Mon, 06 Apr 2009

getrusage() and another example of Linux failure.

So I'm writing some code for work and I need to know the current stack usage. One would think getrusage() would be nice to use. Think again. To quote the manual page on Ubuntu: 

           struct rusage {
               struct timeval ru_utime; /* user time used */
               struct timeval ru_stime; /* system time used */
               long   ru_maxrss;        /* maximum resident set size */
               long   ru_ixrss;         /* integral shared memory size */
               long   ru_idrss;         /* integral unshared data size */
               long   ru_isrss;         /* integral unshared stack size */
               long   ru_minflt;        /* page reclaims */
               long   ru_majflt;        /* page faults */
               long   ru_nswap;         /* swaps */
               long   ru_inblock;       /* block input operations */
               long   ru_oublock;       /* block output operations */
               long   ru_msgsnd;        /* messages sent */
               long   ru_msgrcv;        /* messages received */
               long   ru_nsignals;      /* signals received */
               long   ru_nvcsw;         /* voluntary context switches */
               long   ru_nivcsw;        /* involuntary context switches */
           };

[...]

       The  above struct was taken from 4.3BSD Reno.  Not all fields are mean-
       ingful under Linux.  In linux 2.4 only the fields  ru_utime,  ru_stime,
       ru_minflt, and ru_majflt are maintained.  Since Linux 2.6, ru_nvcsw and
       ru_nivcsw are also maintained.

I can understand having the same struct for portability reasons. It's nice that code can be portable between the two at compile time but this is opening itself up to run-time problems if code assumes all the members of the struct will be populated.

I'm not sure why it's only barely implemented but it's quite annoying. The same function call on FreeBSD gives all the information, and the manual page under FreeBSD has useful information on what each of the members of the struct means. Linux fails, again.

gdb output on Linux: 

(gdb) p/x *usage
$1 = {ru_utime = {tv_sec = 0x0, tv_usec = 0xfa0}, ru_stime = {tv_sec = 0x0,
    tv_usec = 0xbb83}, ru_maxrss = 0x0, ru_ixrss = 0x0, ru_idrss = 0x0,
  ru_isrss = 0x0, ru_minflt = 0x2fe, ru_majflt = 0x0, ru_nswap = 0x0,
  ru_inblock = 0x0, ru_oublock = 0x0, ru_msgsnd = 0x0, ru_msgrcv = 0x0,
  ru_nsignals = 0x0, ru_nvcsw = 0x7b, ru_nivcsw = 0x1}
(gdb)

gdb output on FreeBSD: 

(gdb) p/x *usage
$3 = {ru_utime = {tv_sec = 0x0, tv_usec = 0x0}, ru_stime = {tv_sec = 0x0,
    tv_usec = 0x3ac7}, ru_maxrss = 0x8e8, ru_ixrss = 0x3f8, ru_idrss = 0xec,
  ru_isrss = 0x100, ru_minflt = 0xeb, ru_majflt = 0x0, ru_nswap = 0x0,
  ru_inblock = 0x0, ru_oublock = 0x0, ru_msgsnd = 0x0, ru_msgrcv = 0x0,
  ru_nsignals = 0x0, ru_nvcsw = 0xc, ru_nivcsw = 0x1}
(gdb)

posted at: 16:46 | tags: , , | path: /entries/geek | permanent link to this entry