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Introduction to Socket ProgrammingJeong, EunYoung (notav at ndsl.kaist.edu)

EE324 Programming Tu-torial

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Socket programming Editor (emacs) Compiling (Makefile) Debugging (gdb)

Outline

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Sockets Abstraction for data communication A process can send/receive data

through sockets

Sockets

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TCP socket API Stream sockets Connection-oriented 4 tuples (source IP, port, destination IP, port) Reliable

UDP socket API Datagram sockets 2 tuples (source IP, port) Unreliable

Socket API

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TCP socket programming Server first listens for incoming connections Client initiates the connection to the server Server accepts the connection Exchange data

Server sends “Hello, world!” to the client Close connection

Reference: Beej’s guide to network program-ming http://

beej.us/guide/bgnet/output/html/multipage/clientserver.html

TCP Socket Programming

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/* create a socket */sockfd = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);if (sockfd < 0) { perror("socket"); return -1;}

/* set the address family (IPv4), address (any address), port */saddr.sin_family = AF_INET;saddr.sin_addr.s_addr = INADDR_ANY;saddr.sin_port = htons(PORT);/* bind the address to the socket */if (bind(sockfd, (struct sockaddr *)&saddr, sizeof(struct sockaddr_in))) {perror("bind");close(sockfd);return -1;}

TCP Server Example

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/* listen connections */if (listen(sockfd, BACKLOG) < 0) {perror("listen");exit(1);}

/* reap all dead processes */sa.sa_handler = sigchld_handler;sigemptyset(&sa.sa_mask);sa.sa_flags = SA_RESTART;if (sigaction(SIGCHLD, &sa, NULL) == -1) {perror("sigaction");exit(1);}

printf("server: waiting for connections...\n");

TCP Server Example

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while (1) { // main accept() loop /* accept new connections from clients */ sin_size = sizeof(their_addr); new_fd = accept(sockfd, (struct sockaddr *)&their_addr, &sin_size); if (new_fd == -1) { perror("accept"); continue; }

inet_ntop(AF_INET, &their_addr.sin_addr, s, sizeof(s)); printf("server: got connection from %s\n", s);

if (!fork()) { // this is the child process close(sockfd); // child doesn't need the listener if (send(new_fd, "Hello, world!", 13, 0) == -1) perror("send"); close(new_fd); exit(0); } close(new_fd); // parent doesn't need this}

TCP Server Example

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/* get the server host entry */hp = gethostbyname(argv[1]);if (hp == NULL) {perror("gethostbyname");return -1;}

/* create stream socket */sockfd = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);if (sockfd < 0) {perror("socket");return -1;}

/* set the destination address */daddr.sin_family = AF_INET;memcpy(&daddr.sin_addr.s_addr, hp->h_addr, hp->h_length);daddr.sin_port = htons(PORT);

TCP Client Example

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/* connect to the destination */if (connect(sockfd, (struct sockaddr *)&daddr, sizeof(struct sockaddr_in))) { close(sockfd); perror("connect"); return -1;}inet_ntop(AF_INET, &daddr.sin_addr, s, sizeof(s));printf("client: connecting to %s\n", s);

/* receive message sent from the server */if ((numbytes = recv(sockfd, buf, MAXDATASIZE-1, 0)) == -1) { perror("recv"); exit(1);}/* print the received message */buf[numbytes] = '\0';printf("client: received '%s'\n", buf);close(sockfd);

TCP Client Example

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UDP socket programming No connection establishment! Server binds to a port (bind()) Exchange data

Server waits for incoming messages (recvfrom())

Client sends messages (sendto()) Simple!

But there can be loss of data

UDP Socket Programming

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/* create a socket */sockfd = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);if (sockfd < 0) {perror("socket");return -1;}

/* set the address family (IPv4), address (any address), port */saddr.sin_family = AF_INET;saddr.sin_addr.s_addr = INADDR_ANY;saddr.sin_port = htons(PORT);

/* bind the address to the socket */if (bind(sockfd, (struct sockaddr *)&saddr, sizeof(struct sockaddr_in))) {perror("bind");close(sockfd);return -1;}

UDP Listener Example

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printf("listener: waiting to recvfrom...\n");

addr_len = sizeof their_addr;if ((numbytes = recvfrom(sockfd, buf, MAXBUFLEN - 1 , 0,(struct sockaddr *)&their_addr, &addr_len)) == -1) {perror("recvfrom");exit(1);}

printf("listener: got packet from %s\n",inet_ntop(AF_INET, &their_addr.sin_addr, s, sizeof(s)));printf("listener: packet is %d bytes long\n", numbytes);buf[numbytes] = '\0';printf("listener: packet contains \"%s\"\n", buf);

close(sockfd);

