wait(2)wait(2)NAME
wait, waitpid, wait3, wait4 - Wait for a child process to stop or ter‐
minate
SYNOPSIS
#include <sys/wait.h>
pid_t wait(
int *status_location ); pid_t waitpid(
pid_t process_id,
int *status_location,
int options ); pid_t wait3(
int *status_location,
int options,
struct rusage *resource_usage );
The following function declarations do not conform to current standards
and are supported only for backward compatibility: #include
<sys/resource.h>
pid_t wait3(
union wait *status_location,
int options,
struct rusage *resource_usage ); pid_t wait4(
pid_t process_id,
union wait *status_location,
int options,
struct rusage *resource_usage );
Application developers may want to specify an #include statement for
<sys/types.h> before the one for <sys/wait.h> if programs are being
developed for multiple platforms. The additional #include statement is
not required on Tru64 UNIX systems or by industry standards, but may be
required on other vendors' systems that conform to these standards.
See NOTES for further information on the backward-compatible versions
of wait3() and wait4().
STANDARDS
Interfaces documented on this reference page conform to industry stan‐
dards as follows:
wait(): XSH4.0, XSH4.2, XSH5.0
waitpid(), wait3(): XSH4.2, XSH5.0
Refer to the standards(5) reference page for more information about
industry standards and associated tags.
PARAMETERS
Points to a location that contains the termination status of the child
process as defined in the <sys/wait.h> header file. Specifies the
child process or set of child processes. Modifies the behavior of the
function. The flags for the options parameter are described in DESCRIP‐
TION. Specifies the location of a structure that will contain the
resource usage information for child processes that have terminated.
DESCRIPTION
The wait(), waitpid(), and wait3() functions allow the calling process
to obtain status information pertaining to one of its child processes.
Various options permit status information to be obtained for child pro‐
cesses that have terminated or stopped. If status information is avail‐
able for two or more child processes, the order in which their status
is reported is unspecified.
The wait() function suspends execution of the calling process until
status information for one of its terminated child processes is avail‐
able, or until delivery of a signal whose action is either to execute a
signal-catching function or to terminate the process. If status infor‐
mation is available prior to the call to wait(), return is immediate.
The effect of the wait() function can be modified by setting the
SIGCHLD signal. See sigaction(2) for more information.
The waitpid() function behaves identically to wait() if the process_id
parameter has a value of (pid_t)-1 and the options parameter specifies
a value of zero (0). Should these parameters contain other values, the
waitpid() function is changed as specified by those values.
The wait3() function is similar to waitpid() in allowing the calling
process to obtain status information for specified child processes. The
wait3() function also includes a resource_usage parameter. If this
parameter is not a null pointer, the rusage structure pointed to by the
third argument is filled in for the child process identified by the
return value.
[Tru64 UNIX] The wait4() function is similar to wait3(), except that
wait4() waits for a specific child as indicated by the process_id
parameter. A program that calls wait4() must be compiled with the _BSD
switch defined. See NOTES for further information.
In a multithreaded environment, the wait(), waitpid(), wait3(), and
wait4() functions block only the calling thread.
The process_id parameter allows the calling process to gather status
from a specific set of child processes. The waitpid() and wait4() func‐
tions return the status only of child processes from this set. The fol‐
lowing rules define the set: If the process_id parameter is equal to
(pid_t)-1, status is requested for any child process. In this respect,
the waitpid() function is equivalent to the wait() function. If the
process_id parameter is greater than zero (0), it specifies the process
ID of a single child process for which status is requested. If the
process_id parameter is equal to zero (0), status is requested for any
child process whose process group ID is equal to that of the calling
process. If the process_id parameter is less than (pid_t)-1, status is
requested for any child process whose process group ID is equal to the
absolute value of the process_id parameter.
The options parameter to the waitpid(), wait3(), and wait4() functions
modifies the behavior of the function. The flags for the options param‐
eter can be combined by specifying them in a bitwise-inclusive OR oper‐
ation. The flags are as follows: [XSH4.2] Specifies reporting the sta‐
tus of any child process identified by the process_id parameter whose
status has not been reported since it continued from a job control
stop. [Tru64 UNIX] Specifies that the process whose status is
returned in status_location will be kept in a waitable state. The
process can be waited for again with the same results. Prevents the
calling process from being suspended. If there are child processes that
have been stopped or terminated, one is chosen and waitpid() returns
its pid, just as when the WNOHANG flag is not specified. If there are
no such processes (that is, if conditions are such that waitpid() with‐
out the WNOHANG flag would have suspended the calling process), zero
(0) is returned. Because you can never wait for process 0, there is no
confusion arising from this return. Specifies that the call return
additional information when the child process is stopped because it
received a SIGTTIN, SIGTTOU, SIGSTOP, or SIGTSTOP signal.
[XSH4.2] If the calling process has SA_NOCLDWAIT set or has SIGCHLD
set to SIG_IGN, and the process has no unwaited-for child processes
that were transformed into zombie processes, it will block until all of
its child processes terminate, and wait() and waitpid() will fail and
set errno to [ECHILD].
