The Linux Programming Interface

Sockets: Advanced Topics 1281

Listing 61-4: Setting the SO_REUSEADDR socket option

int sockfd, optval;

sockfd = socket(AF_INET, SOCK_STREAM, 0);

if (sockfd == -1)

errExit("socket");

optval = 1;

if (setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &optval,

sizeof(optval)) == -1)

errExit("socket");

if (bind(sockfd, &addr, addrlen) == -1)

errExit("bind");

if (listen(sockfd, backlog) == -1)

errExit("listen");

61.11 Inheritance of Flags and Options Across accept()

Various flags and settings can be associated with open file descriptions and file

descriptors (Section 5.4). Furthermore, as described in Section 61.9, various options

can be set for a socket. If these flags and options are set on a listening socket, are

they inherited by the new socket returned by accept()? We describe the details here.

On Linux, the following attributes are not inherited by the new file descriptor

returned by accept():

z The status flags associated with an open file description—the flags that can be

altered using the fcntl() F_SETFL operation (Section 5.3). These include flags

such as O_NONBLOCK and O_ASYNC.

z The file descriptor flags—the flags that can be altered using the fcntl() F_SETFD

operation. The only such flag is the close-on-exec flag (FD_CLOEXEC, described in

Section 27.4).

z The fcntl() F_SETOWN (owner process ID) and F_SETSIG (generated signal) file

descriptor attributes associated with signal-driven I/O (Section 63.3).

On the other hand, the new descriptor returned by accept() inherits a copy of most

of the socket options that can be set using setsockopt() (Section 61.9).

SUSv3 is silent on the details described here, and the inheritance rules for the

new connected socket returned by accept() vary across UNIX implementations.

Most notably, on some UNIX implementations, if open file status flags such as

O_NONBLOCK and O_ASYNC are set on a listening socket, then they are inherited by the

new socket returned by accept(). For portability, it may be necessary to explicitly

reset these attributes on a socket returned by accept().