getpwnam getpwnam_r

Question

I am trying to update a password structure in a function. This function gets a pointer to password structure, into which I have to update details.

I get a struct passwd* to a function as an argument and my requirement is to update it's members to someother user's(user2) information and caller of this function should have all these updated details.

I tried following, can you please help me by suggesting what is an ideal way to do it.

My function is like this:

struct passwd* myfunc(struct passwd *pw)

To this pw , I have to update details.

Try1:

Using getpwnam() get details of user2 into passed struct passwd *pw .

pw = getpwnam("user2");
return pw;

Does not work, as we are not updating pw members rather we are just pointing pw to some other struct, pointer members of pw , still point to older data. And thus parent of myfunc() does not get updated details.

We have to write into members.

Try2:

Using getpwnam() get details of user2 into a temp struct passwd* . Do a memcpy(pw, temppasswd, sizeof(struct passwd))

Does not work as memcpy is a shallow copy and consequent getpwnam() (although the return is not given to temppasswd ) will over write static buffer area, and pw may have different values.

Try3:

Tried getpwnam_r() which writes details into buffer passed as an argument to it rather than into a static data area (like getpwnam ).

Can not use this, as I don't have pw , I have *pw . So, I can not pass address of memory allocated to pw to getpwnam_r .

Try 4:

I can do member by member copy in Try2, but it can result in memory leaks if I don't free pw structure pointer members before copying data into them.

Is there any other way to copy details to passed pw* without free-ing all struct pw* pointer members.

It is a bit confusing, I hope I make sense to someone.

Answer 1

You could combine the first two alternatives as *pw = *getpwnam("user2"); . It's still a shallow copy though.

As for your problem, if you can change the prototype to struct passwd* myfunc(struct passwd **pw) then you could allocate the structure and all needed fields yourself. Then have a separate function that the user must call to free all allocated data.