Author : BracaMan
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bellow 1024 are reserved.
You can set a port above 1024 and bellow 65535 (unless the ones being used by other
programs).
6.3. connect()
#include <sys/types.h>
#include <sys/socket.h>
int connect(int fd, struct sockaddr *serv_addr, int addrlen);
Let's see the arguments:
fd -> is the socket file descriptor returned by socket() call
serv_addr -> is a pointer to struct sockaddr that contains destination IP
address and port
addrlen -> set it to sizeof(struct sockaddr)
connect() is used to connect to an IP address on a defined port. It returns -1 on
error.
6.4. listen()
#include <sys/types.h>
#include <sys/socket.h>
int listen(int fd,int backlog);
Let's see the arguments:
fd -> is the socket file descriptor returned by socket() call
backlog -> is the number of allowed connections
listen() is used if you're waiting for incoming connections, this is, if you want
someone to connect to your machine. Before calling listen(), you must call bind()
or you'll be listening on a random port =) After calling listen() you must call
accept() in order to accept incoming connection. Resuming, the sequence of system calls
is:
1. socket()
2. bind()
3. listen()
4. accept() /* In the next section I'll explain how to use accept() */
As all functions above described, listen() returns -1 on error.
6.5. accept()
#include <sys/socket.h>
int accept(int fd, void *addr, int *addrlen);
Let's see the arguments:
fd -> is the socket file descriptor returned by listen() call
addr -> is a pointer to struct sockaddr_in where you can determine which host
is calling you from which port
addrlen -> set it to sizeof(struct sockaddr_in) before its address is passed
to accept()
When someone is trying to connect to your computer, you must use accept() to get the
connection. It's very simple to understand: you just get a connection if you accept =)
Next, I'll give you a little example of accept() use because it's a little different
from other functions.
(...)
sin_size=sizeof(struct sockaddr_in);
if ((fd2 = accept(fd,(struct sockaddr *)&client,&sin_size))==-1){ /* calls accept() */
printf("accept() error\n");
exit(-1);
}
(...)
From now on, fd2 will be used for add send() and recv() calls.
6.6. send()
int send(int fd,const void *msg,int len,int flags);
Let's see the arguments:
fd -> is the socket descriptor where you want to send data to
msg -> is a pointer to the data you want to send
len -> is the length of the data you want to send (in bytes)
flags -> set it to 0
This function is used to send data over stream sockets or CONNECTED datagram sockets.
If you want to send data over UNCONNECTED datagram sockets you must use sendto().
send() returns the number of bytes sent out and it will return -1 on error.
6.7. recv()
int recv(int fd, void *buf, int len, unsigned int flags);
Let's see the arguments:
fd -> is the socket descriptor to read from
buf -> is the buffer to read the information into
len -> is the maximum length of the buffer
flags -> set it to 0
As I said above about send(), this function is used to send data over stream sockets or
CONNECTED datagram sockets. If you want to send data over UNCONNECTED datagram sockets
you must use recvfrom().
recv() returns the number of bytes read into the buffer and it'll return -1 on error.
6.8. sendto()
int sendto(int fd,const void *msg, int len, unsigned int flags,
const struct sockaddr *to, int tolen);
Let's see the arguments:
fd -> the same as send()
msg -> the same as send()
len -> the same as send()
flags -> the same as send()
to -> is a pointer to struct sockaddr
tolen -> set it to sizeof(struct sockaddr)
As you can see, sendto() is just like send(). It has only two more arguments : "to"
and "tolen" =)
sendto() is used for UNCONNECTED datagram sockets and it returns the number of bytes
sent out and it will return -1 on error.
6.9. recvfrom()
int recvfrom(int fd,void *buf, int len, unsigned int flags
struct sockaddr *from, int *fromlen);
Let's see the arguments:
fd -> the same as recv()
buf -> the same as recv()
len -> the same as recv()
flags -> the same as recv()
from -> is a pointer to struct sockaddr
fromlen -> is a pointer to a local int that should be initialised
to sizeof(struct sockaddr)
recvfrom() returns the number of bytes received and it'll return -1 on error.
6.10. close()
close(fd);
close() is used to close the connection on your socket descriptor. If you call close(),
it won't be no more writes or reads and if someone tries to read/write will receive an
error.
