《UNUX环境高级编程》(15)进程间通信

2、管道

• 实例：图15-5

#include "apue.h"

int
main(void)
{
	int		n;
	int		fd[2];
	pid_t	pid;
	char	line[MAXLINE];

	if (pipe(fd) < 0)
		err_sys("pipe error");
	if ((pid = fork()) < 0) {
		err_sys("fork error");
	} else if (pid > 0) {		/* parent */
		close(fd[0]);
		write(fd[1], "hello world\n", 12);
	} else {					/* child */
		close(fd[1]);
		n = read(fd[0], line, MAXLINE);
		write(STDOUT_FILENO, line, n);
	}
	exit(0);
}

• 实例：图15-6

#include "apue.h"
#include 

#define	DEF_PAGER	"/bin/more"		/* default pager program */

int
main(int argc, char *argv[])
{
	int		n;
	int		fd[2];
	pid_t	pid;
	char	*pager, *argv0;
	char	line[MAXLINE];
	FILE	*fp;

	if (argc != 2)
		err_quit("usage: a.out ");

	if ((fp = fopen(argv[1], "r")) == NULL)
		err_sys("can't open %s", argv[1]);
	if (pipe(fd) < 0)
		err_sys("pipe error");

	if ((pid = fork()) < 0) {
		err_sys("fork error");
	} else if (pid > 0) {								/* parent */
		close(fd[0]);		/* close read end */

		/* parent copies argv[1] to pipe */
		while (fgets(line, MAXLINE, fp) != NULL) {
			n = strlen(line);
			if (write(fd[1], line, n) != n)
				err_sys("write error to pipe");
		}
		if (ferror(fp))
			err_sys("fgets error");

		close(fd[1]);	/* close write end of pipe for reader */

		if (waitpid(pid, NULL, 0) < 0)
			err_sys("waitpid error");
		exit(0);
	} else {										/* child */
		close(fd[1]);	/* close write end */
		if (fd[0] != STDIN_FILENO) {
			if (dup2(fd[0], STDIN_FILENO) != STDIN_FILENO)
				err_sys("dup2 error to stdin");
			close(fd[0]);	/* don't need this after dup2 */
		}

		/* get arguments for execl() */
		if ((pager = getenv("PAGER")) == NULL)
			pager = DEF_PAGER;
		if ((argv0 = strrchr(pager, '/')) != NULL)
			argv0++;		/* step past rightmost slash */
		else
			argv0 = pager;	/* no slash in pager */

		if (execl(pager, argv0, (char *)0) < 0)
			err_sys("execl error for %s", pager);
	}
	exit(0);
}

• 实例：图15-7

#include "apue.h"

static int	pfd1[2], pfd2[2];

void
TELL_WAIT(void)
{
	if (pipe(pfd1) < 0 || pipe(pfd2) < 0)
		err_sys("pipe error");
}

void
TELL_PARENT(pid_t pid)
{
	if (write(pfd2[1], "c", 1) != 1)
		err_sys("write error");
}

void
WAIT_PARENT(void)
{
	char	c;

	if (read(pfd1[0], &c, 1) != 1)
		err_sys("read error");

	if (c != 'p')
		err_quit("WAIT_PARENT: incorrect data");
}

void
TELL_CHILD(pid_t pid)
{
	if (write(pfd1[1], "p", 1) != 1)
		err_sys("write error");
}

void
WAIT_CHILD(void)
{
	char	c;

	if (read(pfd2[0], &c, 1) != 1)
		err_sys("read error");

	if (c != 'c')
		err_quit("WAIT_CHILD: incorrect data");
}

3、函数popen和pclose

• 实例：图15-11

#include "apue.h"
#include 

#define	PAGER	"${PAGER:-more}" /* environment variable, or default */

int
main(int argc, char *argv[])
{
	char	line[MAXLINE];
	FILE	*fpin, *fpout;

	if (argc != 2)
		err_quit("usage: a.out ");
	if ((fpin = fopen(argv[1], "r")) == NULL)
		err_sys("can't open %s", argv[1]);

	if ((fpout = popen(PAGER, "w")) == NULL)
		err_sys("popen error");

	/* copy argv[1] to pager */
	while (fgets(line, MAXLINE, fpin) != NULL) {
		if (fputs(line, fpout) == EOF)
			err_sys("fputs error to pipe");
	}
	if (ferror(fpin))
		err_sys("fgets error");
	if (pclose(fpout) == -1)
		err_sys("pclose error");

	exit(0);
}

• 实例：图15-12

#include "apue.h"
#include 
#include 
#include 

/*
 * Pointer to array allocated at run-time.
 */
static pid_t	*childpid = NULL;

/*
 * From our open_max(), {Prog openmax}.
 */
static int		maxfd;

FILE *
popen(const char *cmdstring, const char *type)
{
	int		i;
	int		pfd[2];
	pid_t	pid;
	FILE	*fp;

	/* only allow "r" or "w" */
	if ((type[0] != 'r' && type[0] != 'w') || type[1] != 0) {
		errno = EINVAL;
		return(NULL);
	}

	if (childpid == NULL) {		/* first time through */
		/* allocate zeroed out array for child pids */
		maxfd = open_max();
		if ((childpid = calloc(maxfd, sizeof(pid_t))) == NULL)
			return(NULL);
	}

	if (pipe(pfd) < 0)
		return(NULL);	/* errno set by pipe() */
	if (pfd[0] >= maxfd || pfd[1] >= maxfd) {
		close(pfd[0]);
		close(pfd[1]);
		errno = EMFILE;
		return(NULL);
	}

	if ((pid = fork()) < 0) {
		return(NULL);	/* errno set by fork() */
	} else if (pid == 0) {							/* child */
		if (*type == 'r') {
			close(pfd[0]);
			if (pfd[1] != STDOUT_FILENO) {
				dup2(pfd[1], STDOUT_FILENO);
				close(pfd[1]);
			}
		} else {
			close(pfd[1]);
			if (pfd[0] != STDIN_FILENO) {
				dup2(pfd[0], STDIN_FILENO);
				close(pfd[0]);
			}
		}

		/* close all descriptors in childpid[] */
		for (i = 0; i < maxfd; i++)
			if (childpid[i] > 0)
				close(i);

		execl("/bin/sh", "sh", "-c", cmdstring, (char *)0);
		_exit(127);
	}

	/* parent continues... */
	if (*type == 'r') {
		close(pfd[1]);
		if ((fp = fdopen(pfd[0], type)) == NULL)
			return(NULL);
	} else {
		close(pfd[0]);
		if ((fp = fdopen(pfd[1], type)) == NULL)
			return(NULL);
	}

	childpid[fileno(fp)] = pid;	/* remember child pid for this fd */
	return(fp);
}

int
pclose(FILE *fp)
{
	int		fd, stat;
	pid_t	pid;

	if (childpid == NULL) {
		errno = EINVAL;
		return(-1);		/* popen() has never been called */
	}

	fd = fileno(fp);
	if (fd >= maxfd) {
		errno = EINVAL;
		return(-1);		/* invalid file descriptor */
	}
	if ((pid = childpid[fd]) == 0) {
		errno = EINVAL;
		return(-1);		/* fp wasn't opened by popen() */
	}

	childpid[fd] = 0;
	if (fclose(fp) == EOF)
		return(-1);

	while (waitpid(pid, &stat, 0) < 0)
		if (errno != EINTR)
			return(-1);	/* error other than EINTR from waitpid() */

	return(stat);	/* return child's termination status */
}