WAIT(2) Linux Programmer's Manual WAIT(2)
wait, waitpid - wait for process termination
pid_t wait(int *status);
pid_t waitpid(pid_t pid, int *status, int options);
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
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-
which means to return immediately if no child has exited.
which means to also return for children which are stopped, and
whose status has not been reported.
(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
is non-zero if the child exited normally.
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 as the argument for a return
statement in the main program. This macro can only be evaluated
if WIFEXITED returned non-zero.
returns true if the child process exited because of a signal
which was not caught.
returns the number of the signal that caused the child process
to terminate. This macro can only be evaluated if WIFSIGNALED
returns true if the child process which caused the return is
currently stopped; this is only possible if the call was done
returns the number of the signal which caused the child to stop.
This macro can only be evaluated if WIFSTOPPED returned
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.
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).
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
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 igored,
that is, the call blocks until the next child terminates and then
returns the PID and status of that child.
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 pro-
cess, 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
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 termi-
nation.) 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").
(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.
clone(2), signal(2), wait4(2), pthread_create(3), signal(7)
Linux 2000-07-24 WAIT(2)