If the wait(), waitpid(), wait3(), or wait4() functions return because
the status of a child process is available, the process ID of the child
process is returned. In this instance, information is stored in the
location pointed to by the status_location parameter if the value of
the status_location is not null. The value stored in the location
pointed to by the status_location parameter is zero (0) if, and only
if, the status is returned from a terminated child process that did one
of the following: Returned zero (0) from the main() routine. Passed
zero (0) as the status argument to the _exit() or exit() function.
Regardless of its value, this information can be defined using the
macros defined in the <sys/wait.h> header file that evaluate to inte‐
gral expressions. In the following macro descriptions, the status_value
parameter is equal to the integer value pointed to by the status_loca‐
tion parameter: Evaluates to a nonzero value if status was returned for
a child process that terminated normally. If the value of WIFEX‐
ITED(status_value) is nonzero, this macro evaluates to the low-order 8
bits of the status parameter that the child process passed to the
_exit() or exit() function, or the value the child process returned
from the main() routine. Evaluates to nonzero value if status was
returned for a child process that terminated due to the receipt of a
signal that was not caught. If the value of WIFSIGNALED(status_value)
is nonzero, this macro evaluates to the number of the signal that
caused the termination of the child process. Evaluates to a nonzero
value if status was returned for a child process that is currently
stopped. If the value of WIFSTOPPED(status_value) is nonzero, this
macro evaluates to the number of the signal that caused the child
process to stop. [XSH4.2] Evaluates to a non-zero value if status was
returned for a child process that has continued from a job control
stop.
Note
[Tru64 UNIX] The following behavior described for waitpid() and
wait3() also applies to the libbsd versions of wait3() and wait4(),
except the parameter to the macros should be the w_status member of the
union pointed to by the status_location parameter.
If the information pointed to by the status_location parameter was
stored by a call to waitpid() or wait3() that specified the WUNTRACED
flag and did not specify the WCONTINUED flag, one of the following
macros evaluates to a nonzero value: WIFEXITED(*status_location)
WIFSIGNALED(*status_location) WIFSTOPPED(*status_location)
If the information pointed to by status_location was stored by a call
to waitpid() or wait3() that specified both the WUNTRACED and WCONTIN‐
UED flags, one of the following macros evaluates to a nonzero value:
WIFEXITED(*status_location) WIFSIGNALED(*status_location) WIF‐
STOPPED(*status_location) WIFCONTINUED(*status_location)
If the information pointed to by status_location was stored by a call
to waitpid() or wait3() that specified neither the WUNTRACED nor the
WCONTINUED flag, or by a call to the wait() function, one of the fol‐
lowing macros evaluates to a nonzero value: WIFEXITED(*status_location)
WIFSIGNALED(*status_location)
If the information pointed to by status_location was stored by a call
to waitpid() or wait3() that did not specify the WUNTRACED flag and
specified the WCONTINUED flag, or by a call to wait(), one of the fol‐
lowing macros evaluates to a nonzero value: WIFEXITED(*status_location)
WIFSIGNALED(*status_location) WIFCONTINUED(*status_location)
If a parent process terminates without waiting for all of its child
processes to terminate, the remaining child processes will be assigned
a parent process ID equal to the process ID of the init process.
NOTES
[Tru64 UNIX] Compiling with the _BSD switch defined and then linking
with the libbsd compatibility library redefines the status_location
parameter as the type union wait * instead of int *. Programs using
the backward-compatible versions of wait3() and wait4() functions must
be compiled in this manner.
Paragraphs flagged with [XSH4.2] apply to applications built in com‐
pilation environments conforming to XSH4.2 and higher versions of the
XSH specification.
RETURN VALUES
If the wait(), waitpid(), wait3(), or wait4() function returns because
the status of a child process is available, the process ID of the child
is returned to the calling process. If the function returns because a
signal was caught by the calling process, -1 is returned and errno is
set to [EINTR].
If the waitpid(), wait3(), or wait4() function was invoked with the
WNOHANG flag specified, 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, a value of 0 (zero) is returned. Other‐
wise, (pid_t)-1 is returned, and errno is set to indicate the error.
ERRORS
The wait(), waitpid(), wait3(), and wait4() functions set errno to the
following values: The calling process has no existing unwaited-for
child processes. The function was terminated by receipt of a signal.
[Tru64 UNIX] The status_location or resource_usage parameter points to
a location outside of the address space of the process.
The waitpid() function also fails if the following is true: The process
or process group ID specified by the process_id parameter does not
exist or is not a child process of the calling process.
The waitpid(), wait3(), and wait4() functions also fail if the follow‐
ing is true: The value of the options parameter is not valid.
SEE ALSO
Functions: exec(2), exit(2), fork(2), getrusage(2), ptrace(2), sigac‐
tion(2), pause(3)
Standards: standards(5)wait(2)
