The Linux Programming Interface

Alternative I/O Models 1351

system call returns in the range –1 to –4095 as indicating an error, copies this (abso-

lute) value into errno, and returns –1 as the function result for the application pro-

gram. This technique is generally sufficient for dealing with the few system call

service routines that can return a valid negative result; the fcntl() F_GETOWN operation

is the only practical case where it fails. This limitation means that an application

that uses process groups to receive “I/O possible” signals (which is unusual) can’t

reliably use F_GETOWN to discover which process group owns a file descriptor.

Since glibc version 2.11, the fcntl() wrapper function fixes the problem of F_GETOWN

with process group IDs less than 4096. It does this by implementing F_GETOWN in

user space using the F_GETOWN_EX operation (Section 63.3.2), which is provided

by Linux 2.6.32 and later.

63.3.1 When Is “I/O Possible” Signaled?

We now consider the details of when “I/O possible” is signaled for various file types.

Terminals and pseudoterminals

For terminals and pseudoterminals, a signal is generated whenever new input becomes

available, even if previous input has not yet been read. “Input possible” is also sig-

naled if an end-of-file condition occurs on a terminal (but not on a pseudoterminal).

There is no “output possible” signaling for terminals. A terminal disconnect is

also not signaled.

Starting with kernel 2.4.19, Linux provides “output possible” signaling for the

slave side of a pseudoterminal. This signal is generated whenever input is consumed on

the master side of the pseudoterminal.

Pipes and FIFOs

For the read end of a pipe or FIFO, a signal is generated in these circumstances:

z Data is written to the pipe (even if there was already unread input available).

z The write end of the pipe is closed.

For the write end of a pipe or FIFO, a signal is generated in these circumstances:

z A read from the pipe increases the amount of free space in the pipe so that it is

now possible to write PIPE_BUF bytes without blocking.

z The read end of the pipe is closed.

Sockets

Signal-driven I/O works for datagram sockets in both the UNIX and the Internet

domains. A signal is generated in the following circumstances:

z An input datagram arrives on the socket (even if there were already unread

datagrams waiting to be read).

z An asynchronous error occurs on the socket.