3.2 What do parent/child processes share?

Using fork, the child process is a clone of the parent. In other words, the two processes are almost identical. The first difference is that as they continue from the returning of fork, the parent gets a non-zero child PID as the return value, whereas the child get 0 as a return value.

man 2 fork explains the differences between the parent and child processes. Particularly,

• The PIDs are different.
• The parent’s PID is the child’s parent PID (PPID).
• Resource utilizations are reset in the child.
• The child starts with no pending signals (even if the parent has pending signals).
• The child does not inherit timers.
• The child does not inherit outstanding asynchronous I/O operations.
• The child does not receive a signal when the parent dies.
• When the child process dies, SIGCHLD is sent to the parent.
• The child is created with a single thread.
• The child does inherit open file descriptors.

Furthermore, the parent and child processes do not share any memory space. This means that all global variables and duplicated, along with the stack of the thread that invokes fork.