管道与popen函数与重定向

转自：http://www.tldp.org/LDP/lpg/node12.html

Pipes the Easy Way!

  LIBRARY FUNCTION: popen();                                                    

  PROTOTYPE: FILE *popen ( char *command, char *type);                          
    RETURNS: new file stream on success                                         
             NULL on unsuccessful fork() or pipe() call                         

  NOTES: creates a pipe, and performs fork/exec operations using "command"

Direction of data flow is determined by the second argument, "type". It can be "r" or "w", for "read" or "write". It cannot be both!

popen函数相当于，创建了管道文件，通过参数  char *type （"r"  "w"）来决定是从管道读取数还是 向管道写数据。

fputs(strings[cntr], pipe_fp)将  strings  写入到管道文件中， 在管道文件读端 收到数据后后，调用

execlp( "sort", "sort", NULL, NULL )

int main(void)
{
        int  cntr;
        FILE *pipe_fp;
        char *strings[MAXSTRS] = { "echo", "bravo", "alpha",
                                  "charlie", "delta"};

        /* Create one way pipe line with call to popen() */
        if (( pipe_fp = popen("sort", "w")) == NULL)
        {
                perror("popen");
                exit(1);
        }

        /* Processing loop */
        for(cntr=0; cntr<MAXSTRS; cntr++) {
                fputs(strings[cntr], pipe_fp);
                fputc('
', pipe_fp);
        }

        /* Close the pipe */
        pclose(pipe_fp);
        
        return(0);
}

？？？
必须在调用了  pclose(pipe_fp);  之后。对端管道才会收到数据？