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!!NAME 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 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()'' . 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 ''ioctl'' s 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. 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. !!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)
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vfork(2)
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csh(1)
strace(1)
clone(2)
fork(2)
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