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WAIT(2) Linux Programmer’s Manual WAIT(2)
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 immedi-
ately. 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 pro-
gram. 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 wait-
pid() 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" chil-
dren 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)
Linux 2000-07-24 WAIT(2)
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