FILE* file = fopen("/sdcard/hello.txt","w+");
if (file != NULL)
{
    fputs("HELLO WORLD!\n", file);
    fflush(file);
    fclose(file);
}
Dont forget to place a permission for it. in the Android manifest place a
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE">;
http://stackoverflow.com/questions/1992953/file-operations-in-android-ndk
