Originální popis anglicky:
wait, waitpid - wait for process termination
Návod, kniha: Linux Programmer's Manual
#include <sys/types.h>
#include <sys/wait.h>
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 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.
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 (for wait)
- The calling process does not have any unwaited-for
children.
- ECHILD (for waitpid)
- 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.)
- EINTR
- WNOHANG was not set and an unblocked signal or a
SIGCHLD was caught.
- EINVAL
- The options argument was invalid.
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 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 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.
SVr4, POSIX.1
clone(2),
ptrace(2),
signal(2),
wait4(2),
pthread_create(3),
signal(7)
