wait, waitpid - wait for process termination
#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 [Toc] [Back]
which means to return immediately if no child has exited.
WUNTRACED [Toc] [Back]
which means to also return for children which are stopped, and
whose status has not been reported.
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)
is non-zero if the child exited normally.
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 as the argument for a return
statement in the main program. This macro can only be evaluated
if WIFEXITED returned non-zero.
WIFSIGNALED(status)
returns true if the child process exited 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.
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 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
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.
The Single Unix Specification describes a flag SA_NOCLDWAIT (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 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.
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.
SVr4, POSIX.1
clone(2), signal(2), wait4(2), pthread_create(3), signal(7)
Linux 2000-07-24 WAIT(2)
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