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FORK(2) 		 DragonFly System Calls Manual		       FORK(2)


fork -- create a new process


Standard C Library (libc, -lc)


#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: * The child process has a unique process ID. * The child process has a different parent process ID (i.e., the process ID of the parent process). * 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(2) or write(2) by the parent. This descriptor copying is also used by the shell to establish standard input and output for newly created processes as well as to set up pipes. * The child process' resource utilizations are set to 0; see setrlimit(2). * All interval timers are cleared; see setitimer(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. MULTI-THREADING CONSIDERATIONS fork() can create severe issues for multi-threaded programs due to the fact that the memory state of the child process will record the asynchro- nous state of the threads that are running in the parent. Fork() will only be synchronous for the specific thread making the call. In particu- lar, locks used internally by pthread(3) and rtld(1) can be caught in a bad state. To deal with these issues, the pthreads library goes to great lengths to synchronize internal locks when a fork() call is issued. The threaded program itself as well as third party libraries used by the pro- gram might or might not properly handle these issues when it comes to their own internal state. If at all possible, programs should use vfork(2) instead of fork() when forking for the purposes of issuing an exec of some sort. Attempting to fork a threaded program without issuing an exec is not recommended. Attempting to bypass pthreads and implement threading manually is also not recommended as it is doubtful that homegrown implementations could properly deal with rtld races.


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. The limit is given by the sysctl(3) MIB variable KERN_MAXPROC. (The limit is actually ten less than this except for the super user). [EAGAIN] The user is not the super user, and the system-imposed limit on the total number of processes under execution by a single user would be exceeded. The limit is given by the sysctl(3) MIB variable KERN_MAXPROCPERUID. [EAGAIN] The user is not the super user, and the soft resource limit corresponding to the resource parameter RLIMIT_NPROC would be exceeded (see getrlimit(2)). [ENOMEM] There is insufficient swap space for the new process.


execve(2), rfork(2), setitimer(2), setrlimit(2), vfork(2), wait(2)


A fork() function call appeared in Version 6 AT&T UNIX. DragonFly 5.3 June 4, 1993 DragonFly 5.3 FORK(3am) GNU Awk Extension Modules FORK(3am)


fork, wait, waitpid - basic process management


@load "fork" pid = fork() ret = waitpid(pid) ret = wait();


The fork extension adds three functions, as follows. fork() This function creates a new process. The return value is the zero in the child and the process-id number of the child in the parent, or -1 upon error. In the latter case, ERRNO indicates the problem. In the child, PROCINFO["pid"] and PROCINFO["ppid"] are updated to reflect the correct values. waitpid() This function takes a numeric argument, which is the process-id to wait for. The return value is that of the waitpid(2) system call. wait() This function waits for the first child to die. The return value is that of the wait(2) system call.


There is no corresponding exec() function. The interfaces could be enhanced to provide more facilities, including pulling out the various bits of the return status.


@load "fork" ... if ((pid = fork()) == 0) print "hello from the child" else print "hello from the parent"


GAWK: Effective AWK Programming, filefuncs(3am), fnmatch(3am), inplace(3am), ordchr(3am), readdir(3am), readfile(3am), revoutput(3am), rwarray(3am), time(3am). fork(2), wait(2), waitpid(2).


Arnold Robbins, arnold@skeeve.com.


Copyright (C) 2012, 2013, Free Software Foundation, Inc. Permission is granted to make and distribute verbatim copies of this manual page provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual page under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this man- ual page into another language, under the above conditions for modified versions, except that this permission notice may be stated in a trans- lation approved by the Foundation. Free Software Foundation Jan 15 2013 FORK(3am)

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