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Chapter 63. Alternative I/O Models > Signal-Driven I/O
63.3. Signal-Driven I/O
With I/O multiplexing, a process makes a system call (select() or poll()) in order to check whether I/O is possible on a file descriptor. With signal-driven I/O, a process requests that the kernel send it a signal when I/O is possible on a file descriptor. The process can then perform any other activity until I/O is possible, at which time the signal is delivered to the process. To use signal-driven I/O, a program performs the following steps:
Establish a handler for the signal delivered by the signal-driven I/O mechanism. By default, this notification signal is SIGIO.
Set the owner of the file descriptor—that is, the process or process group that is to receive signals when I/O is possible on the file descriptor. Typically, we make the calling process the owner. The owner is set using an fcntl() F_SETOWN operation of the following form:
fcntl(fd, F_SETOWN, pid);
Enable nonblocking I/O by setting the O_NONBLOCK open file status flag.
Enable signal-driven I/O by turning on the O_ASYNC open file status flag. This can be combined with the previous step, since they both require the use of the fcntl() F_SETFL operation (Section 5.3), as in the following example:
flags = fcntl(fd, F_GETFL); /* Get current flags */ fcntl(fd, F_SETFL, flags | O_ASYNC | O_NONBLOCK);
The calling process can now perform other tasks. When I/O becomes possible, the kernel generates a signal for the process and invokes the signal handler established in step 1.
Signal-driven I/O provides edge-triggered notification (Section 63.1.1). This means that once the process has been notified that I/O is possible, it should perform as much I/O (e.g., read as many bytes) as possible. Assuming a nonblocking file descriptor, this means executing a loop that performs I/O system calls until a call fails with the error EAGAIN or EWOULDBLOCK.
