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#include <unistd.h>
int pipe(int filedes[2]);    //成功：0 出错：-1

#include <stdio.h>
#include <unistd.h>

int
main(void)
{
        int n;
        int fd[2];
        pid_t pid;
        char line[128];

        if (pipe(fd) < 0) {
                printf("pipe error\n");
                return (-1);
        }
        if ((pid = fork()) < 0) {
                printf("fork error\n");
                return (-1);
        } else if (pid > 0) {
                        close(fd[0]);
                        write(fd[1], "hello world\n" , 12);
                } else {
                        close(fd[1]);
                        n = read(fd[0], line, 128);
                        write(1, line, n);
                }

        return (0);
}

#include <stdio.h>
FILE *popen(const char *cmdstring, const char *type);   //成功：文件指针  出错：NULL

int pclose(FILE *fp);    //成功：终止状态   出错：-1

#include <stdio.h>

int
main(void)
{
        char buf[128];
        FILE *fp = popen("ls", "r");
        if (NULL == fp) {
                printf("popen error\n");
                return (-1);
        }

        while (fgets(buf, 128, fp) != NULL) {
                if (fputs(buf, stdout) == EOF) {
                        printf("over\n");
                }
        }

        if (pclose(fp) == -1) {
                printf("pclose error\n");
                return (-1);
        }

        FILE *fp2 = popen("cat", "w");
        if (NULL == fp2) {
                printf("popen error\n");
                return (-1);
        }
        fputs("hello\n", fp2);

        if (pclose(fp2) == -1) {
                printf("pclose error\n");
                return (-1);
        }

        return (0);
}