Problem with system() call

I am using a system() call in a program , that is in c library. For 1st 9 calls it returns '0'(zero) after 10th call it returns 256. I do not know what does it 开发者_如何学编程mean. Please anybody help me. Following is the line of code

int returnValue= system("/system/bin/cat /dev/graphics/fb0 > /tmpdata/Screenshot/screenshot.bin");

According to this man page dealing with the general unix cat command, an error code >0 simply means an error occurred.

The following exit values shall be returned:

 0
    All input files were output successfully.
>0
    An error occurred.

Your system() call is attempting to concatentate two files, so perhaps there is a space issue or maybe the source file does not exist.

You may also wish to take a look at some recent source code for Android cat (cat.c) which gives some indicatations of the kind of things that trigger errors within cat.