system: Resource temporarily unavailable, which one?

I search for answer and so far haven't found a clear one.

I am doing testing which launches many threads calling "system()", like below.

for (int i = 0; i < 3000; ++i)
  pthread_create(&thread[i], NULL, thread_func, NULL);

for (int i = 0; i < 3000; ++i)
  pthread_join(thread[i], NULL);

...
void* thread_func(void* arg)
{
  if (system('test.sh') == -1)
  {
    perror("system");
    exit(1);
  }
  pthread_exit(NULL);
}

test.sh

#!/bin/bash

sleep 100

When I run the program, at certain point it will display.

system: Resource temporarily unavailable

Is there way to know which resource? I fix the max processes issue so I think it may be due to something else.

This error means that some system call called by the system library function returned EGAIN. Most likely is the fork call, which can fail with EAGAIN for a number of reasons:

   EAGAIN A system-imposed limit on the number of threads was encountered.  There are a  num‐
          ber of limits that may trigger this error:

          *  the  RLIMIT_NPROC  soft  resource limit (set via setrlimit(2)), which limits the
             number of processes and threads for a real user ID, was reached;

          *  the  kernel's  system-wide  limit  on  the  number  of  processes  and  threads,
             /proc/sys/kernel/threads-max, was reached (see proc(5));

          *  the maximum number of PIDs, /proc/sys/kernel/pid_max, was reached (see proc(5));
             or

          *  the PID limit (pids.max) imposed by the  cgroup  "process  number"  (PIDs)  con‐
             troller was reached.