloc(sizeof(structThreadArgs)))) == NULL) DieWithError(“…”); threadArgs-> clntSock = clntSock; /* Create client thread */ if (pthread_create (&threadID, NULL, ThreadMain, (void *) threadArgs) != 0) DieWithError(“…”); } /* NOT REACHED */ } void *ThreadMain(void*threadArgs) { int clntSock; /* Socket descriptor for client connection */ pthread_detach(pthread_self()); /* Guarantees that thread resources are deallocatedupon return */ /* Extract socket file descriptor from argument */ clntSock= ((struct ThreadArgs*) threadArgs) -> clntSock; free(threadArgs); /* Deallocatememory for argument */ HandleTCPClient(clntSock); return (NULL); }
Multitasking -Constrained
Both process and thread incurs(承受) overhead(开销): creation, scheduling and context switchingAs their numbers increases
this overhead increases
after some point it would be better if a client was blocked
Solution: Constrained multitasking. The server:
1. begins, creating, binding and listening to a socket
2. creates a number of processes, each loops forever and accept connections from the same socket
3. when a connection is established
- the client socket descriptor is returned to only one process
- the other remain blocked
Multitasking -Constrained -Example: echo using stream socket
void ProcessMain(int servSock); /* Main program of process */ int main(intargc, char *argv[]) { int servSock; /* Socket descriptor for server*/ unsigned short echoServPort; /* Server port */ pid_t processID; /* Process ID */ unsigned int processLimit; /* Number of child processes to create */ unsigned int processCt; /* Process counter */ if (argc != 3) { /* Test for correct number of arguments*/ fprintf(stderr,"Usage: %s <SERVER PORT> <FORK LIMIT>\n", argv[0]); exit(1); } echoServPort = atoi(argv[1]); /* First arg: local port */ processLimit= atoi(argv[2]); /* Second arg: number of child processes */ servSock = CreateTCPServerSocket(echoServPort); for (processCt=0; processCt< processLimit; processCt++) if ((processID= fork()) < 0) DieWithError("fork() failed"); /* Fork child process */ else if (processID== 0) ProcessMain(servSock); /* If this is the child process */ exit(0); /* The children will carry on */ } void ProcessMain(int servSock) { int clntSock; /* Socket descriptor for client connection */ for (;;) { /* Run forever */ clntSock = AcceptTCPConnection(servSock); printf("withchild process: %d\n", (unsigned int) getpid()); HandleTCPClient(clntSock); } }
Multiplexing
So far, we have dealt with a single I/O channel;We may need to cope with multiple I/O channels;
e.g., supporting the echo service over multiple ports
Problem: from which socket the server should accept connections or receive messages?
it can be solved using non-blocking sockets, but it requires polling
Solution: select()
specifies a list of descriptors to check for pending I/O operations
blocks until one of the descriptors is ready
returns which descriptors are ready
int select (maxDescPlus1, &readDescs, &writeDescs, &exceptionDescs, &timeout);
maxDescsPlus1: integer, hint of the maximum number of descriptors
readDescs: fd_set, checked for immediate input availability
writeDescs: fd_set, checked for the ability to immediately write data
exceptionDescs: fd_set, checked for pending exceptions
timeout: struct timeval, how long it blocks (NULL forever)
returns the total number of ready descriptors, -1 in case of error
changes the descriptor lists so that only the corresponding positions are set
int FD_ZERO (fd_set*descriptorVector); /* removes all descriptors from vector */ int FD_CLR (int descriptor, fd_set*descriptorVector); /* remove descriptor from vector */ int FD_SET (int descriptor, fd_set*descriptorVector); /* add descriptor to vector */ int FD_ISSET (int descriptor, fd_set*descriptorVector); /* vector membership check */
struct timeval{ time_ttv_sec;/* seconds */ time_ttv_usec;/* microseconds */ };
Multiplexing -Example: echo using stream socket
#include <sys/time.h> /* for struct timeval{} */ int main(intargc, char *argv[]) { int *servSock; /* Socket descriptors for server */ int maxDescriptor; /* Maximum socket descriptor value */ fd_setsockSet; /* Set of socket descriptors for select() */ long timeout; /* Timeout value given on command-line */ struct timevalselTimeout; /* Timeout for select() */ int running = 1; /* 1 if server should be running; 0 otherwise */ int noPorts; /* Number of port specified on command-line */ int port; /* Looping variable for ports */ unsigned short portNo; /* Actual port number */ if (argc < 3) { /* Test for correct number of arguments */ fprintf(stderr, "Usage: %s <Timeout (secs.)> <Port 1> ...\n", argv[0]); exit(1); } timeout= atol(argv[1]); /* First arg: Timeout */ noPorts= argc -2; /* Number of ports is argument count minus 2 */ servSock = (int *) malloc(noPorts* sizeof(int)); /* Allocate list of sockets for incoming connections */ maxDescriptor= -1; /* Initialize maxDescriptorfor use by select() */ for (port = 0; port < noPorts; port++) { /* Create list of ports and sockets to handle ports */ portNo= atoi(argv[port+ 2]); /* Add port to port list. Skip first two arguments */ servSock[port] = CreateTCPServerSocket(portNo); /* Create port socket */ if (servSock[port] > maxDescriptor)/* Determine if new descriptor is the largest */ maxDescriptor= servSock[port]; } printf("Startingserver: Hit return to shutdown\n"); while (running) { /* Zero socket descriptor vector and set for server sockets */ /* This must be reset every time select() is called */ FD_ZERO(&sockSet); FD_SET(STDIN_FILENO, &sockSet); /* Add keyboard to descriptor vector */ for (port = 0; port < noPorts; port++) FD_SET(servSock[port], &sockSet); /* Timeout specification */ /* This must be reset every time select() is called */ selTimeout.tv_sec= timeout; /* timeout (secs.) */ selTimeout.tv_usec= 0; /* 0 microseconds */ /* Suspend program until descriptor is ready or timeout */ if (select(maxDescriptor+ 1, &sockSet, NULL, NULL, &selTimeout) == 0) printf("Noecho requests for %ld secs...Server still alive\n", timeout); else { if (FD_ISSET(0, &sockSet)) { /* Check keyboard */ printf("Shuttingdown server\n"); getchar(); running = 0; } for (port = 0; port < noPorts; port++) if (FD_ISSET(servSock[port], &sockSet)) { printf("Requeston port %d: ", port); HandleTCPClient(AcceptTCPConnection(servSock[port])); } } } for (port = 0; port < noPorts; port++) close(servSock[port]); /* Close sockets */ free(servSock);/* Free list of sockets */ exit(0); }
Multiple Recipients(接收者)
So far, all sockets have dealt with unicast(单播) communication i.e., an one-to-one communication, where one copy (“uni”) of the data is sent (“cast”).what if we want to send data to multiple recipients?
