socket.h File Reference

The getsockopt and setsockopt system calls manipulate the options associated with a socket. Options may exist at multiple protocol levels; they are always present at the uppermost "socket" level. When manipulating socket options the level at which the option resides and the name of the option must be specified. To manipulate options at the socket level, level is specified as SOL_SOCKET. To manipulate options at any other level the protocol number of the appropriate protocol controlling the option is supplied. For example, to indicate that an option is to be interpreted by the TCP protocol, level should be set to the protocol number of TCP;

The optval and optlen arguments are used to access option values for setsockopt. For getsockopt they identify a buffer in which the value for the requested option(s) are to be returned. For getsockopt, optlen is a value-result argument, initially containing the size of the buffer pointed to by optval, and modified on return to indicate the actual size of the value returned. If no option value is to be supplied or returned, optval may be NULL.

  #include <sys/socket.h,> which specifies the desired state of the option and the linger interval (see below). SO_SNDTIMEO and SO_RCVTIMEO use a struct timeval argument, defined in  
  #include <sys/time.h.> 
The following options are recognized at the socket level. Except as noted, each may be examined with getsockopt and set with setsockopt. SO_DEBUG            enables recording of debugging information 
SO_REUSEADDR       Allows a socket to be bound to an local address that is already in use. 
SO_REUSEPORT       enables duplicate address and port bindings 
SO_KEEPALIVE       enables keep connections alive 
SO_DONTROUTE       enables routing bypass for outgoing messages 
SO_BROADCAST       enables permission to transmit broadcast messages (enable only)
SO_OOBINLINE       enables reception of out-of-band data in band 
SO_SNDBUF          set buffer size for output 
SO_RCVBUF          set buffer size for input 
SO_SNDLOWAT        set minimum count for output 
SO_RCVLOWAT        set minimum count for input 
SO_SNDTIMEO        set timeout value for output 
SO_RCVTIMEO        set timeout value for input 
SO_ACCEPTFILTER            set accept filter on listening socket 
SO_TYPE            get the type of the socket (get only) 
SO_ERROR           get and clear error on the socket (get only)  

SO_DEBUG enables debugging in the underlying protocol modules. SO_REUSEADDR indicates that the rules used in validating addresses supplied in a bind system call should allow reuse of local addresses. SO_REUSEPORT allows completely duplicate bindings by multiple processes if they all set SO_REUSEPORT before binding the port. This option permits multiple instances of a program to each receive UDP/IP multicast or broadcast datagrams destined for the bound port. SO_KEEPALIVE enables the periodic transmission of messages on a connected socket. SO_DONTROUTE indicates that outgoing messages should bypass the standard routing facilities. Instead, messages are directed to the appropriate network interface according to the network portion of the destination address.

The option SO_BROADCAST requests permission to send broadcast datagrams on the socket. Broadcast was a privileged operation in earlier versions of the system. With protocols that support out-of-band data, the SO_OOBINLINE option requests that out-of-band data be placed in the normal data input queue as received; it will then be accessible with recv or read calls without the MSG_OOB flag. Some protocols always behave as if this option is set. SO_SNDBUF and SO_RCVBUF are options to adjust the normal buffer sizes allocated for output and input buffers, respectively. The buffer size may be increased for high-volume connections, or may be decreased to limit the possible backlog of incoming data. The system places an absolute maximum on these values, which is accessible through the sysctl MIB variable "kern.ipc.maxsockbuf."

SO_SNDLOWAT is an option to set the minimum count for output operations. Most output operations process all of the data supplied by the call, delivering data to the protocol for transmission and blocking as necessary for flow control. Nonblocking output operations will process as much data as permitted subject to flow control without blocking, but will process no data if flow control does not allow the smaller of the low water mark value or the entire request to be processed. A select operation testing the ability to write to a socket will return true only if the low water mark amount could be processed. The default value for SO_SNDLOWAT is set to a convenient size for network efficiency, often 1024. SO_RCVLOWAT is an option to set the minimum count for input operations. In general, receive calls will block until any (non-zero) amount of data is received, then return with the smaller of the amount available or the amount requested. The default value for SO_RCVLOWAT is 1. If SO_RCVLOWAT is set to a larger value, blocking receive calls normally wait until they have received the smaller of the low water mark value or the requested amount.

