Advanced Programming in the UNIX® Environment

ptg10805159

232 Process Control Chapter 8

parent process table entry

...

fd 0:

fd 1:

fd 2:

flagsfd pointerfile

child process table entry

...

fd 0:

fd 1:

fd 2:

flagsfd pointerfile

file status flags

current file offset

v-node pointer

file status flags

current file offset

v-node pointer

file status flags

current file offset

v-node pointer

file table

v-node information

v_data

i-node information

current file size

i_vnode

v-node information

v_data

i-node information

current file size

i_vnode

v-node information

v_data

i-node information

current file size

i_vnode

v-node table

Figure 8.2 Sharing of open files between parent and child afterfork

If both parent and child write to the same descriptor,without any form of

synchronization, such as having the parentwaitfor the child, their output will be

intermixed (assuming it’s a descriptor that was open beforethefork). Although this is

possible — we saw it in Figure8.2 — it’s not the normal mode of operation.

Thereare two normal cases for handling the descriptors after afork.

1. The parent waits for the child to complete. In this case, the parent does not need
   to do anything with its descriptors. When the child terminates, any of the
   shared descriptors that the child read from or wrote to will have their file offsets
   updated accordingly.


2. Both the parent and the child go their own ways. Here, after thefork,the
   parent closes the descriptors that it doesn’t need, and the child does the same
   thing. This way,neither interferes with the other’s open descriptors. This
   scenario is often found with network servers.