how to redirect /dev/null to stdout/stderr in running process

Question

Test cmd

$ ping localhost > /dev/null & disown
# [3] 20757

$ ls -l /proc/20757/fd

total 0
lrwx------ 1 root root 64  3월 23 14:28 0 -> /dev/pts/5
l-wx------ 1 root root 64  3월 23 14:28 1 -> /dev/null
lrwx------ 1 root root 64  3월 23 14:28 2 -> /dev/pts/5
lrwx------ 1 root root 64  3월 23 14:28 3 -> 'socket:[251223]'
lrwx------ 1 root root 64  3월 23 14:28 4 -> 'socket:[251225]'

current stdout is /dev/null

and i want to redirect /dev/pts/5

so i tried to

$ gdb -p 20757

...

(gdb) p dup2(open("/dev/pts/5", 1089, 0777), 1) # try 1, return false
# $1 = -1

(gdb) p dup2(open("/dev/pts/5", 0), 1) # try 2 return false
# $2 = -1

(gdb) p dup2(open("/dev/null", 0), 1)
# $3 = 1

but i fail to redirect /dev/pts/5

what can i do?

---

I found one solution

(gdb) p dup2(0, 1)

but it just duplicate stdin to stdout

there tis in status

$ ls -l /proc/20757/fd

total 0
lrwx------ 1 root root 64  3월 23 14:28 0 -> /dev/null
l-wx------ 1 root root 64  3월 23 14:28 1 -> /dev/null
lrwx------ 1 root root 64  3월 23 14:28 2 -> /dev/null
lrwx------ 1 root root 64  3월 23 14:28 3 -> 'socket:[251223]'
lrwx------ 1 root root 64  3월 23 14:28 4 -> 'socket:[251225]'

how can i change /dev/null to /dev/pts/5