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Annotated edit history of vfork(2) version 2, including all changes. View license author blame.
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1 perry 1 !!NAME
2 JohnMcPherson 2 vfork - create a child process and block parent
1 perry 3
4 !!SYNOPSIS
2 JohnMcPherson 5 __#include <sys/types.h>__
6 __#include <unistd.h>__
1 perry 7
2 JohnMcPherson 8 __pid_t vfork(void);__
1 perry 9
10 !!STANDARD DESCRIPTION
2 JohnMcPherson 11 (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.
1 perry 12
13 !!ERRORS
14
2 JohnMcPherson 15 ;__[EAGAIN]__: Too many processes - try again.
16 ;__[ENOMEM]__: There is insufficient swap space for the new process.
1 perry 17
18 !!LINUX DESCRIPTION
2 JohnMcPherson 19 __vfork__ , just like fork(2), creates a child process of the calling process. For details and return value and errors, see fork(2).
1 perry 20
2 JohnMcPherson 21 __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()'' .
1 perry 22
2 JohnMcPherson 23 __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 memory 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()'' .
1 perry 24
2 JohnMcPherson 25 Signal handlers are inherited, but not shared. Signals to the parent arrive after the child releases the parent.
1 perry 26
27 !!HISTORIC DESCRIPTION
2 JohnMcPherson 28 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.
1 perry 29
30 !!BUGS
2 JohnMcPherson 31 It is rather unfortunate that Linux revived this spectre from the past. The BSD manpage states:
32 ;:"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''"
1 perry 33
2 JohnMcPherson 34 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.
1 perry 35
2 JohnMcPherson 36 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 ''ioctl'' s are allowed and input attempts result in an end-of-file indication."
1 perry 37
2 JohnMcPherson 38 Currently (Linux 2.3.25), strace(1) cannot follow ''vfork()'' and requires a kernel patch.
1 perry 39
40 !!HISTORY
2 JohnMcPherson 41 The ''vfork'' () system call appeared in 3.0BSD. In [BSD] 4.4 it was made synonymous to ''fork'' (), but [NetBSD] introduced it again, cf. http://www.netbsd.org/Documentation/kernel/vfork.html . 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.
1 perry 42
43 !!CONFORMING TO
2 JohnMcPherson 44 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.
1 perry 45
46 !!SEE ALSO
2 JohnMcPherson 47 clone(2), execve(2), fork(2), wait(2)
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