The Linux Programming Interface

520 Chapter 24

Figure 24-2: Duplication of file descriptors during fork(), and closing of unused descriptors

24.2.2 Memory Semantics of fork()...................................................................

Conceptually, we can consider fork() as creating copies of the parent’s text, data,

heap, and stack segments. (Indeed, in some early UNIX implementations, such

duplication was literally performed: a new process image was created by copying the

parent’s memory to swap space, and making that swapped-out image the child pro-

cess while the parent kept its own memory.) However, actually performing a simple

copy of the parent’s virtual memory pages into the new child process would be

wasteful for a number of reasons—one being that a fork() is often followed by an

immediate exec(), which replaces the process’s text with a new program and reinitializes

descriptor y

descriptor x

Parent file descriptors

(close-on-exec flag)

Open file table

(file offset, status flags)

Parent file descriptors Open file table

Child file descriptors

Child file descriptors

Parent file descriptors Open file table

a) Descriptors and open

file table entries

before fork()

b) Descriptors after

fork()

c) After closing unused

descriptors in parent

(y) and child (x)

Descriptors

duplicated

in child

descriptor y

descriptor x

descriptor y

descriptor x

descriptor y

descriptor x

descriptory

descriptor x

OFT entry m

OFT entry n

OFT entry m

OFT entry n

OFT entry m

OFT entry n