*nix伪终端如何工作？什么是主/从频道？

Question

我想在Linux系统上用C编写一个简单,愚蠢的X终端模拟器.

起初,我只是想我必须打开一个shell并显示它的输出.我检查了xterm和rxvt代码,看起来有点复杂.

首先,我必须用openpty打开一个伪终端.所以我看一下手册页,看看openpty填充了2个文件描述符,即master和slave.xterm和rxvt代码都很混乱,因为这些特殊文件的系统依赖性.

我理解termios的东西:它只是关于终端转义码的一堆信息.我真正没有得到的是:我想对主/从文件描述符做什么？

打开终端,登录,在shell上执行"ls"的示例程序将是非常棒的.

(英语不是我的母语,原谅我最终的错误)

编辑:这是我提出的示例代码:

#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <pty.h>
#include <utmp.h>
#include <ctype.h>

void
safe_print (char* s)
{
    while(*s) { 
        if(*s == '\n')
            putchar("\n");
        else if(iscntrl(*s))
            printf("\\e(%d)", *s);
        else
            putchar(*s);
        s++;
    }
}


int
main (int argc, char** argv)
{
    char buf[BUFSIZ] = {0};
    int master;
    int ret = forkpty(&master, NULL, NULL, NULL);

    if(ret == -1)
        puts("no fork"), exit(0);

    if(!ret) { 
        execl("/bin/sh", "sh", NULL);
        exit(0);
    }

    sleep(1); /* let the shell run */


    if(argc >= 2) {
        write(master, argv[1], strlen(argv[1]));
        write(master, "\n", 1);
    } else {
        write(master, "date\n", sizeof "date\n");
    }


    while(1) {
        switch(ret = read(master, buf, BUFSIZ)) {
        case -1:
            puts("error!"); 
            exit(1);
            break;
        case 0:
            puts("nothing.."), sleep(1);
            break;
        default:
            buf[ret] = '\0';
            safe_print(buf);

        }
    }

    close(master);

    return 0;
}    

Answer 1

关于问题的主/从部分,从pty(4)手册页(从我系统上的openpty(3)手册页引用):

伪终端是一对字符设备,主设备和从设备.从设备向进程提供与tty(4)中描述的接口相同的接口.然而,尽管提供tty(4)中描述的接口的所有其他设备在它们后面具有某种类型的硬件设备,但是从设备具有通过伪终端的主半部操纵它的另一个过程. 也就是说,在主设备上写入的任何内容都作为输入提供给从设备,并且在从设备上写入的任何内容都作为输入呈现在主设备上.

手册页是你的朋友.

Answer 2

我刚刚尝试了本教程中的示例，它们对我来说效果很好，我认为它们是解决问题的一个有趣的起点。

编辑：本教程简要解释了伪终端功能。解释是一步一步完成的，然后是例子。

下面的示例示出了如何建立一个新的伪终端，和叉两个部分，在所述一个写过程主从伪终端，另一读取侧从所述伪终端的侧。

#define _XOPEN_SOURCE 600 
#include <stdlib.h> 
#include <fcntl.h> 
#include <errno.h> 
#include <unistd.h> 
#include <stdio.h> 
#define __USE_BSD 
#include <termios.h> 


int main(void) 
{ 
int fdm, fds, rc; 
char input[150]; 

fdm = posix_openpt(O_RDWR); 
if (fdm < 0) 
{ 
fprintf(stderr, "Error %d on posix_openpt()\n", errno); 
return 1; 
} 

rc = grantpt(fdm); 
if (rc != 0) 
{ 
fprintf(stderr, "Error %d on grantpt()\n", errno); 
return 1; 
} 

rc = unlockpt(fdm); 
if (rc != 0) 
{ 
fprintf(stderr, "Error %d on unlockpt()\n", errno); 
return 1; 
} 

// Open the slave PTY
fds = open(ptsname(fdm), O_RDWR); 
printf("Virtual interface configured\n");
printf("The master side is named : %s\n", ptsname(fdm));

// Creation of a child process
if (fork()) 
{ 
  // Father
 
  // Close the slave side of the PTY 
  close(fds); 
  while (1) 
  { 
    // Operator's entry (standard input = terminal) 
    write(1, "Input : ", sizeof("Input : ")); 
    rc = read(0, input, sizeof(input)); 
    if (rc > 0) 
    {
      // Send the input to the child process through the PTY 
      write(fdm, input, rc); 

      // Get the child's answer through the PTY 
      rc = read(fdm, input, sizeof(input) - 1); 
      if (rc > 0) 
      { 
        // Make the answer NUL terminated to display it as a string
        input[rc] = '\0'; 

        fprintf(stderr, "%s", input); 
      } 
      else 
      { 
        break; 
      } 
    } 
    else 
    { 
      break; 
    } 
  } // End while 
} 
else 
{ 
struct termios slave_orig_term_settings; // Saved terminal settings 
struct termios new_term_settings; // Current terminal settings 

  // Child

  // Close the master side of the PTY 
  close(fdm); 

  // Save the default parameters of the slave side of the PTY 
  rc = tcgetattr(fds, &slave_orig_term_settings); 

  // Set raw mode on the slave side of the PTY
  new_term_settings = slave_orig_term_settings; 
  cfmakeraw (&new_term_settings); 
  tcsetattr (fds, TCSANOW, &new_term_settings); 

  // The slave side of the PTY becomes the standard input and outputs of the child process 
  close(0); // Close standard input (current terminal) 
  close(1); // Close standard output (current terminal) 
  close(2); // Close standard error (current terminal) 

  dup(fds); // PTY becomes standard input (0) 
  dup(fds); // PTY becomes standard output (1) 
  dup(fds); // PTY becomes standard error (2) 

  while (1) 
  { 
    rc = read(fds, input, sizeof(input) - 1); 

    if (rc > 0) 
    { 
      // Replace the terminating \n by a NUL to display it as a string
      input[rc - 1] = '\0'; 

      printf("Child received : '%s'\n", input); 
    } 
    else 
    { 
      break; 
    } 
  } // End while 
} 

return 0; 
} // main