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 pro
cess until a child has exited, or until a signal is deliv
ered 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 "zom
bie" process), the function returns immediately. Any sys
tem 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" pro
cess), 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
-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
> 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 fol
which means to return immediately if no child has
which means to also return for children which are
mation 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!):
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 evalu
ated 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 returned non-zero.
returns true if the child process which caused the
return is currently stopped; this is only possible
if the call was done using WUNTRACED.
returns the number of the signal which caused the
child to stop. This macro can only be evaluated if
WIFSTOPPED returned non-zero.
The process ID of the child which exited, -1 on error or
zero if WNOHANG was used and no child was available (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 NOTES sec
tion about threads.)
EINVAL if the options argument was invalid.
if WNOHANG was not set and an unblocked signal or a
to return ERESTARTSYS, but will return EINTR.
The Single Unix Specification describes a flag SA_NOCLD
WAIT (not present under Linux) such that if either this
flag is set, or the action for SIGCHLD is set to SIG_IGN
(which, by the way, is not allowed by POSIX), then chil
dren 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.
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).
Thus, if two threads A and B are siblings, then thread A
cannot wait on any processes forked by thread B or its
descendents, because an uncle cannot wait on his nephews.
In some other Unix-like systems, where multiple threads
are implemented as belonging to a single process, thread A
can wait on any processes forked by sibling thread B; you
will have to rewrite any code that makes this assumption
for it to work on Linux.
clone(2), signal(2), wait4(2), pthread_create(3), sig