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vfork(2) [freebsd man page]

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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VFORK(2)						     Linux Programmer's Manual							  VFORK(2)

vfork - create a child process and block parent SYNOPSIS
#include <sys/types.h> #include <unistd.h> pid_t vfork(void); STANDARD DESCRIPTION
(From XPG4 / SUSv2 / POSIX draft.) The vfork() function has the same effect as fork(), except that the behaviour is undefined if the process created by vfork() either modifies any data other than a variable of type pid_t used to store the return value from vfork(), or returns from the function in which vfork() was called, or calls any other function before successfully calling _exit() or one of the exec family of functions. ERRORS
EAGAIN Too many processes - try again. ENOMEM There is insufficient swap space for the new process. LINUX DESCRIPTION
vfork, just like fork(2), creates a child process of the calling process. For details and return value and errors, see fork(2). vfork() is a special case of clone(2). It is used to create new processes without copying the page tables of the parent process. It may be useful in performance sensitive applications where a child will be created which then immediately issues an execve(). vfork() differs from fork in that the parent is suspended until the child makes a call to execve(2) or _exit(2). The child shares all mem- ory with its parent, including the stack, until execve() is issued by the child. The child must not return from the current function or call exit(), but may call _exit(). Signal handlers are inherited, but not shared. Signals to the parent arrive after the child releases the parent. HISTORIC DESCRIPTION
Under Linux, fork() is implemented using copy-on-write pages, so the only penalty incurred by fork() is the time and memory required to duplicate the parent's page tables, and to create a unique task structure for the child. However, in the bad old days a fork() would require making a complete copy of the caller's data space, often needlessly, since usually immediately afterwards an exec() is done. Thus, for greater efficiency, BSD introduced the vfork system call, that did not fully copy the address space of the parent process, but borrowed the parent's memory and thread of control until a call to execve() or an exit occurred. The parent process was suspended while the child was using its resources. The use of vfork was tricky - for example, not modifying data in the parent process depended on knowing which variables are held in a register. BUGS
It is rather unfortunate that Linux revived this spectre from the past. The BSD manpage states: "This system call will be eliminated when proper system sharing mechanisms are implemented. Users should not depend on the memory sharing semantics of vfork as it will, in that case, be made synonymous to fork." Formally speaking, the standard description given above does not allow one to use vfork() since a following exec might fail, and then what happens is undefined. Details of the signal handling are obscure and differ between systems. The BSD manpage states: "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 ioctls are allowed and input attempts result in an end-of-file indication." Currently (Linux 2.3.25), strace(1) cannot follow vfork() and requires a kernel patch. HISTORY
The vfork() system call appeared in 3.0BSD. In BSD 4.4 it was made synonymous to fork(), but NetBSD introduced it again, cf. . In Linux, it has been equivalent to fork() until 2.2.0-pre6 or so. Since 2.2.0-pre9 (on i386, somewhat later on other architectures) it is an independent system call. Support was added in glibc 2.0.112. CONFORMING TO
The vfork call may be a bit similar to calls with the same name in other operating systems. The requirements put on vfork by the standards are weaker than those put on fork, so an implementation where the two are synonymous is compliant. In particular, the programmer cannot rely on the parent remaining blocked until a call of execve() or _exit() and cannot rely on any specific behaviour w.r.t. shared memory. SEE ALSO
clone(2), execve(2), fork(2), wait(2) Linux 2.2.0 1999-11-01 VFORK(2)

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