We’ve seen how a program can fork a child process.The child process is initially

running its parent’s program, with its parent’s virtual memory, file descriptors, and so

on copied.The child process can modify its memory, close file descriptors, and the like

without affecting its parent, and vice versa.When a program creates another thread,

though, nothing is copied.The creating and the created thread share the same memory

space, file descriptors, and other system resources as the original. If one thread changes

the value of a variable, for instance, the other thread subsequently will see the modi-fied

value. Similarly, if one thread closes a file descriptor, other threads may not read

from or write to that file descriptor. Because a process and all its threads can be exe-cuting only one program at a time, if any thread inside a process calls one of the exec

functions, all the other threads are ended (the new program may, of course, create new

threads).