Asynchronous Open

Occasionally, an application might be required to dynamically open a regular file in a file system mounted from a remote host, or on a device whose initialization might be prolonged. However, while such a request to open a file is being processed, the application is unable to achieve real-time response to other events. The SunOS software solves this problem by having a second process handle the actual opening of the file, then passes the file descriptor to the real-time process.

Transferring a File Descriptor

The STREAMS interface provided by the SunOS platform provides a mechanism for passing an open file descriptor from one process to another. The process with the open file descriptor uses ioctl(2) with a command argument of I_SENDFD. The second process obtains the file descriptor by calling ioctl(2) with a command argument of I_RECVFD.

In the following example, the parent process prints out information about the test file, and creates a pipe. Next, the parent creates a child process that opens the test file and passes the open file descriptor back to the parent through the pipe. The parent process then displays the status information on the new file descriptor.

Example 9–6 File Descriptor Transfer

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stropts.h>
#include <stdio.h>

#define TESTFILE "/dev/null"
main(int argc, char *argv[])
{
	int fd;
	int pipefd[2];
	struct stat statbuf;

	stat(TESTFILE, &statbuf);
	statout(TESTFILE, &statbuf);
	pipe(pipefd);
	if (fork() == 0) {
		close(pipefd[0]);
		sendfd(pipefd[1]);
	} else {
		close(pipefd[1])
		recvfd(pipefd[0]);
	}
}

sendfd(int p)
{
	int tfd;

	tfd = open(TESTFILE, O_RDWR);
	ioctl(p, I_SENDFD, tfd);
}

recvfd(int p)
{
	struct strrecvfd rfdbuf;
	struct stat statbuf;
	char			fdbuf[32];

	ioctl(p, I_RECVFD, &rfdbuf);
	fstat(rfdbuf.fd, &statbuf);
	sprintf(fdbuf, "recvfd=%d", rfdbuf.fd);
	statout(fdbuf, &statbuf);	
}

statout(char *f, struct stat *s)
{
	printf("stat: from=%s mode=0%o, ino=%ld, dev=%lx, rdev=%lx\n",
		f, s->st_mode, s->st_ino, s->st_dev, s->st_rdev);
	fflush(stdout);
}