Question

I am checking my program against memory leaks and corruptions and
I have a problem using setpwent.
consider the simple program as:

#include<stdio.h>
#include<stdlib.h>

main()
{
    struct passwd *ent=NULL;
    setpwent();
        while ((ent = getpwent()) != NULL) { }
    endpwent();
}

when I run this piece of code in valgrind, I get this:

> valgrind  --track-origins=yes
> --tool=memcheck --leak-check=yes --show-reachable=yes --num-callers=20 --track-fds=yes ./a.out . . . 160 (40 direct, 120 indirect) bytes in 1
> blocks are definitely lost in loss
> record 11 of 11
> ==6471==    at 0x4025BD3: malloc (vg_replace_malloc.c:236)
> ==6471==    by 0x411CA9C: nss_parse_service_list
> (nsswitch.c:622)
> ==6471==    by 0x411D216: __nss_database_lookup (nsswitch.c:164)
> ==6471==    by 0x459BEAB: ???
> ==6471==    by 0x459C1EC: ???
> ==6471==    by 0x411D864: __nss_setent (getnssent_r.c:84)
> ==6471==    by 0x40D304F: setpwent (getXXent_r.c:127)
> ==6471==    by 0x8048469: main (in /root/workspace/cdk-examples/MMC-0.64/a.out)
> ==6471== 
> ==6471== LEAK SUMMARY:
> ==6471==    definitely lost: 40 bytes in 1 blocks
> ==6471==    indirectly lost: 120 bytes in 10 blocks
> ==6471==      possibly lost: 0 bytes in 0 blocks
> ==6471==    still reachable: 0 bytes in 0 blocks
> ==6471==         suppressed: 0 bytes in 0 blocks

should I worry about it?
how can I remove this problem?

Second question:
Do I need to free the password entry as:

main()
{
    struct passwd *ent=NULL;
    setpwent();
        while ((ent = getpwent()) != NULL) {
            free(ent);
        }
        endpwent();
}

thank you for helping me.

Answer 1

For the first question. I don’t think you need to call setpwent anyway. It’s the first call to getpwent (after process start or after endpwent) which goes back to the start.

You only need setpwent if you want to rewind after calling getpwent but without calling endpwent.

A single set is probably faster than an end/get pair, especially if, as I suspect, the whole (or a sizable proportion of the) file may be cached in memory ^(a).

---

For the second question, no, you don’t free it. See here. It will most likely use a static buffer (for single-threading) or thread-local storage (for multi-threading).

In both cases, it’s the calls themselves that manage the buffer, not your code ^(a).

---

^(a) Interestingly, the values of ent passed back are different on each iteration which certainly looks like separate allocations.

However, since the addresses are only 32 bytes apart and the size of a struct pwd is 32 bytes, that leaves no room for intervening malloc housekeeping information.

So, either the housekeeping information is not inline (unlikely) or you’re actually working with an array of structures rather than individual allocations.

The following program shows this in action:

#include <stdio.h>
#include <stdlib.h>
#include <pwd.h>

int main (void) {
    struct passwd *ent = NULL;
    printf ("ent struct is %d bytes\n", sizeof(*ent));
    while ((ent = getpwent()) != NULL) {
        printf ("pointer is %p, user is %s\n", ent, ent->pw_name);
        // free (ent);
    }
    endpwent();
    return 0;
}

It outputs:

ent struct is 32 bytes
pointer is 0x4708d0, user is alan
pointer is 0x4708f0, user is bill
pointer is 0x470910, user is carl
pointer is 0x470930, user is dawn
pointer is 0x470950, user is ella
pointer is 0x470970, user is fran

which is what leads me to the conclusions above. In any case, the instant I uncomment that free line in my code, I get a core dump, giving more support to my theory.

Now I suppose I could have just gone and had a look at the getpwent source code but I love a good puzzle 🙂