mirror of https://github.com/mkerrisk/man-pages
114 lines
4.2 KiB
Plaintext
114 lines
4.2 KiB
Plaintext
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.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved
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.TH "VFORK" P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
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.\" vfork
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.SH NAME
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vfork \- create a new process; share virtual memory
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.SH SYNOPSIS
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.LP
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\fB#include <unistd.h>
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.br
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.sp
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pid_t vfork(void); \fP
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\fB
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.br
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\fP
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.SH DESCRIPTION
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.LP
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The \fIvfork\fP() function shall be equivalent to \fIfork\fP(), except
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that the behavior
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is undefined if the process created by \fIvfork\fP() either modifies
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any data other than a variable of type \fBpid_t\fP used to
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store the return value from \fIvfork\fP(), or returns from the function
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in which \fIvfork\fP() was called, or calls any other
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function before successfully calling \fI_exit\fP() or one of the \fIexec\fP
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family of functions.
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.SH RETURN VALUE
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.LP
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Upon successful completion, \fIvfork\fP() shall return 0 to the child
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process and return the process ID of the child process to
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the parent process. Otherwise, -1 shall be returned to the parent,
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no child process shall be created, and \fIerrno\fP shall be set
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to indicate the error.
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.SH ERRORS
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.LP
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The \fIvfork\fP() function shall fail if:
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.TP 7
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.B EAGAIN
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The system-wide limit on the total number of processes under execution
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would be exceeded, or the system-imposed limit on the
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total number of processes under execution by a single user would be
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exceeded.
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.TP 7
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.B ENOMEM
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There is insufficient swap space for the new process.
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.sp
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.LP
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\fIThe following sections are informative.\fP
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.SH EXAMPLES
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.LP
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None.
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.SH APPLICATION USAGE
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.LP
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Conforming applications are recommended not to depend on \fIvfork\fP(),
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but to use \fIfork\fP() instead. The \fIvfork\fP() function may be
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withdrawn in a future version.
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.LP
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On some implementations, \fIvfork\fP() is equivalent to \fIfork\fP().
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.LP
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The \fIvfork\fP() function differs from \fIfork\fP() only in that
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the child process can
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share code and data with the calling process (parent process). This
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speeds cloning activity significantly at a risk to the
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integrity of the parent process if \fIvfork\fP() is misused.
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.LP
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The use of \fIvfork\fP() for any purpose except as a prelude to an
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immediate call to a function from the \fIexec\fP family, or to \fI_exit\fP(),
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is not advised.
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.LP
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The \fIvfork\fP() function can be used to create new processes without
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fully copying the address space of the old process. If a
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forked process is simply going to call \fIexec\fP, the data space
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copied from the parent to
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the child by \fIfork\fP() is not used. This is particularly inefficient
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in a paged
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environment, making \fIvfork\fP() particularly useful. Depending upon
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the size of the parent's data space, \fIvfork\fP() can give
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a significant performance improvement over \fIfork\fP().
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.LP
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The \fIvfork\fP() function can normally be used just like \fIfork\fP().
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It does not work,
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however, to return while running in the child's context from the caller
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of \fIvfork\fP() since the eventual return from
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\fIvfork\fP() would then return to a no longer existent stack frame.
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Care should be taken, also, to call \fI_exit\fP() rather than \fIexit\fP()
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if \fIexec\fP cannot be used, since \fIexit\fP() flushes and closes
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standard I/O channels, thereby damaging the parent process' standard
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I/O data structures. (Even with \fIfork\fP(), it is wrong to call
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\fIexit\fP(), since buffered
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data would then be flushed twice.)
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.LP
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If signal handlers are invoked in the child process after \fIvfork\fP(),
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they must follow the same rules as other code in the
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child process.
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.SH RATIONALE
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.LP
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None.
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.SH FUTURE DIRECTIONS
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.LP
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This function may be withdrawn in a future version.
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.SH SEE ALSO
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.LP
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\fIexec\fP() , \fIexit\fP() , \fIfork\fP() , \fIwait\fP() , the Base
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Definitions volume of
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IEEE\ Std\ 1003.1-2001, \fI<unistd.h>\fP
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.SH COPYRIGHT
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Portions of this text are reprinted and reproduced in electronic form
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from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
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-- Portable Operating System Interface (POSIX), The Open Group Base
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Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
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Electrical and Electronics Engineers, Inc and The Open Group. In the
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event of any discrepancy between this version and the original IEEE and
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The Open Group Standard, the original IEEE and The Open Group Standard
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is the referee document. The original Standard can be obtained online at
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http://www.opengroup.org/unix/online.html .
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