Transcript The ‘wait4()’ function Looking at how the various

The ‘wait4()’ function
Looking at how the various
‘wait()’ system-calls rely on ‘wait4()’
in the x86_64 Linux kernel
‘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:
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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()’
resource usage
‘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
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’
From assembly language
• We’ve 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’ )