getsockopt(2)

NAME

getsockopt, setsockopt − get and set options on sockets

SYNOPSIS

#include <sys/socket.h>

int getsockopt(

int s,
int level,
int optname,
void *optval,
int *optlen);

int setsockopt(

int s,
int level,
int optname,
const void *optval,
int optlen);

DESCRIPTION

getsockopt() and setsockopt() manipulate options associated with a socket. The socket is identified by the socket descriptor s. Options can exist at multiple protocol levels, and they are always present at the uppermost “socket” level (see socket(2)).

When manipulating socket options, the level at which the option resides (level) and the name of the option (optname) must be specified. To manipulate options at the “socket” level, level is specified as SOL_SOCKET.

There are two kinds of options: boolean and non-boolean. Boolean options are either set or not set and also can use optval and optlen (see below) to pass information. Non-boolean options always use optval and optlen to pass information.

To determine whether boolean option optname is set, the return value of getsockopt() must be examined. If the option is set, getsockopt() returns without error. If the boolean option is not set, getsockopt() returns −1 and errno is set to indicate the error.

For setsockopt(), the parameters optval and optlen are used to pass option information from the system to the calling process. optval is the address of a location in memory that contains the option information to be passed to the system. optlen is an integer that specifies the size in bytes of the option information.

For getsockopt(), optval and optlen are used to pass option information from the system to the calling process. optval is the address of a location in memory that contains the option information to be passed to the calling process, or (char ∗) NULL if the option information is not of interest and not to be passed to the calling process. optlen is an address of an integer initially used to specify the maximum number of bytes of option information to be passed. If optval is not (char ∗) NULL, optlen is set on return to the actual number of bytes of option information passed. If the getsockopt() call fails, no option information is passed.

optname and any specified options are passed uninterpreted to the appropriate protocol module for interpretation. The include file <sys/socket.h> contains definitions for “socket” level options (see socket(2)). Options at other protocol levels vary in format and name. Consult the appropriate entries in Section 7P, such as tcp(7P).

The “socket” level options defined in the include file <sys/socket.h> are explained below:

SO_DEBUG (boolean option) no functionality; included only for compatibility.

SO_DONTROUTE (boolean option; SOCK_STREAM sockets only) causes outgoing messages to bypass standard routing facilities and to be routed by the network portion of the Internet address.

SO_ERROR returns the current contents of the variable so_error for this socket and then clears the variable (so_error is defined in <sys/socketvar.h>. The contents match those found in errno.

SO_REUSEADDR (boolean option; AF_INET sockets only) allows local address reuse.

SO_KEEPALIVE (boolean option; SOCK_STREAM and AF_INET <<<<<<< getsockopt.2 sockets only) keeps otherwise idle connections active. If a connection has been idle for two hours, transmissions are forced every 75 seconds until a response is received or 10 minutes expires, whichever occurs first. If 10 minutes expires with no response, the connection is dropped.

SO_LINGER (boolean option; SOCK_STREAM and AF_INET sockets only) lingers on close if data is present. For SO_LINGER, optval points to a struct linger , defined in /usr/include/sys/socket.h. The linger structure contains an integer boolean flag to toggle behavior on/off and an integer linger value.

SO_BROADCAST (boolean option; SOCK_DGRAM and AF_INET sockets only) toggles permission to transmit broadcast messages.

SO_RCVBUF (non-boolean option) For stream sockets it changes the buffer size of a socket’s receive socket buffer. For datagram sockets it changes the maximum size message a socket can receive. A stream socket’s buffer size can be increased at any time but decreased only prior to establishing a connection. For datagram sockets, the inbound maximum message size can be increased or decreased at any time. The default and maximum values for SO_RCVBUF are protocol-specific. Refer to the appropriate entries in Sections 7F and 7P.

SO_SNDBUF (non-boolean option) For stream sockets, it changes the buffer size of a socket’s send socket buffer. For datagram sockets it changes the maximum size message that can be sent. A stream socket’s buffer size can be increased at any time but decreased only prior to establishing a connection. For datagram sockets, the maximum outbound message size can be increased or decreased at any time. The default and maximum values for SO_SNDBUF are protocol-specific. Refer to the appropriate entries in Sections 7F and 7P.

