wait 4(2) - Waikato Linux Users Group

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).; 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