waitpid(2) -- Linux man page
NAME
wait, waitpid - wait for process termination
SYNOPSIS
#include <sys/types.h>
#include <sys/wait.h>
pid_t wait(int *status);
pid_t waitpid(pid_t pid, int *status, int options);
DESCRIPTION
The
wait
function suspends execution of the current process until a child has
exited, or until a signal is delivered whose action is to terminate
the current process or to call a signal handling function. If a child
has already exited by the time of the call (a so-called "zombie"
process), the function returns immediately. Any system resources used
by the child are freed.
The
waitpid
function suspends execution of the current process until a
child as specified by the
pid
argument has exited, or until a signal is delivered whose action is to
terminate the current process or to call a signal handling function.
If a child as requested by
pid
has already exited by the time of the call (a so-called "zombie"
process), the function returns immediately. Any system resources used
by the child are freed.
The value of
pid
can be one of:
- < -1
-
which means to wait for any child process whose process group ID is
equal to the absolute value of
pid.
- -1
-
which means to wait for any child process; this is the same
behaviour which
wait
exhibits.
- 0
-
which means to wait for any child process whose process group ID is
equal to that of the calling process.
- > 0
-
which means to wait for the child whose process ID is equal to the
value of
pid.
The value of
options
is an OR of zero or more of the following constants:
- WNOHANG
-
which means to return immediately if no child has exited.
- WUNTRACED
-
which means to also return for children which are stopped
(but not traced), and whose status has not been reported.
Status for traced children which are stopped is provided
also without this option.
(For Linux-only options, see below.)
If
status
is not
NULL,
wait
or
waitpid
store status information in the location pointed to by
status.
This status can be evaluated with the following macros (these macros take
the stat buffer (an int) as an argument --- not a pointer to the
buffer!):
- WIFEXITED(status)
-
returns true if the child terminated normally, that is,
by calling exit() or _exit(), or by returning from main().
- WEXITSTATUS(status)
-
evaluates to the least significant eight bits of the return code of
the child which terminated, which may have been set as the argument to
a call to exit() or _exit() or as the argument for a return statement
in the main program. This macro can only be evaluated if
WIFEXITED
returned true.
- WIFSIGNALED(status)
-
returns true if the child process terminated because of a signal
which was not caught.
- WTERMSIG(status)
-
returns the number of the signal that caused the child process to
terminate. This macro can only be evaluated if
WIFSIGNALED
returned non-zero.
- WIFSTOPPED(status)
-
returns true if the child process which caused the return is currently
stopped; this is only possible if the call was done using
WUNTRACED
or when the child is being traced (see
ptrace(2)).
- WSTOPSIG(status)
-
returns the number of the signal which caused the child to stop. This
macro can only be evaluated if
WIFSTOPPED
returned non-zero.
Some versions of Unix (e.g. Linux, Solaris, but not AIX, SunOS)
also define a macro
WCOREDUMP(status)
to test whether the child process dumped core. Only use this
enclosed in #ifdef WCOREDUMP ... #endif.
RETURN VALUE
The process ID of the child which exited, or zero if
WNOHANG
was used and no child was available, or -1 on error (in which case
errno
is set to an appropriate value).
ERRORS
- ECHILD
-
if the process specified in
pid
does not exist or is not a child of the calling process.
(This can happen for one's own child if the action for SIGCHLD
is set to SIG_IGN. See also the LINUX NOTES section about threads.)
- EINVAL
-
if the
options
argument was invalid.
- EINTR
-
if
WNOHANG
was not set and an unblocked signal or a
SIGCHLD
was caught.
NOTES
The Single Unix Specification describes a flag SA_NOCLDWAIT (not supported
under Linux) such that if either this flag is set, or the action for
SIGCHLD is set to SIG_IGN
then children that exit do not become zombies and a call to
wait()
or
waitpid()
will block until all children have exited, and then fail with
errno
set to ECHILD.
The original POSIX standard left the behaviour of setting SIGCHLD to
SIG_IGN unspecified.
Later standards, including SUSv2 and POSIX 1003.1-2001 specify the
behaviour just described as an XSI-compliance option.
Linux does not conform to the second of the two points just described:
if a
wait() or waitpid()
call is made while SIGCHLD is being ignored,
the call behaves just as though SIGCHLD were not being ignored, that is,
the call blocks until the next child terminates and then returns the
PID and status of that child.
LINUX NOTES
In the Linux kernel, a kernel-scheduled thread is not a distinct
construct from a process. Instead, a thread is simply a process
that is created using the Linux-unique
clone(2)
system call; other routines such as the portable
pthread_create(3)
call are implemented using
clone(2).
Before Linux 2.4, a thread was just a special case of a process,
and as a consequence one thread could not wait on the children
of another thread, even when the latter belongs to the same thread group.
However, POSIX prescribes such functionality, and since Linux 2.4
a thread can, and by default will, wait on children of other threads
in the same thread group.
The following Linux-specific
options
are for use with children created using
clone(2).
- __WCLONE
-
Wait for "clone" children only. If omitted then wait
for "non-clone" children only. (A "clone" child is one
which delivers no signal, or a signal other than
SIGCHLD
to its parent upon termination.)
This option is ignored if
__WALL
is also specified.
- __WALL
-
(Since Linux 2.4) Wait for all children, regardless of
type ("clone" or "non-clone").
- __WNOTHREAD
-
(Since Linux 2.4) Do not wait for children of other threads in
the same thread group. This was the default before Linux 2.4.
CONFORMING TO
SVr4, POSIX.1
SEE ALSO
clone(2),
ptrace(2),
signal(2),
wait4(2),
pthread_create(3),
signal(7)
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