/* sysconf(_SC_GET{PW,GR}_R_SIZE_MAX) */
#define R_SIZE_MAX 1024

int sigar_user_name_get(sigar_t *sigar, int uid, char *buf, int buflen)
{
    struct passwd *pw = NULL;
    /* XXX cache lookup */

# ifdef HAVE_GETPWUID_R
    struct passwd pwbuf;
    char buffer[R_SIZE_MAX];

    if (getpwuid_r(uid, &pwbuf, buffer, sizeof(buffer), &pw) != 0) {
        return errno;
    }
    if (!pw) {
        return ENOENT;
    }
# else
    if ((pw = getpwuid(uid)) == NULL) {
        return errno;
    }
# endif

    strncpy(buf, pw->pw_name, buflen);
    buf[buflen-1] = '\0';

    return SIGAR_OK;
}

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

#define R_SIZE_MAX 1024

int main(void) {

    struct passwd *pw = NULL;
    struct passwd pwbuf;
    char buffer[R_SIZE_MAX];

    // 600 is a userid
    if (getpwuid_r(600, &pwbuf, buffer, sizeof(buffer), &pw) != 0) {
        return errno;
    }
    if (!pw) {
        return ENOENT;
    }

    puts (pw->pw_name);

    return EXIT_SUCCESS;
}

          struct hostent *
          gethostname (char *host)
          {
            struct hostent hostbuf, *hp;
            size_t hstbuflen;
            char *tmphstbuf;
            int res;
            int herr;
          
            hstbuflen = 1024;
            /* Allocate buffer, remember to free it to avoid memory leakage.  */
            tmphstbuf = malloc (hstbuflen);
          
            while ((res = gethostbyname_r (host, &hostbuf, tmphstbuf, hstbuflen,
                                           &hp, &herr)) == ERANGE)
              {
                /* Enlarge the buffer.  */
                hstbuflen *= 2;
                tmphstbuf = realloc (tmphstbuf, hstbuflen);
              }
            /*  Check for errors.  */
            if (res || hp == NULL)
              return NULL;
            return hp;
          }

// Copyright 2013 Free Software Foundation, Inc.
// Verbatim copying and distribution of this entire article is permitted in any medium, provided this notice is preserved.