Content-type: text/html Man page of wait

wait

Section: System Calls (2)
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NAME

wait, waitpid, wait3, wait4 - Waits for a child process to stop or terminate  

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 ISO or X/Open standards, but may be required on other vendors' systems that conform to these standards.

See the NOTES section for further information on the backward-compatible versions of wait3() and wait4().  

STANDARDS

Interfaces documented on this reference page conform to industry standards as follows:

wait():  XPG4, XPG4-UNIX

waitpid():  POSIX.1, XPG4-UNIX

wait3():  XPG4-UNIX

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 defined in the DESCRIPTION section. 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 processes that have terminated or stopped. If status information is available 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 available, or until delivery of a signal whose action is either to execute a signal-catching function or to terminate the process. If status information 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 the sigaction(2) reference page 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 struct rusage structure pointed to by the third argument is filled in for the child process identified by the return value.

[Digital]  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 the NOTES section 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() functions return the status only of child processes from this set. The following 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 parameter can be combined by specifying them in a bitwise-inclusive OR operation. The flags are as follows: [XPG4-UNIX]  Specifies reporting the status of any child process identified by the process_id parameter whose status has not been reported since it continued from a job control stop. [Digital]  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 WNOHANGn flag is not specified. If there are no such processes (that is, if conditions are such that waitpid() without 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.

[XPG4-UNIX]  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 parameter 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 integral expressions. In the following macro descriptions, the status_value parameter is equal to the integer value pointed to by the status_location parameter: Evaluates to a nonzero value if status was returned for a child process that terminated normally. If the value of WIFEXITED(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. [XPG4-UNIX]  Evaluates to a non-zero value if status was returned for a child process that has continued from a job control stop.

[Digital]  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 WCONTINUED flags, one of the following macros evaluates to a nonzero value: WIFEXITED(*status_location) WIFSIGNALED(*status_location) WIFSTOPPED(*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 following 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 and specified the WCONTINUED flag, or by a call to wait(), one of the following 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

[Digital]  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.  

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 they return 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 option 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. Otherwise, (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 specified values for the following conditions: The calling process has no existing unwaited-for child processes. The function was terminated by receipt of a signal. [Digital]  The status_location or resource_usage parameter points to a location outside of the address space of the process.

The waitpid() function fails if one or both of the following are 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(), or wait4() functions fail if the following is true: The value of the options parameter is not valid.  

RELATED INFORMATION

Functions: exec(2), exit(2), fork(2), ptrace(2), getrusage(2), sigaction(2)

Routines: pause(3)

Standards: standards(5) delim off


 

Index

NAME
SYNOPSIS
STANDARDS
PARAMETERS
DESCRIPTION
NOTES
RETURN VALUES
ERRORS
RELATED INFORMATION

This document was created by man2html, using the manual pages.
Time: 02:40:16 GMT, October 02, 2010