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fork(2) [freebsd man page]

FORK(2) 						      BSD System Calls Manual							   FORK(2)

NAME
fork -- create a new process LIBRARY
Standard C Library (libc, -lc) SYNOPSIS
#include <unistd.h> pid_t fork(void); DESCRIPTION
The fork() system call 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(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. o The child process' resource utilizations are set to 0; see setrlimit(2). o All interval timers are cleared; see setitimer(2). o The child process has only one thread, corresponding to the calling thread in the parent process. If the process has more than one thread, locks and other resources held by the other threads are not released and therefore only async-signal-safe functions (see sigaction(2)) are guaranteed to work in the child process until a call to execve(2) or a similar function. RETURN VALUES
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 indicate the error. ERRORS
The fork() system call will fail and no child process will be created if: [EAGAIN] The system-imposed limit on the total number of processes 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 argument RLIMIT_NPROC would be exceeded (see getrlimit(2)). [ENOMEM] There is insufficient swap space for the new process. SEE ALSO
execve(2), rfork(2), setitimer(2), setrlimit(2), sigaction(2), vfork(2), wait(2) HISTORY
The fork() function appeared in Version 6 AT&T UNIX. BSD
May 31, 2013 BSD

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RFORK(2)						      BSD System Calls Manual							  RFORK(2)

NAME
rfork -- manipulate process resources LIBRARY
Standard C Library (libc, -lc) SYNOPSIS
#include <unistd.h> pid_t rfork(int flags); DESCRIPTION
Forking, vforking or rforking are the only ways new processes are created. The flags argument to rfork() selects which resources of the invoking process (parent) are shared by the new process (child) or initialized to their default values. The resources include the open file descriptor table (which, when shared, permits processes to open and close files for other processes), and open files. The flags argument is the logical OR of some subset of: RFPROC If set a new process is created; otherwise changes affect the current process. RFNOWAIT If set, the child process will be dissociated from the parent. Upon exit the child will not leave a status for the parent to collect. See wait(2). RFFDG If set, the invoker's file descriptor table (see intro(2)) is copied; otherwise the two processes share a single table. RFCFDG If set, the new process starts with a clean file descriptor table. Is mutually exclusive with RFFDG. RFTHREAD If set, the new process shares file descriptor to process leaders table with its parent. Only applies when neither RFFDG nor RFCFDG are set. RFMEM If set, the kernel will force sharing of the entire address space, typically by sharing the hardware page table directly. The child will thus inherit and share all the segments the parent process owns, whether they are normally shareable or not. The stack segment is not split (both the parent and child return on the same stack) and thus rfork() with the RFMEM flag may not generally be called directly from high level languages including C. May be set only with RFPROC. A helper function is provided to assist with this problem and will cause the new process to run on the provided stack. See rfork_thread(3) for information. RFSIGSHARE If set, the kernel will force sharing the sigacts structure between the child and the parent. RFLINUXTHPN If set, the kernel will return SIGUSR1 instead of SIGCHILD upon thread exit for the child. This is intended to mimic certain Linux clone behaviour. File descriptors in a shared file descriptor table are kept open until either they are explicitly closed or all processes sharing the table exit. If RFPROC is set, the value returned in the parent process is the process id of the child process; the value returned in the child is zero. Without RFPROC, the return value is zero. Process id's range from 1 to the maximum integer (int) value. The rfork() system call will sleep, if necessary, until required process resources are available. The fork() system call can be implemented as a call to rfork(RFFDG | RFPROC) but is not for backwards compatibility. RETURN VALUES
Upon successful completion, rfork() 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 indicate the error. ERRORS
The rfork() system call will fail and no child process will be created if: [EAGAIN] The system-imposed limit on the total number of processes 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 argument RLIMIT_NOFILE would be exceeded (see getrlimit(2)). [EINVAL] Both the RFFDG and the RFCFDG flags were specified. [EINVAL] Any flags not listed above were specified. [ENOMEM] There is insufficient swap space for the new process. SEE ALSO
fork(2), intro(2), minherit(2), vfork(2), rfork_thread(3) HISTORY
The rfork() function first appeared in Plan9. BUGS
FreeBSD does not yet implement a native clone() library call, and the current pthreads implementation does not use rfork() with RFMEM. A native port of the linux threads library, /usr/ports/devel/linuxthreads, contains a working clone() call that utilizes RFMEM. The rfork_thread(3) function can often be used instead of clone(). BSD
May 14, 2007 BSD

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