进程通信笔记

来自apue  fig15.5 fig 15.6

#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);
}

调用/bin/more

#include "apue.h"
#include <sys/wait.h>

#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 <pathname>");

	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);
}

popen和pclose函数 fig15.11

#include "apue.h"
#include <sys/wait.h>

#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 <pathname>");
        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);
}