NAME

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

SYNOPSIS

#include <sys/types.h> #include <sys/wait.h> pid_t wait (
int ∗status_location ); pid_t waitpid (
pid_t process_id,
int ∗status_location,
int options ); #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 );

PARAMETERS

status_location
Points to a location that contains the termination status of the child process as defined in the <sys/wait.h> header file. 

process_idSpecifies the child process or set of child processes. 

optionsModifies the behavior of the function. The flags for the options parameter are defined in the DESCRIPTION section. 

resource_usage
Specifies the location of a structure that will contain the resource utilization information for child processes that have terminated.

DESCRIPTION

The wait function suspends the calling process until it receives a signal that is to be caught, or until any one of the calling process’s child processes stops or terminates.  Any condition that leads to an ECHILD error causes all the wait variants to return immediately with a status of −1.  The wait function also returns immediately if a child process (not waited for) has stopped, terminated, or continued prior to the call. 

The effect of the wait function can be modified by setting the SIGCHLD signal.  (See the sigaction(2) for more information). 

The waitpid function behaves the same as the wait function only if the process_id parameter has a value of −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 wait, waitpid, wait3, and wait4 functions, which suspend the calling process until the request is completed, are redefined so that only the calling thread is suspended. 

The process_id parameter allows the calling process to gather status from a specific set of child processes, according to the following rules:

       •If the process_id parameter is equal to−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−1, status is requested for any child process whose process group ID is equal to the absolute value of the process_id parameter. 

The waitpid and wait4 functions will only return the status of a child process from this set. 

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 their bitwise-inclusive OR.  The flags are as follows:

WCONTINUED
Specifies that the status of any continued child process that was specified by the process_id parameter whose status has not been reported since it continued will be reported to the calling process. 

WNOWAIT
Specifies that the process whose status is returned in status_location will be kept in a waitable state will be kept.  The process can be waited for again with the same results. 

WNOHANG
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 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. 

WUNTRACED
Specifies that the call return additional information when the child processes of the current process are stopped because they received a SIGTTIN, SIGTTOU, SIGSTOP, or SIGTSTOP signal. 

If the wait, waitpid, wait3, or wait4 function returns 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:

WIFEXITED(status_value)
Evaluates to a nonzero value if status was returned for a child process that terminated normally.

WEXITSTATUS(status_value)
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. 

WIFSIGNALED(status_value)
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.

WTERMSIG(status_value)
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. 

WIFSTOPPED(status_value)
Evaluates to a nonzero value if status was returned for a child process that is currently stopped.

WSTOPSIG(status_value)
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. 

WIFCONTINUED(status_value)
Evaluates to a non-zero value if status was returned for a child process that has continued.

If the information stored at the location pointed to by the status_location parameter was stored there by a call to the waitpid, wait3, or the wait4 function that specified the WUNTRACED flag, one of the following macros evaluates to a nonzero value:

       •WIFEXITED(∗status_value)

       •WIFSIGNALED(∗status_value)

       •WIFSTOPPED(∗status_value)

       •WIFCONTINUED(∗status_value)

If the information stored in the buffer pointed to by the status_location parameter resulted from a call to the wait function or a call to the waitpid, wait3, or wait4 function without the WUNTRACED flag specified, one of the following macros evaluates to a nonzero value:

       •WIFEXITED(∗status_value)

       •WIFSIGNALED(∗status_value)

The wait3 function provides compatibility with BSD systems.  A program that calls wait3 must be compiled with the _BSD switch defined.  In this case, the parameter to the macros described previously should be the w_status member of the union pointed to by the status_location parameter.  The wait3 and wait4 functions also include a resource_usage parameter.  This parameter points to a location that contains resource usage information for the child process as defined in the <sys/resource.h> header file. 

The wait4 function is similar to wait3.  However, the wait4 function 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 previous paragraph for further information. 

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

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 wait3 and wait4 functions must be compiled and linked in this manner. 

AES Support Level:Full use (wait, waitpid)

ERRORS

If the wait, waitpid, wait3, or wait4 function fails, errno can be set to one of the following values:

[ECHILD]The calling process has no existing unwaited-for child processes. 

[EINTR]The function was terminated by receipt of a signal. 

[EFAULT]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:

[ECHILD]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:

[EINVAL]The value of the options parameter is not valid. 

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 WNOHANG option was specified, and there are no stopped or exited child processes, the waitpid, wait3, and wait4 functions return a value of zero (0). Otherwise,−1 is returned and errno is set to indicate the error. 

RELATED INFORMATION

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

Routines: pause(3).