fork() is the name of the
system call that the
parent process uses to "divide" itself ("fork") into two identical processes. After calling fork(), the created
child process is an exact copy of the parent except for the return value of the fork() call. This includes open files, register state, and all memory allocations, which includes the program's executable code. In some cases the two continue to run the same binary, but often one (usually the child) switches to running another binary executable using the
exec() system call. When a process forks, a complete copy of the executing program is made into the new process. This new process is a child of the parent process, and has a new
process identifier (PID). The fork() function returns the child's PID to the parent process. The fork() function returns 0 to the child process. This enables the two otherwise identical processes to distinguish themselves from each other. The parent process can either continue execution or wait for the child process to complete. The child, after discovering that it is the child, will most often then replace itself completely with another program, so that the
code and
address space of the original program are lost. This replacement is, however, a choice of the architecture one builds the given program on, and is therefore not an obligatory step in the child process' life. If the parent chooses to wait for the child to die, then the parent will receive the
exit code of the program that the child executed. To prevent the child becoming a
zombie the parent should call
wait on its children, either periodically or upon receiving the
SIGCHLD signal, which indicates a child process has terminated. One can also asynchronously
wait on their children to finish, by using a signal handler for
SIGCHLD, if they need to ensure everything is cleaned up. Here's an example of a signal handler that catches any incoming
SIGCHLD signals and handles multiple concurrent signals received. void cleanup(int signal) { while (waitpid((pid_t)(-1), 0, WNOHANG) > 0) { // ... } } When the child process calls exec(), all data in the original program is lost, and it is replaced with a running copy of the new program. This is known as
overlaying. Although all data are replaced, the
file descriptors that were open in the parent are closed only if the program has explicitly marked them
close-on-exec. This allows for the common practice of the parent creating a
pipe prior to calling fork() and using it to communicate with the executed program.
Microsoft Windows does not support the fork–exec model, as it does not have a system call analogous to fork(). The
spawn() family of functions declared in
process.h can replace it in cases where the call to fork() is followed directly by exec(). ==References==