ref: c6d4cd09a395a24ee993c1edf1031e5b01adddde
dir: /sys/man/2/wait/
.TH WAIT 2
.SH NAME
await, wait, waitpid \- wait for a process to exit
.SH SYNOPSIS
.B #include <u.h>
.br
.B #include <libc.h>
.PP
.B
Waitmsg* wait(void)
.PP
.B
int waitpid(void)
.PP
.B
int await(char *s, int n)
.SH DESCRIPTION
.I Wait
causes a process to wait for any child process (see
.IR fork (2))
to exit.
It returns a
.B Waitmsg
holding
information about the exited child.
A
.B Waitmsg
has this structure:
.IP
.EX
.ta 6n +\w'long 'u +\w'msg[ERRLEN]; 'u
typedef
struct Waitmsg
{
int pid; /* of loved one */
ulong time[3]; /* of loved one & descendants */
char *msg;
} Waitmsg;
.EE
.PP
.B Pid
is the child's
process id.
The
.B time
array contains the time the child and its descendants spent in user code,
the time spent in system calls, and the child's elapsed real time,
all in units of milliseconds.
.B Msg
contains the message that the child specified in
.IR exits (2).
For a normal exit,
.B msg[0]
is zero,
otherwise
.B msg
is the exit string
prefixed by the process name, a blank, the process id, and a colon.
.PP
If there are no more children to wait for,
.I wait
returns immediately, with return value nil.
.PP
The
.B Waitmsg
structure is allocated by
.IR malloc (2)
and should be freed after use.
For programs that only need the pid of the exiting program,
.I waitpid
returns just the pid and discards the rest of the information.
.PP
The underlying system call is
.IR await ,
which fills in the n-byte buffer
.I s
with a textual representation of the pid, times, and exit string.
There is no terminal NUL.
The return value is the length, in bytes, of the data.
.PP
The buffer filled in by
.I await
may be parsed (after appending a NUL) using
.IR tokenize
(see
.IR getfields (2));
the resulting fields are, in order, pid, the three times, and the exit string,
which will be
.B ''
for normal exit.
If the representation is longer than
.I n
bytes, it is truncated but, if possible, properly formatted.
The information that does not fit in the buffer is discarded, so
a subsequent call to
.I await
will return the information about the next exiting child, not the remainder
of the truncated message.
In other words, each call to
.I await
returns the information about one child, blocking if necessary if no child has exited.
.PP
If the calling process has no living children,
.I await
and
.I waitpid
return
.BR -1 .
.SH SOURCE
.B /sys/src/libc/9syscall
.br
.B /sys/src/libc/9sys
.SH "SEE ALSO"
.IR fork (2),
.IR exits (2),
the
.B wait
file in
.IR proc (3)
.SH DIAGNOSTICS
These routines set
.IR errstr .