Program with pseudo-tty behaving differently in single-step

Andrea Monaco
Fri Nov 18 19:54:13 GMT 2022

I have a program that behaves very differently when single-stepping and
when running freely in gdb.

The first part of the program opens a pseudo-terminal pair in the usual
way.  This pair includes a master (that sort of simulates the user: you
can read from master and write to master like a human would do) and a
slave (that the child process uses as a terminal for stdin and stdout,
receiving what you send to master and sending back to master).

The child process runs bc (a calculator).  The parent sends the line
"1+1\n" and reads back the subsequent line.

#define _GNU_SOURCE

#include <stdlib.h>
#include <stdio.h>
#include <fcntl.h>
#include <sys/types.h>
#include <unistd.h>
#include <string.h>

char buffer [1024];

int master, slave;

readl (int fd, char *buf)
  ssize_t i = 0;

      read (fd, buf+i, 1);
  while (buf [i++] != '\n');

  return i;

main (int argc, char *argv [])
  int i;
  char *name;
  pid_t child;

  master = getpt ();

  if (master < 0)
      perror ("getpt() failed");
      return 1;

  if (grantpt (master) < 0 || unlockpt (master) < 0)
      perror ("grantpt() or unlockpt() failed");
      return 1;

  name = ptsname (master);
  if (!name)
    return 1;

  slave = open (name, O_RDWR);
  if (slave < 0)
      perror ("could not open slave");
      return 1;

  /* until now, we opened a pseudo-tty pair in the standard way */

  child = fork ();

  if (child < 0)
      perror ("fork failed");
      return 1;

  if (!child)
      /* in the child, we close stdin and stdout and substitute them
	 both with the slave pseudo-terminal for input and output */
      close (1); 
      close (0);

      if (dup (slave) == -1 || dup (slave) == -1)
	  perror ("dup failed");
	  return 1;
      execlp ("bc", "bc", NULL);
      perror ("could not exec bc");

  /* in the parent, we skip the startup message of bc */
  for (i = 0; i < 4; i++)
      readl (master, buffer);

  write (master, "1+1\n", strlen ("1+1\n"));
  readl (master, buffer); /* we skip the echo line of 1+1 */
  read (master, buffer, 3);  /* now we read the result line */

  buffer [3] = '\0';
  printf ("%s", buffer);
  return 0;

In gdb, when I do a run I get "1+1" as output.  When I single-step, I
get 2 from the last printf instead, which is the desired output.

Can anyone replicate this?  Is there a known explanation?

Andrea Monaco

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