ash ulimit problem

[lukgeeker(lulu)]

dear busybox:
  
 busybox version is v1.23.2 
  
 The following command is not valid :
 1,busybox sh -c 'ulimit -c 1024 true'
2,busybox sh -c 'ulimit -c 1024'

  
 Result:
  
  
 [root at lukgeeker]# busybox ash -c 'ulimit -a'

-c: core file size (blocks)        0

  
 why?
 thk!!!
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