WAITPID(3C) Standard C Library Functions WAITPID(3C)

NAME


waitpid - wait for child process to change state

SYNOPSIS


#include <sys/types.h>
#include <sys/wait.h>

pid_t waitpid(pid_t pid, int *stat_loc, int options);


DESCRIPTION


The waitpid() function will suspend execution of the calling thread
until status information for one of its terminated child processes is
available, or until delivery of a signal whose action is either to
execute a signal-catching function or to terminate the process. If
more than one thread is suspended in waitpid(), wait(3C), or
waitid(2) awaiting termination of the same process, exactly one
thread will return the process status at the time of the target
process termination. If status information is available prior to the
call to waitpid(), return will be immediate.


The pid argument specifies a set of child processes for which status
is requested, as follows:

o If pid is less than (pid_t)-1, status is requested for any
child process whose process group ID is equal to the
absolute value of pid.

o If pid is equal to (pid_t)-1, status is requested for any
child process.

o If pid is equal to (pid_t)0 status is requested for any
child process whose process group ID is equal to that of
the calling process.

o If pid is greater than (pid_t)0, it specifies the process
ID of the child process for which status is requested.


One instance of a SIGCHLD signal is queued for each child process
whose status has changed. If waitpid() returns because the status of
a child process is available, and WNOWAIT was not specified in
options, any pending SIGCHLD signal associated with the process ID of
that child process is discarded. Any other pending SIGCHLD signals
remain pending.


If the calling process has SA_NOCLDWAIT set or has SIGCHLD set to
SIG_IGN and the process has no unwaited children that were
transformed into zombie processes, it will block until all of its
children terminate, and waitpid() will fail and set errno to ECHILD.


If waitpid() returns because the status of a child process is
available, then that status may be evaluated with the macros defined
by wait.h(3HEAD) If the calling process had specified a non-zero
value of stat_loc, the status of the child process will be stored in
the location pointed to by stat_loc.


The options argument is constructed from the bitwise-inclusive OR of
zero or more of the following flags, defined in the header
<sys/wait.h>:

WCONTINUED
The status of any continued child process specified by
pid, whose status has not been reported since it
continued, is also reported to the calling process.


WNOHANG
The waitpid() function will not suspend execution of
the calling process if status is not immediately
available for one of the child processes specified by
pid.


WNOWAIT
Keep the process whose status is returned in stat_loc
in a waitable state. The process may be waited for
again with identical results.


WUNTRACED
The status of any child processes specified by pid that
are stopped, and whose status has not yet been reported
since they stopped, is also reported to the calling
process. WSTOPPED is a synonym for WUNTRACED.


RETURN VALUES


If waitpid() returns because the status of a child process is
available, it returns a value equal to the process ID of the child
process for which status is reported. If waitpid() returns due to
the delivery of a signal to the calling process, -1 is returned and
errno is set to EINTR. If waitpid() was invoked with WNOHANG set in
options, it has at least one child process specified by pid for
which status is not available, and status is not available for any
process specified by pid, then 0 is returned. Otherwise, -1 is
returned and errno is set to indicate the error.

ERRORS


The waitpid() function will fail if:

ECHILD
The process or process group specified by pid does not
exist or is not a child of the calling process or can never
be in the states specified by options.


EINTR
The waitpid() function was interrupted due to the receipt
of a signal sent by the calling process.


EINVAL
An invalid value was specified for options.


USAGE


With options equal to 0 and pid equal to (pid_t)-1, waitpid() is
identical to wait(3C). The waitpid() function is implemented as a
call to the more general waitid(2) function.

ATTRIBUTES


See attributes(7) for descriptions of the following attributes:


+--------------------+-------------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-------------------+
|Interface Stability | Committed |
+--------------------+-------------------+
|MT-Level | Async-Signal-Safe |
+--------------------+-------------------+
|Standard | See standards(7). |
+--------------------+-------------------+

SEE ALSO


Intro(2), exec(2), exit(2), fork(2), pause(2), sigaction(2),
ptrace(3C), signal(3C), wait(3C), siginfo.h(3HEAD), wait.h(3HEAD),
attributes(7), standards(7)

December 7, 2007 WAITPID(3C)

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