Unix Prog: File Sharing(4)

1. ioctl
For some I/O, like terminal I/O, socket, mag tape,etc. We can not operate them with system calls like: read, write, lseek. We need to use ioctl to handle them.

Definition:

 ubuntu@ip-172-31-23-227:~$ less /usr/include/x86_64-linux-gnu/bits/ioctls.h  
 /* Routing table calls. */  
 #define SIOCADDRT    0x890B     /* add routing table entry   */  
 #define SIOCDELRT    0x890C     /* delete routing table entry  */  
 #define SIOCRTMSG    0x890D     /* call to routing system    */  
   
 /* Socket configuration controls. */  
 #define SIOCGIFNAME   0x8910     /* get iface name        */  
 #define SIOCSIFLINK   0x8911     /* set iface channel      */  
 #define SIOCGIFCONF   0x8912     /* get iface list        */  
 ......  
 ubuntu@ip-172-31-23-227:~$ less /usr/include/x86_64-linux-gnu/sys/ioctl.h  
 ......  
 /* Perform the I/O control operation specified by REQUEST on FD.  
   One argument may follow; its presence and type depend on REQUEST.  
   Return value depends on REQUEST. Usually -1 indicates error. */  
 extern int ioctl (int __fd, unsigned long int __request, ...) __THROW;  
 ......  

2. /dev/fd/n
opening the /dev/fd/n is equivalent to duplicating file descriptor n, assuming n is already open.

fileio.c:

 #include<unistd.h>  
 #include<fcntl.h>  
 #include<stdio.h>  
 #include<stdlib.h>  
 #include<errno.h>  
   
 int main(int argc, char *argv[])  
 {  
  int fd;  
  char buff[] = "Hello world!";  
   
  // We are duplicating the file descriptor 0, after this call  
  // file descriptor 0 and new fd will point to the same file  
  // table entry  
  if((fd = open("/dev/fd/0", O_RDWR)) == -1) {  
   printf("open error!\n");  
   exit(1);  
  }  
   
  printf("fd: %d\n", fd);  
   
  // read from the standard input  
  if(read(fd, buff, 12) < 0) {  
   printf("read error!\n");  
   exit(2);  
  }  
   
  // Write information to standard output  
  if(write(STDOUT_FILENO, buff, 12) != 12) {  
   printf("write error!\n");  
   exit(2);  
  }  
   
  exit(0);  
 }  

shell:
After runnign io.out, the program stops to let us input. We put in "Hello fire!", it then output the string to standard output.

 ubuntu@ip-172-31-23-227:~$ ./io.out  
 fd: 3  
 Hello fire!  
 Hello fire!  

Current implementation:
At the current system, we only have 3 file descriptors existing at /dev/fd.

 ubuntu@ip-172-31-23-227:~$ sudo ls -lrt /dev/fd/*  
 ls: cannot access /dev/fd/255: No such file or directory  
 ls: cannot access /dev/fd/3: No such file or directory  
 lrwx------ 1 root root 64 Aug 19 00:51 /dev/fd/2 -> /dev/pts/0  
 lrwx------ 1 root root 64 Aug 19 00:51 /dev/fd/1 -> /dev/pts/0  
 lrwx------ 1 root root 64 Aug 19 00:51 /dev/fd/0 -> /dev/pts/0  

Other usage:
1) "cat -" means outputting the content from standard input, but it is very ugly and confusing
2) "cat /dev/fd/0" means same thing, but this way is more straightforward. It specify that it will output the content from file descriptor 0, which is standard input.

 ubuntu@ip-172-31-23-227:~$ cat -  
 Hello world!  
 Hello world!  
 ^C  
 ubuntu@ip-172-31-23-227:~$ cat /dev/fd/0  
 Hello world!  
 Hello world!  
 ^C