View Source: wait4(2) - Waikato Linux Users Group

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!!NAME
wait4 - wait for process termination, BSD style
!!SYNOPSIS
 #include <sys/types.h>
 #include <sys/time.h>
 #include <sys/resource.h>
 #include <sys/wait.h>
 __pid_t wait4(pid_t__ ''pid''__, int *__''status''__, int__ ''options''__, struct rusage *__''rusage''__)__
!!DESCRIPTION
The __wait4__ 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 equivalent to calling wait3(2).
;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 a bitwise OR of zero or more of the following constants:
;__WNOHANG__: which means to return immediately if no child is there to be waited for.
;__WUNTRACED__: which means to also return for children which are stopped, and whose status has not been reported.

If ''status'' is not __NULL__, __wait3__ or __wait4__ 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(3) 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.

If ''rusage'' is not __NULL__, the __struct rusage__ as defined in ''<sys/resource.h>'' it points to will be filled with accounting information. See
getrusage(2) for details.

!!RETURN VALUE
The process ID of the child which exited, -1 on error (in particular, when no unwaited-for child processes of the specified kind exist) or zero if __WNOHANG__ was used and no child was available yet. In the latter two cases ''errno'' will be set appropriately.

!!ERRORS
;[ECHILD]: No unwaited-for child process as specified does exist.
;[ERESTARTSYS]: if __WNOHANG__ was not set and an unblocked signal or a [SIGCHLD] was caught. This error is returned by the system call. The library interface is not allowed to return [ERESTARTSYS], but will return [EINTR].

!!NOTE
Including ''<sys/time.h>'' is not required these days, but increases portability. (Indeed, ''<sys/resource.h>'' defines the ''rusage'' structure with fields of type ''struct timeval'' defined in ''<sys/time.h>''.)
!!CONFORMING TO
SVr4, POSIX.1

!!SEE ALSO
signal(2), getrusage(2), wait(2), signal(7)
12 pages link to wait4(2):
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Last edited on Wednesday, February 12, 2003 12:46:24 am by PerryLorier
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