socket connect vs bind

Question

Both connect   and bind   system calls  associate  the socket file descriptor to an address  typically an ip port combination   Their prototypes are like -  int connect int sockfd  const struct sockaddr  addr                 socklen t addrlen     and   int bind int sockfd  const struct sockaddr  addr              socklen t addrlen     What is the exact difference between 2 calls  When should one use connect   and when bind     Specifically  in some sample server client codes  found that client is using connect   and server is using the bind   call  Reason was not fully clear to me

User · Accepted Answer

To make understanding better   lets find out where exactly bind and connect comes into picture     Further to positioning of two calls   as clarified by Sourav   bind   associates the socket with its local address  that s why server side binds  so that clients can use that address to connect to server   connect   is used to connect to a remote  server  address  that s why is client side  connect  read as  connect to server  is used   We cannot use them interchangeably  even when we have client server on same machine  because of specific roles and corresponding implementation   I will further recommend to correlate these calls TCP IP handshake       So   who will send SYN here   it will be connect     While bind   is used for defining the communication end point   Hope this helps

User · Answer

The one liner   bind   to own address  connect   to remote address   Quoting from the man page of bind       bind   assigns the address specified by addr to the socket referred to by the file descriptor sockfd  addrlen specifies the size  in bytes  of the address structure pointed to by addr  Traditionally  this operation is called  assigning a name to a socket     and  from the same for connect       The connect   system call connects the socket referred to by the file descriptor sockfd to the address specified by addr    To clarify     bind   associates the socket with its local address  that s why server side binds  so that clients can use that address to connect to server   connect   is used to connect to a remote  server  address  that s why is client side  connect  read as  connect to server  is used

User · Answer

bind tells the running process to claim a port  i e  it should bind itself to port 80 and listen for incomming requests  with bind  your process becomes a server  when you use connect  you tell your process to connect to a port that is ALREADY in use  your process becomes a client  the difference is important  bind wants a port that is not in use  so that it can claim it and become a server   and connect wants a port that is already in use  so it can connect to it and talk to the server

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Too Long  Don t Read  The difference is whether the source  local  or the destination address port is being set  In short  bind   set the source and connect   set the destination  Regardless of TCP or UDP   bind    bind   set the socket s local  source  address  This is the address where packets are received  Packets sent by the socket carry this as the source address  so the other host will know where to send back its packets   If receive is not needed the socket source address is useless  Protocols like TCP require receiving enabled in order to send properly  as the destination host send back a confirmation when one or more packets have arrived  i e  acknowledgement    connect     TCP has a  connected  state  connect   triggers the TCP code to try to establish a connection to the other side  UDP has no  connected  state  connect   only set a default address to where packets are sent when no address is specified  When connect   is not used  sendto   or sendmsg   must be used containing the destination address    When connect   or a send function is called  and no address is bound  Linux automatically bind the socket to a random port  For technical details  take a look at inet autobind   in Linux kernel source code   Side notes   listen   is TCP only  In AF INET family  the socket s source or destination address  struct sockaddr in  is composed by an IP address  see IP header   and TCP or UDP port  see TCP and UDP header

User · Answer

I think it would help your comprehension if you think of connect   and listen   as counterparts  rather than connect   and bind    The reason for this is that you can call or omit bind   before either  although it s rarely a good idea to call it before connect    or not to call it before listen     If it helps to think in terms of servers and clients  it is listen   which is the hallmark of the former  and connect   the latter  bind   can be found - or not found - on either   If we assume our server and client are on different machines  it becomes easier to understand the various functions   bind   acts locally  which is to say it binds the end of the connection on the machine on which it is called  to the requested address and assigns the requested port to you  It does that irrespective of whether that machine will be a client or a server  connect   initiates a connection to a server  which is to say it connects to the requested address and port on the server  from a client  That server will almost certainly have called bind   prior to listen    in order for you to be able to know on which address and port to connect to it with using connect     If you don t call bind    a port and address will be implicitly assigned and bound on the local machine for you when you call either connect    client  or listen    server   However  that s a side effect of both  not their purpose  A port assigned in this manner is ephemeral   An important point here is that the client does not need to be bound  because clients connect to servers  and so the server will know the address and port of the client even though you are using an ephemeral port  rather than binding to something specific  On the other hand  although the server could call listen   without calling bind    in that scenario they would need to discover their assigned ephemeral port  and communicate that to any client that it wants to connect to it   I assume as you mention connect   you re interested in TCP  but this also carries over to UDP  where not calling bind   before the first sendto    UDP is connection-less  also causes a port and address to be implicitly assigned and bound  One function you cannot call without binding is recvfrom    which will return an error  because without an assigned port and bound address  there is nothing to receive from  or too much  depending on how you interpret the absence of a binding

