来自fork(2)：

       *  The child inherits copies of the parent’s set of open file
 descrip-
         tors.   Each  file  descriptor  in the child refers to the same
 open
          file description (see open(2)) as the corresponding file
 descriptor
          in  the parent.  This means that the two descriptors share open
 file
          status flags, current file offset, and signal-driven I/O
 attributes
          (see the description of F_SETOWN and F_SETSIG in fcntl(2)).