_SC_GETPW_R_SIZE_MAX undefined in sysconf.c, what is correct value?

[Christopher J. Ruwe]

On Sun, 23 Oct 2011 19:10:34 -0500
Dan Nelson <dnelson at allantgroup.com> wrote:

> In the last episode (Oct 23), Christopher J. Ruwe said:
> > I need to get the maximum size of an pwd-entry to determine the
> > correct buffersize for calling getpwnam_r("uname",&pwd, buf,
> > bufsize, &pwdp).  I would like to use sysconf(_SC_GETPW_R_SIZE_MAX)
> > to determine bufsize, which unfornutately fails (returns -1).
> > Currently, I used 16384, which seems to be too much, bit works for
> > the time being.
> > 
> > From recent mails I get that _SC_GETPW_R_SIZE_MAX is not available
> > on FreeBSD, e.g.,
> > http://lists.freebsd.org/pipermail/freebsd-ports-bugs/2009-September/173081.html
> > and
> > http://www.redhat.com/archives/libvir-list/2011-May/msg01013.html.
> > This assertion seems to be backed
> > by /usr/srclib/libc/gen/sysconf.c, line 374.
> > 
> > From Stevens & Rago, Adavanced Programming in the UNIX Environment,
> > I can get that FreeBSD supports all possible members in the passwd
> > structure, but where can I determine the maximum size of these so
> > that I can calculate the ax size of the pwd struct therefrom?  Does
> > anybody know or know where to look what maximum size a pwd-entry
> > can have?
> > 
> > Knowing the maximum size a pwd structure can have, it seems like to
> > be an idea to define the correct value in sysconf.c.  As that is
> > not done though suggested in the code, are there any obstacles in
> > defining that value so nobody has done that so far?
> 
> >From looking at the libc/gen/getpwent.c file, it looks like a
> >maximum size
> might be 1MB.  The wrapper functions that convert getpw*_r functions
> into ones that simply return a pointer to malloced data all use the
> getpw() helper function, which starts with a 1k buffer and keeps
> doubling its size until the data fits or it hits PWD_STORAGE_MAX
> (1MB).  PWD_STORAGE_MAX is only checked within that getpw() function,
> though, so it's possible that an nss library might return an even
> longer string to a get*_r call.  It's up to you to decide what your
> own limit is :)
> 

Uh ... it's just that I hoped I had not to decide ;-)

However, 1M seems to be rather large to me. Let's see (pwd.h):

    116 struct passwd {
    117 	char	*pw_name;	/* user name */
    118 	char	*pw_passwd;	/* encrypted password */
    119 	uid_t	pw_uid;		/* user uid */
    120 	gid_t	pw_gid;		/* user gid */
    121 	time_t	pw_change;	/* password change time */
    122 	char	*pw_class;	/* user access class */
    123 	char	*pw_gecos;	/* Honeywell login info */
    124 	char	*pw_dir;	/* home directory */
    125 	char	*pw_shell;	/* default shell */
    126 	time_t	pw_expire;	/* account expiration */
    127 	int	pw_fields;	/* internal: fields filled in */
    128 };

So pw_name -> MAXLOGNAME (from param.h) = 17. pw_passwd -> http://www.freebsd.org/doc/handbook/one-time-passwords.html = 129. pw_uid & pw_gid each sizeof(__uint32_t) ?= 32b. time_t -> sizeof(__int64_t) ?= 64b.

At some point, I would just sum it up and reach some size which might be machine dependant, but should be somewhere (guessing/estimating now) between 4k and 16k. I am short on time just now, am I on the right track or am I missing something which should be obvious to someone with experience, but is not to me (lacking experience)?

Thanks and regards,
-- 
Christopher J. Ruwe
TZ GMT + 2
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