c15-4----the implemention of popen

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

/*
 *pointer to array allocatied at run-time
 */
static pid_t * childpid = NULL;
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 */
        maxfd = open_max();
        if (( childpid = calloc(maxfd,sizeof(pid_t))) == NULL)
            return(NULL);
    }
    if (pipe(pfd) <0)
        return(NULL); /* errno set by pipe() */
    if (( pid = fork()) < 0){
        return(NULL);
    } else if( pid == 0) {
        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 descript in childpid[] */
        for (i = 0 ; i < maxfd;i++)
            if (childpid[i] > 0)
                close(childpid[i]);
        execl("/bin/sh","sh","-c",cmdstring,(char *) 0);
        _exit(127);
    }
    /* parent continue */
        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;
    /* remeber childpid for this fd */
    return(fp);
}
int pclose( FILE * fp)
{
    int fd,stat;
    pid_t pid;
    if (childpid == NULL) {
        errno = EINVAL;
        return(-1);
    }
    fd =fileno(fp);
    if (( pid =childpid[fd]) == 0) {
        errno = EINVAL;
        return(-1); /* fp wasn't opened by popen() */
    }
    childpid[fd] = 0;   /* reset the pid to zero */
    if (fclose(fp) == EOF)
        return(-1);
    while(waitpid(pid,&stat,0) <0)
        if (errno != EINVAL)
            return(-1); /* error other than EINVAL from waitpid() */
    return(stat); /* return child's termination status */
}

在每次调用popen的时候，需要记住fork进程的进程id，为此使用childpid来保存，还有就是文件描述符或是文件指针
在着就是要复制文件描述符到标准输入和标准输出

在关闭文件的时候，需要等待子进程结束，然后关闭文件描述符。