trying to learn systems programming, fear I have not understood and thus messed up

[Christopher J. Ruwe]

On Fri, 21 Oct 2011 18:53:33 +0200
"Christopher J. Ruwe" <cjr at cruwe.de> wrote:

> [...]
> 
> I have tried to follow the suggestion from the comment by modifiying
> the else-statement thus:
> 
> } else {
>   struct passwd *trgpwd;
>   if (!(trgpwd = GETPWNAM(arg->val)))
>      errx(EX_DATAERR, "User %s does not exist", arg->val);
>    
>    if (strcmp(a_name->val,"root") == 0)
>       errx(EX_DATAERR, "can't change uid of `root' account");
>    if (strcmp(trgpwd->pw_name, "root") == 0)
>       warnx("WARNING: account `%s' will have a uid of 0 (superuser
> access!)", pwd->pw_name);
> 
>    pwd->pw_uid = (uid_t) (trgpwd->pw_uid);		    
>    edited = 1;
> } 
> 
> What happens is not what I intended. I invoke as "sudo ./pw usermod
> testuser1 -u testuser2". I can get testuser2's pwd-entry by GETPWNAM
> allright, but when I assign the pw_uid, so as to make testuser2's uid
> the same as testuser1's and imgaining to retain all other values, ./pw
> reports "pw: user 'testuser2' disappeared during update" and the
> testuser2's /etc/passwd entry is replaced by testuser1's.
> 
> I fear I have not understood GETPWNAM correctly, as it seems to
> replace the struct pwd as some sort of sideeffect. I could manually
> set all pwd-members to the correct ones (those of testuser2), but I
> fear that I have messed something up beforehand.
> 
> I am grateful for any suggestions and/or correction. 

It seems I have indeed not understood GETPWNAM correctly. I have worked
out a method which works by calling GETPWNAM twice:

   else {
   /*
    * operation as follows:
    * a_name->val is passed as usermod <uname>
    * arg->val is passed as -u <uname>
    *
    * first check if we do someting stupid, i.e., want
    * to set root uid to some other users uid or
    * to set some user accout's uid to root uid.
    * then get pwd to that of uname passed as -u <uname>.
    * store uid from that pwdent.
    * get pwd to that of uname passed as usermod <uname>
    * change uid of that latter uid to the one stored
    */
		    
    if(strcmp(a_name->val,"root") == 0)
      errx(EX_DATAERR, "can't change uid of `root' account");

    if(strcmp(arg->val, "root") == 0)
      warnx("WARNING: account `%s' will have a uid of 0 (superuser
    access!)", pwd->pw_name);

    if(!(pwd = GETPWNAM(arg->val))) /* -u <uname>*/
      errx(EX_DATAERR, "User %s does not exist", arg->val);
    int alias_uid = pwd->pw_uid; 

    if(!(pwd = GETPWNAM(a_name->val))) /*usermod <uname>*/
      errx(EX_DATAERR, "User %s does not exist", a_name->val);

    pwd->pw_uid = (uid_t) alias_uid;
        warnx("User %s's uid changed to %d", pwd->pw_name, pwd->pw_uid);
    edited = 1;
  }

As I stil do not know why the latter variant of my code worked and the
former does not, I would still appreciate any comment or explanation which would help me understanding GETPWNAM and getpwnam.

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

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