The

1
The wait4() function

• Looking at how the various
• wait() system-calls rely on wait4()
• in the x86_64 Linux kernel

2
Waiting for a child-process

• Various standard UNIX library functions may be
  used by a parent-process to wait for one of its
  child-processes to terminate
• pid_t wait( int status )
• pid_t waitpid( pid_t pid, int status, int
  options )
• pid_t wait3( int status, int options, struct
  rusage rusage )
• pid_t wait4( pid_t pid, int status, int options,
  struct rusage rusage )
• But all of these can be built using wait4()

3
resource usage
4
getrusage()

• Linux provides a library-function allowing a
  program to obtain current resource usages for
  itself, or for its child-processes which have
  terminated and been waited for
• int getrusage( int who, struct rusage rusage
  )
• The who argument is one of the symbols
  RUSAGE_SELF or RUSAGE_CHILDREN

5
implementation

• According to the man page, getrusage() does
  not currently support all of the fields defined
  within the struct rusage record
• We have posted a short demo-program, showing
  fields which our current Linux kernel supports,
  named myrusage.cpp

6
From assembly language

• Weve also posted an assembly language example in
  which a parent-process uses the wait4()
  system-call in order to wait until its
  child-process has terminated it does not make
  use of the struct rusage argument however, but
  supplies a NULL pointer instead (see our run.s
  )