Advanced Programming in the UNIX® Environment

ptg10805159

Section 17.3 Unique Connections 635

struct sockaddr_un un;

un.sun_family = AF_UNIX;

strcpy(un.sun_path, "foo.socket");

if ((fd = socket(AF_UNIX, SOCK_STREAM, 0)) < 0)

err_sys("socket failed");

size = offsetof(struct sockaddr_un, sun_path) + strlen(un.sun_path);

if (bind(fd, (struct sockaddr *)&un, size) < 0)

err_sys("bind failed");

printf("UNIX domain socket bound\n");

exit(0);

}

Figure 17.5 Binding an address to a UNIX domain socket

When we run this program, thebindrequest succeeds. If we run the program a

second time, however, we get an error,because the file already exists. The program

won’t succeed again until we remove the file.

$./a.out run the program

UNIX domain socket bound

$ls -l foo.socket look at the socket file

srwxr-xr-x 1 sar 0 May 18 00:44 foo.socket

$./a.out try to run the program again

bind failed: Address already in use

$rm foo.socket remove the socket file

$./a.out run the program a third time

UNIX domain socket bound now it succeeds

The way we determine the size of the address to bind is to calculate the offset of the

sun_pathmember in thesockaddr_unstructureand add to it the length of the

pathname, not including the terminating null byte. Since implementations vary in

which members precede sun_path in the sockaddr_un structure, we use the

offsetofmacrofrom<stddef.h>(included byapue.h) to calculate the offset of the

sun_pathmember from the start of the structure. If you look in<stddef.h>,you’ll

see a definition similar to the following:

#define offsetof(TYPE, MEMBER) ((int)&((TYPE *)0)->MEMBER)

The expression evaluates to an integer,which is the starting address of the member,

assuming that the structurebegins at address 0.

17.3 Unique Connections

Aserver can arrange for unique UNIX domain connections to clients using the standard

bind,listen,andacceptfunctions. Clients useconnectto contact the server; after

the connect request is accepted by the server,aunique connection exists between the

client and the server.This style of operation is the same that we illustrated with

Internet domain sockets in Figures 16.16 and 16.17.