tcdrain() — Wait until output has been transmitted

Standards

Standards / Extensions	C or C++	Dependencies
POSIX.1 XPG4 XPG4.2 Single UNIX Specification, Version 3	both

Format

#define _POSIX_SOURCE
#include <termios.h>

int tcdrain(int fildes);

General description

The tcdrain() function waits until all output sent to fildes has actually been sent to the terminal device.

If tcdrain() is called from a background process group against the caller's controlling terminal, a SIGTTOU signal may be generated depending how the process is handling SIGTTOUs:

Processing for SIGTTOU	System Behavior
Default or signal handler	The SIGTTOU signal is generated, and the function is not performed. tcdrain() returns -1 and sets errno to EINTR.
Ignored or blocked	The SIGTTOU signal is not sent, and the function continues normally.

Returned value

If successful, tcdrain() returns 0.

If unsuccessful, tcdrain() returns -1 and sets errno to one of the following values:

Error Code: Description
EBADF: fildes is not a valid open file descriptor.
EINTR: A signal interrupted tcdrain().
EIO: The process group of the process issuing the function is an orphaned, background process group, and the process issuing the function is not ignoring or blocking SIGTTOU.
ENOTTY: fildes is not associated with a terminal.

Example

CELEBT03

⁄* CELEBT03  *⁄

#define _POSIX_SOURCE
#include <termios.h>
#include <stdio.h>
#include <time.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys⁄stat.h>

main() {
  char Master[]="master.tty";
  char Slave[]="slave.tty";
  char text[]="text to be written to tty";
  char data[80];
  int  master, slave;
  time_t T;

  if (mknod(Master, S_IFCHR|S_IRUSR|S_IWUSR, 0x00010000 + 10) !=0)
    perror("mknod() error for master tty");
  else {
    if (mknod(Slave, S_IFCHR|S_IRUSR|S_IWUSR, 0x00020000 + 10) !=0)
      perror("mknod() error for slave tty");
    else {
      if ((master = open(Master, O_RDWR|O_NONBLOCK)) < 0)
        perror("open() error for master tty");
      else {
        if ((slave = open(Slave, O_RDWR|O_NONBLOCK)) < 0)
          perror("open() error for slave tty");
        else {
          if (fork() == 0) {
            if (write(slave, text, strlen(text)+1) == -1)
              perror("write() error");
            time(&T);
            printf("child has written to tty, tcdrain() started at %s",
                   ctime(&T));
            if (tcdrain(slave) != 0)
              perror("tcdrain() error");
            time(&T);
            printf("tcdrain() returned at %s", ctime(&T));
            exit(0);
          }
          time(&T);
          printf("parent is starting nap at %s", ctime(&T));
          sleep(5);
          time(&T);
          printf("parent is done with nap at %s", ctime(&T));
          if (read(master, data, sizeof(data)) == -1)
            perror("read() error");
          else printf("read '%s' from the tty\n", data);
          sleep(5);
          close(slave);
        }
        close(master);
      }
      unlink(Slave);
    }
    unlink(Master);
  }
}

Output

parent is starting nap at Fri Jun 16 12:46:28 2001
child has written to tty, tcdrain() started at Fri Jun 16 12:46:28 2001
parent is done with nap at Fri Jun 16 12:46:34 2001
read 'text to be written to tty' from the tty
tcdrain() returned at Fri Jun 16 12:46:34 2001