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 an open is in progress, the application is unable to achieve realtime response to other events. Fortunately, SunOS 5.0 through 5.7 provides a means of solving this problem by having a second process perform the actual open and then pass the file descriptor to the realtime process.

Transferring a File Descriptor

The STREAMS interface under SunOS 5.0 through 5.7x provides a mechanism for passing an open file descriptor from one process to another. The process with the open file descriptor uses the ioctl(2) function with a command argument of I_SENDFD. The second process obtains the file descriptor by calling the ioctl() function with a command argument of I_RECVFD.

In this example, the parent process prints out information about the test file, and creates a pipe. Next, the parent creates a child process, which 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 9-1 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);
}