LinuxQuestions.org - How many thread can be created in user space program?

Kernel system wide maximum.

Code:

$ cat /proc/sys/kernel/threads-max

16383

Variable limit set per user during login.

Code:

$ ulimit -a

core file size          (blocks, -c) 0

data seg size          (kbytes, -d) unlimited

file size              (blocks, -f) unlimited

pending signals                (-i) 8191

max locked memory      (kbytes, -l) 32

max memory size        (kbytes, -m) unlimited

open files                      (-n) 1024

pipe size            (512 bytes, -p) 8

POSIX message queues    (bytes, -q) 819200

stack size              (kbytes, -s) 8192

cpu time              (seconds, -t) unlimited

max user processes              (-u) 8191

virtual memory          (kbytes, -v) unlimited

file locks                      (-x) unlimited

$

Note that the ulimit values shown are okay for an interactive user on a workstation. Server processes such as ftp should have lower limits in order to avoid being susceptible to a fork attack. A fork attack is when a bad guy "freezes" your server by causing some remotely accessible process such as a server daemon to spawn a gajillion subprocesses. This overwhelms the kernel's ability to manage resources and the system will hang.