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 8–6 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);
}