GDB doesn't hit catchpoint on the child process forked off from debuggee

I was re-doing what described here about multiprocessing debugging in GDB. The weird thing is that GDB doesn't hit the exec catchpoint on the child process running cat command (the latter is forked off from bash). ubuntu@ubuntu:~$ echo $$ 670639 ubuntu@ubuntu:~$ cat /etc/issue root@ubuntu:~# gdb -q -p 670639 Attaching to process 670639 Reading symbols from /usr/bin/bash... (No debugging symbols found in /usr/bin/bash) Reading symbols from /lib/x86_64-linux-gnu/libtinfo.so.6... (No debugging symbols found in /lib/x86_64-linux-gnu/libtinfo.so.6) Reading symbols from /lib/x86_64-linux-gnu/libc.so.6... Reading symbols from /usr/lib/debug/.build-id/49/0fef8403240c91833978d494d39e537409b92e.debug... Reading symbols from /lib64/ld-linux-x86-64.so.2... Reading symbols from /usr/lib/debug/.build-id/41/86944c50f8a32b47d74931e3f512b811813b64.debug... [Thread debugging using libthread_db enabled] Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1". pselect64_syscall (sigmask=0x564025a0c820 , timeout=, exceptfds=0x0, writefds=0x0, readfds=0x7ffe468736e0, nfds=1) at ../sysdeps/unix/sysv/linux/pselect.c:34 34 ../sysdeps/unix/sysv/linux/pselect.c: No such file or directory. (gdb) catch fork Catchpoint 1 (fork) (gdb) catch exec Catchpoint 2 (exec) (gdb) c Continuing. Catchpoint 1 (forked process 719946), arch_fork (ctid=0x7fbccdc6fa10) at ../sysdeps/unix/sysv/linux/arch-fork.h:52 52 ../sysdeps/unix/sysv/linux/arch-fork.h: No such file or directory. (gdb) info inferiors Num Description Connection Executable * 1 process 670639 1 (native) /usr/bin/bash (gdb) set detach-on-fork off (gdb) nexti [New inferior 2 (process 719946)] [Thread debugging using libthread_db enabled] Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1". 52 in ../sysdeps/unix/sysv/linux/arch-fork.h (gdb) c Continuing. What is the reason behind it? Thanks.