User · Answer

From Wikipedia http   en wikipedia org wiki Berkeley sockets bind 28 29  connect     The connect   system call connects a socket  identified by its file descriptor  to a remote host specified by that host s address in the argument list   Certain types of sockets are connectionless  most commonly user datagram protocol sockets  For these sockets  connect takes on a special meaning  the default target for sending and receiving data gets set to the given address  allowing the use of functions such as send   and recv   on connectionless sockets   connect   returns an integer representing the error code  0 represents success  while -1 represents an error   bind     bind   assigns a socket to an address  When a socket is created using socket    it is only given a protocol family  but not assigned an address  This association with an address must be performed with the bind   system call before the socket can accept connections to other hosts  bind   takes three arguments   sockfd  a descriptor representing the socket to perform the bind on  my addr  a pointer to a sockaddr structure representing the address to bind to  addrlen  a socklen t field specifying the size of the sockaddr structure  Bind   returns 0 on success and -1 if an error occurs   Examples  1  Using Connect   include  lt stdio h gt   include  lt sys socket h gt   include  lt netinet in h gt   include  lt string h gt   int main      int clientSocket    char buffer 1024     struct sockaddr in serverAddr    socklen t addr size       ---- Create the socket  The three arguments are  ----        1  Internet domain 2  Stream socket 3  Default protocol  TCP in this case       clientSocket   socket PF INET  SOCK STREAM  0        ---- Configure settings of the server address struct ----        Address family   Internet      serverAddr sin family   AF INET       Set port number  using htons function to use proper byte order      serverAddr sin port   htons 7891        Set the IP address to desired host to connect to      serverAddr sin addr s addr   inet addr  192 168 1 17         Set all bits of the padding field to 0      memset serverAddr sin zero    0   sizeof serverAddr sin zero          ---- Connect the socket to the server using the address struct ----     addr size   sizeof serverAddr    connect clientSocket   struct sockaddr     amp serverAddr  addr size        ---- Read the message from the server into the buffer ----     recv clientSocket  buffer  1024  0        ---- Print the received message ----     printf  Data received   s  buffer         return 0      2  Bind Example   int main         struct sockaddr in source  destination          two sockets declared as previously     int sock   0      int datalen   0      int pkt   0       uint8 t  send buffer   recv buffer       struct sockaddr storage fromAddr       same as the previous entity struct sockaddr storage serverStorage      unsigned int addrlen     in the previous example socklen t addr size      struct timeval tv      tv tv sec   3      3 Seconds Time-out        tv tv usec   0          creating the socket                 if   sock   socket AF INET  SOCK DGRAM  IPPROTO UDP    lt  0           printf  Failed to create socket n           set the socket options       setsockopt sock  SOL SOCKET  SO RCVTIMEO   char    amp tv  sizeof struct timeval           Inititalize source to zero       memset  amp source  0  sizeof source            source is an instance of sockaddr in  Initialization to zero       Inititalize destinaton to zero       memset  amp destination  0  sizeof destination            ---- Configure settings of the source address struct  WHERE THE PACKET IS COMING FROM ----          Address family   Internet        source sin family   AF INET             Set IP address to localhost           source sin addr s addr   INADDR ANY     INADDR ANY   0 0 0 0        Set port number  using htons function to use proper byte order        source sin port   htons 7005           Set all bits of the padding field to 0        memset source sin zero    0   sizeof source sin zero     optional         bind socket to the source WHERE THE PACKET IS COMING FROM       if  bind sock   struct sockaddr     amp source  sizeof source    lt  0           printf  Failed to bind socket            setting the destination  i e our OWN IP ADDRESS AND PORT        destination sin family   AF INET                       destination sin addr s addr   inet addr  127 0 0 1          destination sin port   htons 7005           Creating a Buffer      send buffer  uint8 t    malloc 350       recv buffer  uint8 t    malloc 250        addrlen sizeof fromAddr        memset  void    recv buffer  0  250       memset  void    send buffer  0  350        sendto sock  send buffer  20  0  struct sockaddr     amp destination  sizeof destination         pkt recvfrom sock  recv buffer  98 0  struct sockaddr    amp destination   amp addrlen       if pkt  gt  0          printf   u bytes received n   pkt           I hope that clarifies the difference  Please note that the socket type that you declare will depend on what you require  this is extremely important

[c] socket connect() vs bind()

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