9.20. setsockopt(), getsockopt()

Set various options for a socket

Prototypes

#include <sys/types.h>
#include <sys/socket.h>

int getsockopt(int s, int level, int optname, void *optval,
               socklen_t *optlen);
int setsockopt(int s, int level, int optname, const void *optval,
               socklen_t optlen);

Description

Sockets are fairly configurable beasts. In fact, they are so configurable, I'm not even going to cover it all here. It's probably system-dependent anyway. But I will talk about the basics.

Obviously, these functions get and set certain options on a socket. On a Linux box, all the socket information is in the man page for socket in section 7. (Type: "man 7 socket" to get all these goodies.)

As for parameters, s is the socket you're talking about, level should be set to SOL_SOCKET. Then you set the optname to the name you're interested in. Again, see your man page for all the options, but here are some of the most fun ones:

SO_BINDTODEVICE

Bind this socket to a symbolic device name like eth0 instead of using bind() to bind it to an IP address. Type the command ifconfig under Unix to see the device names.

SO_REUSEADDR

Allows other sockets to bind() to this port, unless there is an active listening socket bound to the port already. This enables you to get around those "Address already in use" error messages when you try to restart your server after a crash.

SO_BROADCAST

Allows UDP datagram (SOCK_DGRAM) sockets to send and receive packets sent to and from the broadcast address. Does nothing—NOTHING!!—to TCP stream sockets! Hahaha!

As for the parameter optval, it's usually a pointer to an int indicating the value in question. For booleans, zero is false, and non-zero is true. And that's an absolute fact, unless it's different on your system. If there is no parameter to be passed, optval can be NULL.

The final parameter, optlen, should be set to the length of optval, probably sizeof(int), but varies depending on the option. Note that in the case of getsockopt(), this is a pointer to a socklen_t, and it specifies the maximum size object that will be stored in optval (to prevent buffer overflows). And getsockopt() will modify the value of optlen to reflect the number of bytes actually set.

Warning: on some systems (notably Sun and Windows), the option can be a char instead of an int, and is set to, for example, a character value of '1' instead of an int value of 1. Again, check your own man pages for more info with "man setsockopt" and "man 7 socket"!

Return Value

Returns zero on success, or -1 on error (and errno will be set accordingly.)

Example

int optval;
int optlen;
char *optval2;

// set SO_REUSEADDR on a socket to true (1):
optval = 1;
setsockopt(s1, SOL_SOCKET, SO_REUSEADDR, &optval, sizeof optval);

// bind a socket to a device name (might not work on all systems):
optval2 = "eth1"; // 4 bytes long, so 4, below:
setsockopt(s2, SOL_SOCKET, SO_BINDTODEVICE, optval2, 4);

// see if the SO_BROADCAST flag is set:
getsockopt(s3, SOL_SOCKET, SO_BROADCAST, &optval, &optlen);
if (optval != 0) {
    print("SO_BROADCAST enabled on s3!\n");
}

See Also

fcntl()