Fork
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fork -- create a new process
#include <sys/types.h>
#include <unistd.h>
pid_t
fork(void);
Fork() causes creation of a new process. The new process (child process)
is an exact copy of the calling process (parent process) except for the
following:
o The child process has a unique process ID.
o The child process has a different parent process ID (i.e., the
process ID of the parent process).
o The child process has its own copy of the parent's descriptors.
These descriptors reference the same underlying objects, so
that, for instance, file pointers in file objects are shared
between the child and the parent, so that an lseek(2) on a
descriptor in the child process can affect a subsequent read or
write by the parent. This descriptor copying is also used by
the shell to establish standard input and output for newly cre-
ated processes as well as to set up pipes.
o The child processes resource utilizations are set to 0; see
setrlimit(2).
Upon successful completion, fork() returns a value of 0 to the child
process and returns the process ID of the child process to the parent
process. Otherwise, a value of -1 is returned to the parent process, no
child process is created, and the global variable errno is set to indi-
cate the error.
Fork() will fail and no child process will be created if:
[EAGAIN] The system-imposed limit on the total number of pro-
cesses under execution would be exceeded. This limit
is configuration-dependent.
[EAGAIN] The system-imposed limit MAXUPRC (<sys/param.h>) on
the total number of processes under execution by a
single user would be exceeded.
[ENOMEM] There is insufficient swap space for the new process.
execve(2), wait(2)
A fork() function call appeared in Version 6 AT&T UNIX. 4th Berkeley Distribution June 4, 1993 4th Berkeley Distribution
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Example Usage (taken from the excellent Advanced Programming in the UNIX Environment 2nd Ed by Stevens).
1. #include <stdio.h>
2. #include <unistd.h>
3. #include <stdlib.h>
4.
5. int glob = 6;
6. char buf[] = "a write to stdout\n";
7.
8. int main(void) {
9. int var;
10. pid_t pid;
11.
12. var = 88;
13. if (write(STDOUT_FILENO, buf, sizeof(buf)-1) != sizeof(buf)-1) {
14. printf("write error");
15. }
16. printf("before fork\n");
17.
18. if ((pid = fork()) < 0) {
19. printf("fork accept");
20. } else if (pid == 0) {
21. glob ++;
22. var ++;
23. } else {
24. sleep(2);
25. }
26.
27. printf("pid = %d, glob = %d, var = %d\n", getpid(), glob, var);
28. exit(0);
29.
30. }
Note: Fork creates a new child process. Fork returns the process ID of the child process or -1 on error. In this code the parent will execute line 24 and sleep for 2 seconds, in contrast, the child will have a pid of 0 and will execute lines 21 and 22. Basically at the fork() call a child will be started in addition to the parent so the function will return twice having only been called once. The processes do not share memory.
Output:
$ ./forkTest.exe
a write to stdout
before fork
pid = 3028, glob = 7, var = 89
pid = 2216, glob = 6, var = 88
pid = fork()
if (pid == 0) {
// child…
…
exec();
}
else {
// parent
wait();
}
Note: A process can have one or more threads. Threads have their own CPU state (SP, register values etc)
Process Vs Thread - Good set of slides <here>