python program call NimMain@.so different position leads to different result ?

[DL]

i build a.so library for python program but different position to call 
NimMain() , program results are different too.

**a.nim**
    
    
    proc test(name : cstring,address: cstring) : int {. exportc,dynlib,cdecl .} 
=
      var k = newSeq[string]()
      k.add($name)
      k.add($address)
      
      echo len(k),k
      if len($name) > 255:
        return 200
      return len($name)
    

build cmd: nim c --app:lib -d:release --stackTrace:off --noMain a

OS: ubuntu 16.04

**a.py**
    
    
    #coding: utf8
    from ctypes import *
    P = CDLL("./liba.so")
    P.test.argtypes = [c_char_p,c_char_p]
    P.test.restype = c_int
    #P.NimMain()    #Position 1
    

**b.py**
    
    
    #coding: utf8
    from a import P
    if __name__=="__main__":
    #    P.NimMain()           #Position 2
        A="Jerry TOM"
        for x in xrange(4096):
            print "===>",x
            print P.test("Hello",A*(x+1)),x
    

Run 
    
    
     b.py

uncomment Position 1: sometime SIGSEGV: Illegal storage access. (Attempt to 
read from nil?)

uncomment Position 2: It's OK..

debug mode:

nim --lineDir:on --debuginfo c --debugger:native --stackTrace:off --app:lib 
--noMain a
    
    
    gdb /usr/bin/python2
    r b.py
    ......
    bt
    Program received signal SIGSEGV, Segmentation fault.
    0x00007ffff63ee8cf in copyStringRC1 (src0=0x7ffff5f67040) at 
/home/program/nim/lib/system/sysstr.nim:109
    109     result.len = src.len
    (gdb) bt
    #0  0x00007ffff63ee8cf in copyStringRC1 (src0=0x7ffff5f67040) at 
/home/program/nim/lib/system/sysstr.nim:109
    #1  0x00007ffff63fb525 in test (name0=0x7ffff7e94fb4 "Hello",
        address0=0xa5a5e4 "Jerry TOMJerry TOMJerry TOMJerry TOMJerry TOMJerry 
TOMJerry TOMJerry TOMJerry TOMJerry TOMJerry TOMJerry TOMJerry TOMJerry 
TOMJerry TOMJerry TOMJerry TOMJerry TOMJerry TOMJerry TOMJerry TOMJerry 
TOMJe"...) at /home/david/1/a.nim:5
    #2  0x00007ffff6617e40 in ffi_call_unix64 () from 
/usr/lib/x86_64-linux-gnu/libffi.so.6
    
    

looks like, NimMain no work and GC work ?

thanks