Invoke system commands in gdb for application with no debug symbols

Question

I just want to learn how I can dynamically to redirect linux process input/output/err(0/1/2) and I found that answer in in stack overflow How to redirect output of an already running process

As you can see the guide ask me to attach to the process trough gdb and invoke close() on the file descriptor and the create new one. Unfortunately cat (the test app) is not build with debug info and gdb complains that there is no such symbol as close. The same is true for the rest of the system commands that I tried.

What I need to do to have access to those symbols in gdb Also if you are able to provide alternative way to redirect the output of already running process without installing exotic tools or using gdb will be nice. Thanks

edit: gdb session

gdb -p 6836 /bin/cat                                                                                                                                                                                                                                              
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Reading symbols from /bin/cat...
(No debugging symbols found in /bin/cat)
Attaching to program: /usr/bin/cat, process 6836
ptrace: No such process.
(gdb) p close(1)
No symbol table is loaded.  Use the "file" command.
(gdb) file /bin/cat
Reading symbols from /bin/cat...
(No debugging symbols found in /bin/cat)
(gdb) p close(1)
No symbol table is loaded.  Use the "file" command.
(gdb) 

Answer 1

Your problem starts here:

Attaching to program: /usr/bin/cat, process 6836
ptrace: No such process.

This is most likely failing because you've either mistyped the process id, or your /usr/bin/cat exited for other reasons.

Try doing it like this:

cat > /tmp/foo &
[1] 1084976   # Note the PID

gdb /usr/bin/cat -p 1084976  # use the PID printed by the shell

Here is what you should see:

Attaching to program: /bin/cat, process 1084976

Program received signal SIGTTIN, Stopped (tty input).
0x00007f96c36178be in __GI___libc_read (fd=0, buf=0x7f96c36f6000, nbytes=131072) at ../sysdeps/unix/sysv/linux/read.c:26
26      ../sysdeps/unix/sysv/linux/read.c: No such file or directory.

(gdb) info shared
From                To                  Syms Read   Shared Object Library
0x00007f96c354f330  0x00007f96c3696df9  Yes         /lib/x86_64-linux-gnu/libc.so.6
0x00007f96c3718050  0x00007f96c3737cbe  Yes         /lib64/ld-linux-x86-64.so.2

The reason you don't have close defined is that it's defined in libc.so.6, but your GDB has not loaded symbols from libc.so.6 because it failed to attach the process.

Without a process, you can't call anything even if close was defined in the main executable.