使用GDB进行调试时，如何将std :: cin重定向到Linux终端？

Question

In Linux, GDB doesn't allow set new-console on and in stead uses something called tty . With

set inferior-tty /dev/pts/[number of an active console] ,

in a .gdbinit file (requires editing the number every time) it redirects std::cout , but std::cin isn't working properly. It just interprets my input as if I'm sending a bash command and reports an error, and my program continues to wait for input. I can no longer type in the console after that, so I assume std::cin is being redirected, but doesn't work properly.

I tried looking up how to launch a terminal from the application itself. I could only find this answer , which also mentions a bug that it doesn't redirect input.

Is there any way to fix this issue and redirect std::cin (and std::cout ) to a Linux terminal properly when debugging?

---

Background info: What I'm trying to do should be simple. Print a > in front of user input before using std::cin. I have simple code in place that prints the > , flushes cout and then calls getline() . It works when just running the program normally. But sadly, GDB refuses to flush the stream when there isn't a newline, so it doesn't print the > , ignores the first character of the user input and then does prints the > , immediately followed by the error message that my program sends because of the mutilated input string.

In Windows, I've solved it by making a .gdbinit file with set new-console on . This causes GDB to use a Windows console in stead of its own and that works as intended.

Answer 1

If you need to start the debugging after the program has a chance to pause, just run the program in a terminal, and then gdb - your-program-pid in another terminal. Otherwise there's a sequence of steps which is faster to do than to describe.

1. Start two terminal windows.
2. In one terminal, figure out the PID of the shell. ps should tell you.

3. In the other terminal, use these commands

    $ gdb - pid-of-the-other-shell (gdb) br main (gdb) c 

4. Now you have gdb attached to your shell in another terminal. Not very useful.

5. In the first terminal, type at the shell prompt

    $ exec your-program 

Now you have one terminal running gdb and another terminal running your program under gdb , stopped at main . Bear in mind that when the program exits, its terminal window will close. If you don't want this, start a second level shell in the first terminal and attach gdb to it.

You can also use set inferior-tty command, but you must make sure the other tty exists and no other program attempts to read from it. The easiest way is to run a shell in another terminal and give it a while true; do sleep 1000; done while true; do sleep 1000; done while true; do sleep 1000; done command. Note that you may get warning: GDB: Failed to set controlling terminal: Operation not permitted messages. They are harmless.

Answer 2

You can use gdb's multiprocess debugging feature and a terminal emulator like xterm to do this. ( gnome-terminal won't work so well as explained later)

prompt.c

#include <stdio.h>

int main()
{
    enum {
        N = 100,
    };
    char name[N];

    printf("> ");
    scanf("%s", name);
    printf("Hi, %s\n", name);
    return 0;
}

xterm.gdb

set detach-on-fork off
set target-async on
set pagination off
set non-stop on

file /usr/bin/xterm
# -ut tells xterm not to write a utmp log record
# which involves root privileges
set args -ut -e ./prompt
catch exec

run

Sample Session

$ gdb -q -x xterm.gdb
Catchpoint 1 (exec)
<...>
[New process 7073]
<...>
Thread 0x7ffff7fb2780 (LWP 7073) is executing new program: /home/scottt/Dropbox/stackoverflow/gdb-stdin-new-terminal/prompt
Reading symbols from /home/scottt/Dropbox/stackoverflow/gdb-stdin-new-terminal/prompt...done.

Catchpoint 1 (exec'd /home/scottt/Dropbox/stackoverflow/gdb-stdin-new-terminal/prompt), 0x0000003d832011f0 in _start () from /lib64/ld-linux-x86-64.so.2
(gdb) inferior 2
[Switching to inferior 2 [process 7073] (/home/scottt/Dropbox/stackoverflow/gdb-stdin-new-terminal/prompt)]
[Switching to thread 2 (Thread 0x7ffff7fb2780 (LWP 7073))] 
#0  0x0000003d832011f0 in _start () from /lib64/ld-linux-x86-64.so.2
(gdb) break prompt.c:11
Breakpoint 2 at 0x4004b4: file prompt.c, line 11.
(gdb) continue &
Continuing.
(gdb) 
Breakpoint 2, main () at prompt.c:11
11      printf("> ");
next
12      scanf("%s", name);
(gdb) next
13      printf("Hi, %s\n", name);

A new xterm window will appear after executing gdb -q -x xterm.gdb and you can interact with prompt inside it without interfering with GDB.

Related GDB commands:

• inferior 2 switches the process GDB's debugging much like the thread command switches threads. ( GDB: Debugging Multiple Inferiors and Programs )
• continue & means to continue executing in the background. ( GDB: Background Execution )

I used xterm because gnome-terminal uses dbus-launch to ask another process to launch the new terminal outside of the parent-child process relationship that GDB relies on.