Linux – Environment variables of a running process on Unix

An interesting question that had me scratching my head for a while.

First, why you have no Swap line: though in theory that was added in kernel 2.6.16, it didn't show up in RHEL until 5.3 or so (kernel 2.6.18-99.el5). Here is the commit (unfortunately not accessible in the RH Bugzilla; if you happen to have an account with appropriate privileges, the Bugzilla id is 443749). http://sophie2.aero.jussieu.fr/distrib/Scientific%20Linux/5x/i386/by-pkgid/351d529f9beeb4e5d936a6d5e3e7813a/files/645

Second, this implies that the top data is unreliable since it is not reading from the smaps (i.e. it is virt - res):

    case P_SWP:
       MKCOL(scale_num(PAGES_TO_KB(p->size - p->resident), w, s));

Third, I regret to say that the kernel is the One True Oracle on how much swap is used, and as far as I can tell the various patches to make the information available in smaps and status are the only way to expose that information. Unfortunately, I see updating your kernel as the only way to get accurate information; either by patching the source yourself (sample patch, no warranty) or updating to kernel 2.6.18-99.el5.

I wish there were an easier answer; blame Linus.

Well, with one more w you should get it all according to the man page:

ewww

Wraps display from the e flag and displays the ENV list until it reaches the INT_MAX value.

That's a pretty big limit.

If you're looking at creating yourself tools for this, you should look into the getevars and geteargs functions which ps uses for the ewww display.

Here's a naïve usage of getevars, prints its own environment (limit ~1Mb):

#include <sys/types.h>
#include <procinfo.h>
#include <unistd.h>
#include <stdio.h>

#define ENV_MAX (1024*1024)

int main(void)
{
    struct procentry64 pe;
    char buffer[ENV_MAX];
    pe.pi_pid = getpid();
    int rc = getevars(&pe, sizeof(pe), buffer, sizeof(buffer));
    if (rc != 0) {
        perror("getevars");
        return 1;
    }
    buffer[ENV_MAX-2] = buffer[ENV_MAX-1] = 0;
    char *cur = buffer;
    char *end = buffer+sizeof(buffer);
    while ((*cur) && (cur < end)) {
        int len = puts(cur);
        cur += len;
    }
    return 0;
}