wait
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 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 con-
stants:
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 sig-
nal 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 sup-
ported 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 con-
struct 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 chil-
dren 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 speci-
fied.
__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)