UDP Listener Example

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/* get the server host entry */hp = gethostbyname(argv[1]);if (hp == NULL) {perror("gethostbyname");return -1;}

/* create a socket */sockfd = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);if (sockfd < 0) {perror("socket");return -1;}

UDP Talker Example

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/* set the destination address */daddr.sin_family = AF_INET;memcpy(&daddr.sin_addr.s_addr, hp->h_addr, hp->h_length);daddr.sin_port = htons(PORT);

if ((numbytes = sendto(sockfd, argv[2], strlen(argv[2]), 0, (struct sockaddr *)&daddr, sizeof(struct sockaddr))) == -1) {perror("talker: sendto");exit(1);}

printf("talker: sent %d bytes to %s\n", numbytes, argv[1]);close(sockfd);

UDP Talker Example

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Launching emacs $ emacs server.c

Notations C-x: Ctrl + x, M-x: ESC followed by x (or Alt + x)

Moving around in buffers Forward: →, C-f Backward: ←, C-b Next line: ↓, C-n Previous line: ↑, C-p Searching

C-s: forward search, C-r: backward search Jumping

M-g g: go to the line

Emacs

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Region Marking: C-SPC C-x h: Select all M-h: Select paragraph

Killing C-k: kill line, C-u 10 C-k: kill 10 lines

Yanking (Paste) C-y: Yanks last killed text

Undo C-/, C-_, C-x u

Emacs

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Finally exit! C-x C-c

For more detail http://www.gnu.org/software/emacs/tour/ http://cmgm.stanford.edu/classes/unix/emacs.

html

Emacs

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Simple compiling g++ (source codes) –o (object name) Ex) $ g++ server.c –o server Ex) $ g++ a.c b.c d.c –o abd

Compile options -g: for debugging -Wall: show as many warnings as possible -DNDEBUG: remove asserts when compiling Ex) $ g++ -g –Wall server.c –o server

Compiling

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Script for easier compilation g++ main.cpp hello.cpp factorial.cpp -o hello -

> make Basic Makefile

Target: dependencies [tab] system command

Exampleall:

g++ main.cpp hello.cpp factorial.cpp -o hello

Makefile

Reference: http://mrbook.org/tutorials/make/

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Build process Compiler generates object files from source codes Linker creates executable binary from the object files

Exampleall: hellohello: main.o factorial.o hello.og++ main.o factorial.o hello.omain.o: main.cppg++ -c main.cpp…clean:rm -rf *.o hello

Using Dependencies

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ExampleCC=g++CFLAGS=-g –Wall

all: hellohello: main.o factorial.o hello.o$(CC) main.o factorial.o hello.omain.o: main.cpp $(CC) $(CFLAGS) -c main.cpp…clean:rm -rf *.o hello

Using Variables

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CC=g++CFLAGS=-g –WallLDFLAGS=SOURCES=main.cpp hello.cpp factorial.cppOBJECTS=$(SOURCES:.cpp=.o)EXECUTABLE=hello

all: $(SOURCES) $(EXECUTABLE)$(EXECUTABLE): $(OBJECTS) –o $@

$(CC) $(LDFLAGS) $(OBJECTS) –o $@.cpp.o:

$(CC) $(CFLAGS) –c $< -o $@clean:

rm -rf *.o $(EXECUTABLE)

Complete Example

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Debugger Helps pointing problem of your program

gdb

Reference: http://www.cs.cmu.edu/~gilpin/tutorial/

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Running program gdb main (gdb) run

Exiting (gdb) quit

Running with gdb

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Exploring stack frames backtrace (or bt): print backtrace of all stack

frames up: go to the stack frame called by this one down: go to the stack frame called this one help stack for more features

print: print value of expression Ex) print item_to_remove

x: examine memory Ex) x 0xffbef014

Inspecting Crashes

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Breakpoints Make the program break at a certain point break LinkedList<int>::remove Or break main.cc:53

Giving condition Make the breakpoint only works on a certain

condition condition 1 item_to_remove==1

Stepping step: forward a line

Conditional Breakpoints

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Line 77: marker is set to 0 Line 79: marker is accessed Removing line 77 will make it work

For more information http://www.cs.cmu.edu/~gilpin/tutorial/ http://www.unknownroad.com/rtfm/gdbtut/ http://www.cs.umd.edu/~srhuang/teachin

g/cmsc212/gdb-tutorial-handout.pdf

Finding the Bug

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Question?