6.11. shutdown()
int shutdown(int fd, int how);
Let's see the arguments:
fd -> is the socket file descriptor you want to shutdown
how -> you put one of those numbers:
0 -> receives disallowed
1 -> sends disallowed
2 -> sends and receives disallowed
When how is set to 2, it's the same thing as close().
shutdown() returns 0 on success and -1 on error.
6.12. gethostname()
#include <unistd.h>
int gethostname(char *hostname, size_t size);
Let's see the arguments:
hostname -> is a pointer to an array that contains hostname
size -> length of the hostname array (in bytes)
gethostname() is used to get the name of the local machine.
7. SOME WORDS ABOUT DNS
I created this section, because I think you should know what DNS is. DNS stands for "Domain
Name Service" and basically, it's used to get IP addresses. For example, I need to know the
IP address of queima.ptlink.net and through DNS I'll get 212.13.37.13 .
This is important, because functions we saw above (like bind() and connect()) work with IP
addresses.
To see you how you can get queima.ptlink.net IP address on c, I made a little example:
/* <---- SOURCE CODE STARTS HERE ----> */
#include <stdio.h>
#include <netdb.h> /* This is the header file needed for gethostbyname() */
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
int main(int argc, char *argv[])
{
struct hostent *he;
if (argc!=2){
printf("Usage: %s <hostname>\n",argv[0]);
exit(-1);
}
if ((he=gethostbyname(argv[1]))==NULL){
printf("gethostbyname() error\n");
exit(-1);
}
printf("Hostname : %s\n",he->h_name); /* prints the hostname */
printf("IP Address: %s\n",inet_ntoa(*((struct in_addr *)he->h_addr))); /* prints IP address */
}
/* <---- SOURCE CODE ENDS HERE ----> */
8. A STREAM SERVER EXAMPLE
In this section, I'll show you a nice example of a stream server. The source code is all
commented so that you ain't no possible doubts =)
Let's start:
/* <---- SOURCE CODE STARTS HERE ----> */
#include <stdio.h> /* These are the usual header files */
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#define PORT 3550 /* Port that will be opened */
#define BACKLOG 2 /* Number of allowed connections */
main()
{
int fd, fd2; /* file descriptors */
struct sockaddr_in server; /* server's address information */
struct sockaddr_in client; /* client's address information */
int sin_size;
if ((fd=socket(AF_INET, SOCK_STREAM, 0)) == -1 ){ /* calls socket() */
printf("socket() error\n");
exit(-1);
}
server.sin_family = AF_INET;
server.sin_port = htons(PORT); /* Remember htons() from "Conversions" section? =) */
server.sin_addr.s_addr = INADDR_ANY; /* INADDR_ANY puts your IP address automatically */
bzero(&(server.sin_zero),8); /* zero the rest of the structure */
if(bind(fd,(struct sockaddr*)&server,sizeof(struct sockaddr))==-1){ /* calls bind() */
printf("bind() error\n");
exit(-1);
}
if(listen(fd,BACKLOG) == -1){ /* calls listen() */
printf("listen() error\n");
exit(-1);
}
while(1){
sin_size=sizeof(struct sockaddr_in);
if ((fd2 = accept(fd,(struct sockaddr *)&client,&sin_size))==-1){ /* calls accept() */
printf("accept() error\n");
exit(-1);
}
printf("You got a connection from %s\n",inet_ntoa(client.sin_addr) ); /* prints client's IP */
send(fd2,"Welcome to my server.\n",22,0); /* send to the client welcome message */
close(fd2); /* close fd2 */
}
}
/* <---- SOURCE CODE ENDS HERE ----> */
9. A STREAM CLIENT EXAMPLE
/* <---- SOURCE CODE STARTS HERE ----> */
#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h> /* netbd.h is needed for struct hostent =) */
#define PORT 3550 /* Open Port on Remote Host */
#define MAXDATASIZE 100 /* Max number of bytes of data */
int main(int argc, char *argv[])
{
int fd, numbytes; /* files descriptors */
char buf[MAXDATASIZE]; /* buf will store received text */
struct hostent *he; /* structure that will get information about remote host */
struct sockaddr_in server; /* server's address information */
if (argc !=2) { /* this is used because our
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