diff options
Diffstat (limited to 'man2/vfork.2')
| -rw-r--r-- | man2/vfork.2 | 181 |
1 files changed, 181 insertions, 0 deletions
diff --git a/man2/vfork.2 b/man2/vfork.2 new file mode 100644 index 000000000..4c951b636 --- /dev/null +++ b/man2/vfork.2 @@ -0,0 +1,181 @@ +.\" Copyright (c) 1999 Andries Brouwer (aeb@cwi.nl), 1 Nov 1999 +.\" +.\" Permission is granted to make and distribute verbatim copies of this +.\" manual 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 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. +.\" +.\" Since the Linux kernel and libraries are constantly changing, this +.\" manual page may be incorrect or out-of-date. The author(s) assume no +.\" responsibility for errors or omissions, or for damages resulting from +.\" the use of the information contained herein. The author(s) may not +.\" have taken the same level of care in the production of this manual, +.\" which is licensed free of charge, as they might when working +.\" professionally. +.\" +.\" Formatted or processed versions of this manual, if unaccompanied by +.\" the source, must acknowledge the copyright and authors of this work. +.\" +.\" 1999-11-10: Merged text taken from the page contributed by +.\" Reed H. Petty (rhp@draper.net) +.\" +.TH VFORK 2 1999-11-01 "Linux 2.2.0" "Linux Programmer's Manual" +.SH NAME +vfork \- create a child process and block parent +.SH SYNOPSIS +.B #include <sys/types.h> +.br +.B #include <unistd.h> +.sp +.B pid_t vfork(void); +.SH "STANDARD DESCRIPTION" +(From XPG4 / SUSv2 / POSIX draft.) +The +.IR vfork () +function has the same effect as +.IR fork (), +except that the behaviour is undefined if the process created by +.IR vfork () +either modifies any data other than a variable of type pid_t used +to store the return value from +.IR vfork (), +or returns from the function in which +.IR vfork () +was called, or calls any other function before successfully calling +.IR _exit () +or one of the +.I exec +family of functions. +.SH ERRORS +.TP +.B EAGAIN +Too many processes - try again. +.TP +.B ENOMEM +There is insufficient swap space for the new process. +.SH "LINUX DESCRIPTION" +.BR vfork , +just like +.BR fork (2), +creates a child process of the calling process. +For details and return value and errors, see +.BR fork (2). +.PP +.B vfork() +is a special case of +.BR 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 +.IR execve() . +.PP +.B vfork() +differs from fork in that the parent is suspended until the child makes +a call to +.BR execve (2) +or +.BR _exit (2). +The child shares all memory with its parent, including the stack, until +.I execve() +is issued by the child. The child must not return from the +current function or call +.IR exit() , +but may call +.IR _exit() . +.PP +Signal handlers are inherited, but not shared. Signals to the parent +arrive after the child releases the parent. +.SH "HISTORIC DESCRIPTION" +Under Linux, +.IR fork () +is implemented using copy-on-write pages, so the only penalty incurred by +.IR 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 +.IR fork() +would require making a complete copy of the caller's data space, +often needlessly, since usually immediately afterwards an +.IR exec () +is done. Thus, for greater efficiency, BSD introduced the +.B 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 +.IR 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. +.SH 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 +.I vfork +as it will, in that case, be made synonymous to +.IR fork .\c +" + +Formally speaking, the standard description given above does not allow +one to use +.IR vfork () +since a following +.IR 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 +.I vfork +are never sent SIGTTOU or SIGTTIN signals; rather, output or +.IR ioctl s +are allowed and input attempts result in an end-of-file indication." + +Currently (Linux 2.3.25), +.BR strace (1) +cannot follow +.IR vfork() +and requires a kernel patch. +.SH HISTORY +The +.IR vfork () +system call appeared in 3.0BSD. +.\" In the release notes for BSD 4.2 Sam Leffler wrote: `vfork: Is still +.\" present, but definitely on its way out'. +In BSD 4.4 it was made synonymous to +.IR fork (), +but NetBSD introduced it again, +cf. http://www.netbsd.org/Documentation/kernel/vfork.html . +In Linux, it has been equivalent to +.IR 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. +.SH "CONFORMING TO" +The +.B vfork +call may be a bit similar to calls with the same name in other +operating systems. The requirements put on +.B vfork +by the standards are weaker than those put on +.BR 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 +.I execve() +or +.I _exit() +and cannot rely on any specific behaviour w.r.t. shared memory. +.\" In AIXv3.1 vfork is equivalent to fork. +.SH "SEE ALSO" +.BR clone (2), +.BR execve (2), +.BR fork (2), +.BR wait (2) |