SO_USELOOPBACK (boolean option) no functionality; included only for compatibility.

None of the boolean options are supported for SOCK_DGRAM sockets.

If SO_DONTROUTE is set, the system does not use the network routing tables when determining which interface to use to send an outbound message. Instead, the system sends the message out through the interface that has a configured address matching the address to which the message is intended to be sent. If SO_DONTROUTE is not set, the system uses the network routing tables.

SO_REUSEADDR indicates the rules used in validating addresses supplied in a bind() call should allow reuse of local addresses. This allows multiple SOCK_STREAM sockets to be bound to the same local address, as long as all existing sockets at the desired address are in a connected state before the bind() is done on the new socket. The SO_REUSEADDR option has no effect on SOCK_DGRAM sockets.

The SO_KEEPALIVE option defaults to off. If SO_KEEPALIVE is set on and the connection has been idle for two hours, TCP sends a packet to the remote socket to acknowledge that it is still alive. If the remote socket does not respond within 75 seconds, TCP sends another packet. If TCP sends a total of 8 packets without response from the remote socket (i.e., 10 minutes have passed), TCP drops the connection. The next socket call (e.g., recv()) returns an error, and errno is set to ETIMEDOUT.

SO_LINGER controls the actions taken when unsent messages are queued on a SOCK_STREAM socket and a close(2) is performed. If SO_LINGER is toggled on with a non-zero linger interval, the system blocks the process on the close() attempt until it is able to transmit the data or until it decides it is unable to deliver the information. If SO_LINGER is toggled on with a linger interval of zero, the connection is immediately terminated on the close() of the socket, and any unsent data queued on the connection is lost. If SO_LINGER is toggled off (default upon socket creation) and a close() is issued, the call returns immediately. The system still gracefully brings down the connection by transmitting any queued data, if possible. SO_LINGER can be toggled on/off at any time during the life of an established connection. Toggling SO_LINGER does not affect the action of shutdown().

The SO_BROADCAST option requests permission to send Internet broadcast datagrams on the socket.

For stream sockets, SO_RCVBUF and SO_SNDBUF can be used with getsockopt() to find the current sizes (in number of bytes) of the socket’s receive and send buffers, respectively. If supported by the protocol, SO_RCVBUF and SO_SNDBUF can also be used with setsockopt() to set the sizes (in number of bytes) of the socket’s receive and send buffers, respectively. The sizes are passed as integer values using optval and optlen. You can increase a socket’s buffer size at any time, but you can decrease it only prior to establishing a connection. The default and maximum buffer sizes are protocol-specific. See the appropriate entries in Sections 7F and 7P for more information.

For datagram sockets, SO_RCVBUF and SO_SNDBUF can be used with getsockopt() to find the current maximum datagram size (in number of bytes) in the inbound and outbound direction, respectively. SO_RCVBUF and SO_SNDBUF can also be used with setsockopt() to set the maximum datagram size. The default and maximum datagram sizes are protocol-specific. See the appropriate entries in Sections 7F and 7P for more information.

AF_CCITT

SO_SNDBUF and SO_RCVBUF are the only options supported for sockets of the AF_CCITT address family.

RETURN VALUE

Upon successful completion, getsockopt() and setsockopt() return 0; otherwise, they return −1 and set errno to indicate the error.

DIAGNOSTICS

getsockopt() and setsockopt() fail if any of the following conditions are encountered:

[EBADF] The argument s is not a valid descriptor.

[EOPNOTSUPP] The option is not supported by the protocol in use by the socket.

[ENOBUFS] No buffer space is available.

[ENOTSOCK] The argument s is a file, not a socket.

[ENOPROTOOPT] In getsockopt(), the requested option is currently not set.

[EINVAL] The option is unknown at the socket level or the socket has been shut down.

[EFAULT] The optval or, in the case of getsockopt(), optlen parameters are not valid pointers.

AUTHOR

getsockopt() was developed by the University of California, Berkeley.

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