Solution: unicasta(单播) copy of the data to each recipient; inefficient(低效率), e.g.,
consider we are connected to the internet through a 3Mbps line
a video server sends 1-Mbps streams
then, server can support only three clients simultaneously
Solution: using network support
broadcast, all the hosts of the network receive the message
multicast, a message is sent to some subset of the host
for IP: only UDP socketsare allowed to broadcast and multicast
Multiple Recipients -Broadcast
Only the IP address changesLocalbroadcast: to address 255.255.255.255
- send the message to every host on the same broadcast network
- not forwarded by the routers(不被路由转发)
Directed broadcast:
- for network identifier 169.125(i.e., with subnet mask 255.255.0.0)
- the directed broadcast address is 169.125.255.255
No network-wide broadcast address is available, why?
In order to use broadcast the options of socket must change:
int broadcastPermission = 1; setsockopt(sock, SOL_SOCKET, SO_BROADCAST, (void*) &broadcastPermission, sizeof(broadcastPermission));
Using class Daddresses
range from 224.0.0.0to 239.255.255.255
hosts send multicast requestsfor specific addresses
a multicast groupis formed
we need to set TTL (time-to-live), to limit the number of hops -using sockopt()
no need to change the options of socket
Usefull Functions
int atoi(constchar *nptr);
converts the initial portion of the string pointed to by nptrto int
int inet_aton(constchar *cp, struct in_addr*inp);
converts the Internet host address cpfrom the IPv4 numbers-and-dots notation into binary form (in network byte order)
stores it in the structure that inppoints to.
it returns nonzero if the address is valid, and 0 if not
char *inet_ntoa(structin_addrin);
converts the Internet host address in, given in network byte order, to a string in IPv4 dotted-decimal notation
typedef uint32_t in_addr_t; struct in_addr{ in_addr_t s_addr; };
int getpeername(intsockfd, struct sockaddr*addr, socklen_t*addrlen);
returns the address (IP and port) of the peer connected to the socket sockfd, in the buffer pointed to by addr
0 is returned on success; -1 otherwise
int getsockname(intsockfd, struct sockaddr*addr, socklen_t*addrlen);
returns the current address to which the socket sockfdis bound, in the buffer pointed to by addr
0 is returned on success; -1 otherwise
Domain Name Service
struct hostent*gethostbyname(constchar *name);
returns a structure of type hostentfor the given host name
name is a hostname, or an IPv4 address in standard dot notatione.g. gethostbyname(“www.csd.uoc.gr”);
struct hostent*gethostbyaddr(constvoid *addr, socklen_tlen, int type);
returns a structure of type hostentfor the given host address addrof length lenand address type type
struct hostent{ char *h_name; /* official name of host */ char **h_aliases; /* alias list (strings) */ int h_addrtype; /* host address type (AF_INET) */ int h_length; /* length of address */ char **h_addr_list; /* list of addresses (binary in network byte order) */ } #define h_addrh_addr_list[0] /* for backward compatibility */
struct servent*getservbyname(constchar *name, const char *proto);
returns a servent structure for the entry from the database that matches the service name using protocol proto.
if protois NULL, any protocol will be matched.e.g. getservbyname(“echo”, “tcp”);
struct servent *getservbyport(int port, const char *proto);
returns a servent structure for the entry from the database that matches the service name using port port
struct servent{ char *s_name; /* official service name */ char **s_aliases; /* list of alternate names (strings)*/ int s_port; /* service port number */ char *s_proto; /* protocol to use (“tcp”or “udp”)*/ }
Compiling and Executing
milo:~/CS556/sockets> gcc-o TCPEchoServerTCPEchoServer.cDieWithError.cHandleTCPClient.c milo:~/CS556/sockets> gcc-o TCPEchoClientTCPEchoClient.cDieWithError.c milo:~/CS556/sockets> TCPEchoServer3451 & [1] 6273 milo:~/CS556/sockets> TCPEchoClient0.0.0.0 hello! 3451 Handling client 127.0.0.1 Received: hello! milo:~/CS556/sockets> ps PID TTY TIME CMD 5128 pts/9 00:00:00 tcsh 6273 pts/9 00:00:00 TCPEchoServer 6279 pts/9 00:00:00 ps milo:~/CS556/sockets> kill 6273 milo:~/CS556/sockets> [1] Terminated TCPEchoServer3451 milo:~/CS556/sockets>
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