How to synchronize input and output in pipes linux? - linux

Question

I am creating a shell command from the custom shell to do the ssh from one terminal to another terminal.
In order to do the ssh, I am using the inbuilt ssh command of the linux. Here is my code that does the ssh login.
However, I am seeing that the I/O buffers are not in sync.
This is what I am seeing on the terminal. After SSH to the other terminal. I did the following in the terminal.
PRT# ssh 192.168.10.42
PRT# Could not create directory '/root/.ssh'.
root#192.168.10.42's password:
# screen -r
-sh: cen-: not found
# hello
-sh: el: not found
#
I don't what's the reason here. Here is the code.
int sshLogin(chr *destIp)
{
char cmd[CMD_LEN];
char readbuff[CMD_LEN];
pid_t pid;
int ret = 0;
int fd[2];
int result;
memset(cmd,'\0',sizeof(cmd));
int status = 0;
/** --tt required to force pseudowire allocation because we are behind screen app **/
sprintf(cmd,"/usr/bin/ssh -tt %s",destIp);
/** create a pipe this will be shared on fork() **/
pipe(fd);
if((pid = fork()) == -1)
{
perror("fork:");
return -1;
}
if( pid == 0 )
{
/** Child Process of Main APP --Make this parent process for the command**/
if((pid = fork()) == -1)
{
perror("fork:");
return -1;
}
if( pid == 0)
{
/** basically Main APP grand child - this is where we running the command **/
ret = execlp("ssh", "ssh", "-tt", destIp, NULL);
printf("done execlp\r\n");
}
else
{
/** child of Main APP -- keep this blocked until the Main APP grand child is done with the job **/
while( (read(fd[0], readbuff, sizeof(readbuff))))
{
printf("%s",readbuff);
}
waitpid(0,&status,0);
LOG_STRING("SSH CONNC CLOSED");
exit(0);
}
}
else
{
/** Parent process APP MAIN-- **/
/** no need to wait let APP MAIN run -- **/
}
return 0;
}
Based on Patrick Ideas.
POST 2# - It seems that it works when we close the stdin in the parent process. However, it becomes very slugguish, I feel like I am typing the keyboard too slow. The system becomes too sluggish. Also, I have a web-server from this terminal. I see that I can no longer access the web.
So, the solution is somewhere around stdin but I am not sure.
int sshLogin(chr *destIp)
{
char cmd[CMD_LEN];
char readbuff[CMD_LEN];
pid_t pid;
int ret = 0;
int fd[2];
int result;
memset(cmd,'\0',sizeof(cmd));
int status = 0;
/** --tt required to force pseudowire allocation because we are behind screen app **/
sprintf(cmd,"/usr/bin/ssh -tt %s",destIp);
/** create a pipe this will be shared on fork() **/
pipe(fd);
if((pid = fork()) == -1)
{
perror("fork:");
return -1;
}
if( pid == 0 )
{
/** Child Process of Main APP --Make this parent process for the command**/
if((pid = fork()) == -1)
{
perror("fork:");
return -1;
}
if( pid == 0)
{
/** basically Main APP grand child - this is where we running the command **/
ret = execlp("ssh", "ssh", "-tt", destIp, NULL);
printf("done execlp\r\n");
}
else
{
/** child of Main APP -- keep this blocked until the Main APP grand child is done with the job **/
while( (read(fd[0], readbuff, sizeof(readbuff))))
{
printf("%s",readbuff);
}
waitpid(0,&status,0);
LOG_STRING("SSH CONNC CLOSED");
exit(0);
}
}
else
{
/** Parent process APP MAIN-- **/
/** no need to wait let APP MAIN run -- **/
close(stdin);
}
return 0;
}
Basically, I have added - close(stdin);

Answer 1

You have 2 different processes trying to read from STDIN. This causes process 1 to get char 1, process 2 to get char 2, process 1 to get char 3, process 2 to get char 4, etc, alternating back and forth.
Your 2 processes are:
execlp("ssh", "ssh", "-tt", destIp, NULL);.
while( (read(fd[0], readbuff, sizeof(readbuff))))
Basically you need to ditch the read(fd[0],...).

Answer 2

My initial thought is that perhaps it is buffering the output: stdout is buffered, so unless you print a newline, nothing will be printed until a certain number of characters build up. This is because I/O operations are expensive. You can find more detail on this here. The result is that there is a delay because your program is waiting to print.
My suggestion: in your main function, before calling your sshLogin function, try disabling buffering with this line of code:
setbuf(stdout, NULL);
You can also call fflush(stdout); periodically to do the same thing, but the above method is more efficient. Try it and see if that solves your problem.