wait.2

Unix操作系统minix 2.0源码

.\" Copyright (c) 1980 Regents of the University of California.
.\" All rights reserved.  The Berkeley software License Agreement
.\" specifies the terms and conditions for redistribution.
.\"
.\"	@(#)wait.2	6.2 (Berkeley) 6/30/85
.\"
.TH WAIT 2 "June 30, 1985"
.UC 4
.SH NAME
wait, waitpid \- wait for process to terminate
.SH SYNOPSIS
.ft B
.nf
#include <sys/types.h>
#include <sys/wait.h>

pid_t wait(int *\fIstatus\fP)
pid_t waitpid(pid_t \fIpid\fP, int *\fIstatus\fP, int \fIoptions\fP)
.fi
.SH DESCRIPTION
.B Wait
causes its caller to delay until a signal is received or
one of its child
processes terminates.
If any child has died since the last
.BR wait ,
return is immediate, returning the process id and
exit status of one of the terminated
children.
If there are no children, return is immediate with
the value \-1 returned.
.PP
On return from a successful 
.B wait
call, 
.I status
is nonzero, and the high byte of 
.I status
contains the low byte of the argument to
.B exit
supplied by the child process;
the low byte of 
.I status
contains the termination status of the process.
A more precise definition of the
.I status
word is given in
.RI < sys/wait.h >.
If
.B wait
can called with a null pointer argument to indicate that no status need
be returned.
.PP
.B Waitpid
provides an alternate interface for programs
that must not block when collecting the status
of child processes, or that wish to wait for
one particular child.  The pid parameter is
the process ID of the child to wait for, \-1
for any child.  The
.I status
parameter is defined as above.  The
.I options
parameter is used to indicate the call should not block if
there are no processes that wish to report status (WNOHANG),
and/or that children of the current process that are stopped
due to a SIGTTIN, SIGTTOU, SIGTSTP, or SIGSTOP signal should also have
their status reported (WUNTRACED).  (Job control is not implemented for
Minix, but these symbold and signals are.)
.PP
When the WNOHANG option is specified and no processes
wish to report status, 
.B waitpid
returns \-1 with
.B errno
set to
.BR EAGAIN .
The WNOHANG and WUNTRACED options may be combined by 
.IR or 'ing
the two values.
.SH NOTES
The call
.BI "wait(&" status ")"
is equivalent to
.BI "waitpid(\-1, &" status ", 0)\fR."
.PP
See
.BR sigaction (2)
for a list of termination statuses (signals);
0 status indicates normal termination.
A special status (0177) is returned for a stopped process
that has not terminated and can be restarted;
see
.BR ptrace (2).
If the 0200 bit of the termination status
is set,
a core image of the process was produced
by the system.
.PP
If the parent process terminates without
waiting on its children,
the initialization process
(process ID = 1)
inherits the children.
.PP
.I <sys/wait.h>
defines a number of macros that operate on a status word:
.TP 5
.BI "WIFEXITED(" status ")"
True if normal exit.
.TP 5
.BI "WEXITSTATUS(" status ")"
Exit status if the process returned by a normal exit, zero otherwise.
.TP 5
.BI "WTERMSIG(" status ")"
Signal number if the process died by a signal, zero otherwise.
.TP 5
.BI "WIFSIGNALED(" status ")"
True if the process died by a signal.
.TP 5
.BI "WIFSTOPPED(" status ")"
True if the process is stopped.  (Never true under Minix.)
.TP 5
.BI "WSTOPSIG(" status ")"
Signal number of the signal that stopped the process.
.SH "RETURN VALUE
If \fBwait\fP returns due to a stopped
or terminated child process, the process ID of the child
is returned to the calling process.  Otherwise, a value of \-1
is returned and \fBerrno\fP is set to indicate the error.
.PP
.B Waitpid
returns \-1 if there are no children not previously waited for, if
the process that it wants to wait for doesn't exist, or if
WNOHANG is specified and there are no stopped or exited children.
.SH ERRORS
.B Wait
will fail and return immediately if one or more of the following
are true:
.TP 15
[ECHILD]
The calling process has no existing unwaited-for
child processes.
.TP 15
[EFAULT]
The \fIstatus\fP argument points to an illegal address.
.TP 15
[EAGAIN]
.B Waitpid
is called with the WNOHANG option and no child has exited yet.
.SH "SEE ALSO"
.BR execve (2),
.BR exit (2),
.BR sigaction (2).