# I/O redirection 運用 Linux system call 的實作方法 (redirect, pipe, ..)

###### tags: `C LANGUAGE` `I/O redirection` `pipe` `linux`

Author: WhoAmI
email: [e-mail]
Date: 20220726
Copyright: CC BY-NC-SA

**IO 要注意 conggestion 問題 (高速公路 回堵現象)**

[Linux I/O 輸入與輸出重新導向，基礎概念教學, by G. T. Wang](https://blog.gtwang.org/linux/linux-io-input-output-redirection-operators/)
[dup, dup2, dup3 - duplicate a file descriptor](https://www.man7.org/linux/man-pages/man2/dup.2.html)
[Traffic load 與 buffer size 的關係 (QoS)](https://hackmd.io/@pingulinux/load-buffer)

/*
**沒有仔細檢查可能的錯誤! 例如 signal,...等
只是了解用法與現象**

Author: WhoAmI 20220726

*/


/*
redirect stdout to "myout.txt"
log child process to "myappend.txt"
tdup2.c
gcc tdup2.c -o tdup2

The exit() function causes normal process termination and the value
of status & 0377 is returned to the parent
*/
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <time.h>
#include <string.h>
static int runchild(int fw)
{

pid_t childpid;
char writebuf[128];
time_t t;


if((childpid = fork()) == -1)
{
  perror("fork");
  exit(EXIT_FAILURE);
}
if(childpid == 0)
{

  time(&t);
  sprintf(writebuf,"Child: [%d]output$>pid=%d,fd=%d\n",t,getpid(),fw);
  write(fw,writebuf,strlen(writebuf));

  close(fw);

  exit(EXIT_SUCCESS);
}
else
{
}

return(EXIT_SUCCESS);
}


int main(void)
{
int newfd0,newfd1, nbytes;

char string[128] = "Hello stdout!";
char readbuffer[128];
char writebuf[128];
int fw;
int status;
time_t current;
newfd1=10;

fw=open("myout.txt",O_CREAT |O_WRONLY,0666); // Octal 0666 =Binary 110 110 110
if(fw<0){
  printf("fw=%d\n",fw);
  perror("open");
  exit(EXIT_FAILURE);
}




  dup2(fw,1); //Be careful! stdout is silently closed before being reused.

  time(&current);
  fprintf(stderr,"stderr:localtime:[%s]",ctime(&current));
  sprintf(readbuffer,"[%d:%d]output$>%s\n",current,getpid(),string);

  write(1,readbuffer,strlen(readbuffer)); //write to "myout.txt"


if(close(1)<0){

  perror("close(1)");
  exit(EXIT_FAILURE);
}
if( close(fw)<0){
  perror("close(fw)");
  exit(EXIT_FAILURE);
}else{
  fprintf(stderr,"stderr:We close %d\n",fw);
  fw=open("myappend.txt",O_CREAT|O_APPEND|O_WRONLY,0666);
if(fw>0){

  fprintf(stderr,"Open myappend.txt[%d]\n",fw);
  write(fw,readbuffer,strlen(readbuffer)); //write to "myappend.txt"
  runchild(fw);
  time(&current);
  sprintf(writebuf,"Parent:[%d]output$>pid=%d\n",current,getpid());
  write(fw,writebuf,strlen(writebuf));
  close(fw);
  waitpid(-1, &status, 0);
//WNOHANG : return immediately if no child has exited.
  fprintf(stderr,"Get child status=%d\n",status & 0377);
}
}
exit(EXIT_SUCCESS);
}

************************************************

/*
redirect stdout to "myout.txt"
tout.c
gcc tout.c -o tout

*/
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <time.h>
#include <string.h>
/*
The call ctime(t) is equivalent to asctime(localtime(t)). It converts the calendar time t into a null-terminated string of the form
"Wed Jun 30 21:49:08 1993\n"
*/

int main(void)
{
int newfd0,newfd1, nbytes;
pid_t childpid;
char string[] = "Hello stdio! From Child!\n";
char readbuffer[128];
int fw;
time_t current;


fw=open("myout.txt",O_CREAT |O_WRONLY);
if(fw<0){
  printf("fw=%d\n",fw);
  perror("open");
  exit(0);
}

dup2(fw,1);

time(&current);
fprintf(stderr,"stderr:localtime:[%s]",ctime(&current));
sprintf(readbuffer,"localtime:[%s]virtual output$>%s",ctime(&current),string);

write(1,readbuffer,strlen(readbuffer));


if(close(1)<0){

  perror("close(1)");
}
if( close(fw)<0){
  perror("close(fw)");
}else{
  fprintf(stderr,"stderr:We close %d\n",fw);
}
exit(0);
}
***************************************************