wait4
WAIT4(2) Linux Programmer’s Manual WAIT4(2)
NAME
wait3, 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 wait3(int *status, int options,
struct rusage *rusage);
pid_t wait4(pid_t pid, int *status, int options,
struct rusage *rusage);
DESCRIPTION
The wait3 function suspends execution of the current process until a
child 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 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 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.
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() 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 informa-
tion. 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 lat-
ter two cases errno will be set appropriately.
ERRORS
ECHILD No unwaited-for child process as specified does exist.
EINTR if WNOHANG was not set and an unblocked signal or a SIGCHLD was
caught.
EINVAL Invalid value for options given for wait4.
NOTES
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>.)
The prototype for these functions is only available if _BSD_SOURCE is
defined (either explicitly, or implicitly, by not defining
_POSIX_SOURCE or compiling with the -ansi flag).
CONFORMING TO
SVr4, POSIX.1
SEE ALSO
signal(2), getrusage(2), wait(2), signal(7)
Linux 1997-06-23 WAIT4(2)
