1.\" Copyright (c) 1980, 1991 Regents of the University of California. 2.\" All rights reserved. 3.\" 4.\" %sccs.include.redist.man% 5.\" 6.\" @(#)vfork.2 6.3 (Berkeley) 03/10/91 7.\" 8.Dd 9.Dt VFORK 2 10.Os BSD 4 11.Sh NAME 12.Nm vfork 13.Nd spawn new process in a virtual memory efficient way 14.Sh SYNOPSIS 15.Fd #include <unistd.h> 16.Ft int 17.Fn vfork void 18.Sh DESCRIPTION 19.Fn Vfork 20can be used to create new processes without fully copying the address 21space of the old process, which is horrendously inefficient in a paged 22environment. It is useful when the purpose of 23.Xr fork 2 24would have been to create a new system context for an 25.Xr execve . 26.Fn Vfork 27differs from 28.Xr fork 29in that the child borrows the parent's memory and thread of 30control until a call to 31.Xr execve 2 32or an exit (either by a call to 33.Xr exit 2 34or abnormally.) 35The parent process is suspended while the child is using its resources. 36.Pp 37.Fn Vfork 38returns 0 in the child's context and (later) the pid of the child in 39the parent's context. 40.Pp 41.Fn Vfork 42can normally be used just like 43.Xr fork . 44It does not work, however, to return while running in the childs context 45from the procedure that called 46.Fn vfork 47since the eventual return from 48.Fn vfork 49would then return to a no longer existent stack frame. 50Be careful, also, to call 51.Xr _exit 52rather than 53.Xr exit 54if you can't 55.Xr execve , 56since 57.Xr exit 58will flush and close standard I/O channels, and thereby mess up the 59parent processes standard I/O data structures. 60(Even with 61.Xr fork 62it is wrong to call 63.Xr exit 64since buffered data would then be flushed twice.) 65.Sh SEE ALSO 66.Xr fork 2 , 67.Xr execve 2 , 68.Xr sigvec 2 , 69.Xr wait 2 , 70.Sh DIAGNOSTICS 71Same as for 72.Xr fork . 73.Sh BUGS 74This system call will be eliminated when proper system sharing 75mechanisms are implemented. 76Users should not depend on the memory 77sharing semantics of 78.Xr vfork 79as it will, in that case, be made synonymous to 80.Xr fork . 81.Pp 82To avoid a possible deadlock situation, 83processes that are children in the middle 84of a 85.Fn vfork 86are never sent 87.Dv SIGTTOU 88or 89.Dv SIGTTIN 90signals; rather, 91output or 92.Xr ioctl 2 93calls 94are allowed 95and input attempts result in an end-of-file indication. 96.Sh HISTORY 97The 98.Nm 99function call appeared in 100.Bx 3.0 . 101