SO_SNDTIMEO is an option to set a timeout value for output operations. It accepts a struct timeval argument with the number of seconds and microseconds used to limit waits for output operations to complete. If a send operation has blocked for this much time, it returns with a partial count or with the error EWOULDBLOCK if no data were sent. In the current implementation, this timer is restarted each time additional data are delivered to the protocol, implying that the limit applies to output portions ranging in size from the low water mark to the high water mark for output. SO_RCVTIMEO is an option to set a timeout value for input operations. It accepts a struct timeval argument with the number of seconds and microseconds used to limit waits for input operations to complete. In the current implementation, this timer is restarted each time additional data are received by the protocol, and thus the limit is in effect an inactivity timer. If a receive operation has been blocked for this much time without receiving additional data, it returns with a short count or with the error EWOULDBLOCK if no data were received.

SO_ACCEPTFILTER places an accept_filter on the socket, which will filter incoming connections on a listening stream socket before being presented for accept. Once more, listen must be called on the socket before trying to install the filter on it, or else the setsockopt system call will fail.

SO_LINGER option is not supported by this api.

struct  accept_filter_arg {
        char    af_name[16];
        char    af_arg[256-16];
};

The optval argument should point to a struct accept_filter_arg that will select and configure the accept_filter. The af_name argument should be filled with the name of the accept filter that the application wishes to place on the listening socket. The optional argument af_arg can be passed to the accept filter specified by af_name to provide additional configuration options at attach time. Passing in an optval of NULL will remove the filter.

Finally, SO_TYPE and SO_ERROR are options used only with getsockopt. SO_TYPE returns the type of the socket, such as SOCK_STREAM; it is useful for servers that inherit sockets on startup. SO_ERROR returns any pending error on the socket and clears the error status. It may be used to check for asynchronous errors on connected datagram sockets or for other asynchronous errors.

#include <sys/socket.h>
#include <netinet/in.h>
#include <unistd.h>
void SocketOptions()
{
   int sock_fd;
   int optval = 1;
   unsigned int optlen = sizeof(optval);
   int rdoptval;
   sock_fd = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
   setsockopt(sock_fd,SOL_SOCKET,SO_KEEPALIVE,&optval;,optlen);
   getsockopt(sock_fd,SOL_SOCKET,SO_KEEPALIVE,(void*)&rdoptval;,&optlen;);
close(sock_fd);
}

See also: ioctl() socket()

To accept connections a socket is first created with socket . A willingness to accept incoming connections and a queue limit for incoming connections are specified with listen and then the connections are accepted with accept . The listen system call applies only to sockets of type SOCK_STREAM or SOCK_SEQPACKET.

The n argument defines the maximum length the queue of pending connections may grow to. The real maximum queue length will be 1.5 times more than the value specified in the n argument. A subsequent listen system call on the listening socket allows the caller to change the maximum queue length using a new n argument. If a connection request arrives with the queue full the client may receive an error with an indication of ECONNREFUSED , or, in the case of TCP, the connection will be silently dropped.

Note that before BSD 4.5 and the introduction of the syncache the n argument also determined the length of the incomplete connection queue, which held TCP sockets in the process of completing TCP's 3-way handshake. These incomplete connections are now held entirely in the syncache, which is unaffected by queue lengths. Inflated n values to help handle denial of service attacks are no longer necessary.

#include <sys/socket.h>
#include <netinet/in.h>
#include <unistd.h>
void listen_example()
{
   int sock_fd;
   int newsock_fd;
   struct sockaddr_in addr;
   struct sockaddr_in ss;
   struct sockaddr_in new_socket;
   unsigned int len;
   unsigned int addr_len;
   
   sock_fd = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
       
   addr.sin_family = AF_INET;
   addr.sin_addr.s_addr = htonl(INADDR_ANY);
   addr.sin_port = htons(5000);
   bind(sock_fd,(struct sockaddr*)&addr;,sizeof(addr));
   listen(sock_fd,1);
   close(sock_fd);
}

See also: accept() connect() socket()