int getpwuid_r(uid_t uid, struct passwd *pwbuf, char *buf,
    size_t buflen, struct passwd **pwbufp);

struct passwd pwent;
    struct passwd *pwentp;
    char buf[1024];

    if (getpwuid_r(101, &pwent, buf, sizeof buf, &pwentp))
    {
            perror("getpwuid_r");
    }
    else
    {
            printf("Username: %sn", pwent.pw_name);
            printf("Real Name: %sn", pwent.pw_gecos);
            printf("Home Directory: %sn", pwent.pw_dir);
    }

struct passwd *getpwuid(uid_t uid);

int getpwuid_r(uid_t uid, struct passwd *pwbuf, char *buf, size_t buflen, struct passwd **pwbufp);

struct passwd * my_passwd;
my_passwd = getpwuid(uid);
// or:
// my_passwd = getpwnam(username);

if (my_passwd == NULL) {
    // the lookup failed - handle the error!
} else {
    // the lookup succeeded - do your thing
    printf("User name: %sn", my_passwd->pw_name);
    printf("User password: %sn", my_passwd->pw_passwd);
    ...
}

int getpwnam_r(const char *name, struct passwd *pwd,
               char *buf, size_t buflen, struct passwd **result);