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fork(2) [netbsd man page]

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

fork -- create a new process LIBRARY
Standard C Library (libc, -lc) SYNOPSIS
#include <unistd.h> pid_t fork(void); DESCRIPTION
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(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). In general, the child process should call _exit(2) rather than exit(3). Otherwise, any stdio buffers that exist both in the parent and child will be flushed twice. Similarly, _exit(2) should be used to prevent atexit(3) routines from being called twice (once in the parent and once in the child). In case of a threaded program, only the thread calling fork() is still running in the child processes. Child processes of a threaded program have additional restrictions, a child must only call functions that are async-signal-safe. Very few functions are asynchronously safe and applications should make sure they call exec(3) as soon as possible. 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
fork() 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. This limit is configuration-depen- dent. [EAGAIN] The limit RLIMIT_NPROC on the total number of processes under execution by this user id would be exceeded. [ENOMEM] There is insufficient swap space for the new process. SEE ALSO
execve(2), setrlimit(2), vfork(2), wait(2), pthread_atfork(3) STANDARDS
The fork() function conforms to ISO/IEC 9945-1:1990 (``POSIX.1''). HISTORY
A fork() system call appeared in Version 6 AT&T UNIX. BSD
June 10, 2004 BSD

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VFORK(2)						      BSD System Calls Manual							  VFORK(2)

vfork -- create a new process without copying the address space LIBRARY
Standard C Library (libc, -lc) SYNOPSIS
#include <unistd.h> pid_t vfork(void); DESCRIPTION
The vfork() system call can be used to create new processes without fully copying the address space of the old process, which is horrendously inefficient in a paged environment. It is useful when the purpose of fork(2) would have been to create a new system context for an execve(2). The vfork() system call differs from fork(2) in that the child borrows the parent's memory and thread of control until a call to execve(2) or an exit (either by a call to _exit(2) or abnormally). The parent process is suspended while the child is using its resources. The vfork() system call returns 0 in the child's context and (later) the pid of the child in the parent's context. The vfork() system call can normally be used just like fork(2). It does not work, however, to return while running in the child's context from the procedure that called vfork() since the eventual return from vfork() would then return to a no longer existent stack frame. Be careful, also, to call _exit(2) rather than exit(3) if you cannot execve(2), since exit(3) will flush and close standard I/O channels, and thereby mess up the parent processes standard I/O data structures. (Even with fork(2) it is wrong to call exit(3) since buffered data would then be flushed twice.) RETURN VALUES
Same as for fork(2). SEE ALSO
_exit(2), execve(2), fork(2), rfork(2), sigaction(2), wait(2), exit(3) HISTORY
The vfork() system call appeared in 2.9BSD. BUGS
To avoid a possible deadlock situation, processes that are children in the middle of a vfork() are never sent SIGTTOU or SIGTTIN signals; rather, output or ioctl(2) calls are allowed and input attempts result in an end-of-file indication. BSD
November 13, 2009 BSD
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