wait4() suspends execution of the current process until a child (as specified by pid) 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:
wait for any child process whose process group ID is equal to the absolute value of pid.
wait for any child process; this is equivalent to calling wait3().
wait for any child process whose process group ID is equal to that of the calling process.
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:
return immediately if no child is there to be waited for.
return for children that are stopped, and whose status has not been reported.
If status is not NULL, wait4() stores status information in the location status. This status can be evaluated with the following macros:
Note: These macros take the
statusvalue (an int) as an argument -- not a pointer to the value!
is nonzero if the child exited normally.
evaluates to the least significant eight bits of the return code of the child that 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 nonzero.
returns true if the child process exited because of a signal that was not caught.
returns the number of the signal that caused the child process to terminate. This macro can only be evaluated if WIFSIGNALED() returned nonzero.
returns true if the child process that caused the return is currently stopped; this is only possible if the call was done using WUNTRACED().
returns the number of the signal that caused the child to stop. This macro can only be evaluated if WIFSTOPPED() returned nonzero.
If rusage is not NULL, the struct rusage (as defined in sys/resource.h) that it points to will be filled with accounting information. (See getrusage()(2) for details.
On success, the process ID of the child that exited is returned. On
error, -1 is returned (in particular, when
no unwaited-for child processes of the specified kind exist), or
0 if WNOHANG() was used
and no child was available yet. In the latter two cases, the global
errno is set appropriately.
No unwaited-for child process as specified does exist.
A WNOHANG() was not set and an unblocked signal or
SIGCHILD was caught. This error is returned by
the system call. The library interface is not allowed to return
ERESTARTSYS, but will return