Plugin dies with segmentation fault on Ubuntu 16.04

[riccardomurri]

I followed the installation instructions at https://slurm.schedmd.com/faq.html#x11
adapting paths for the local installation.

Compiling the plugin against the SLURM packages provided by Ubuntu
16.04 produces these warnings about implicit function declaration::

    $ gcc -g -I/usr/include/slurm/ -shared -fPIC -o x11.so slurm-spank-x11-plug.c
    slurm-spank-x11-plug.c: In function ‘xpopen’:
    slurm-spank-x11-plug.c:140:7: warning: implicit declaration of function ‘setresgid’ [-Wimplicit-function-declaration]
       if (setresgid(egid,-1,egid) || setresuid(euid,-1,euid))
           ^
    slurm-spank-x11-plug.c:140:34: warning: implicit declaration of function ‘setresuid’ [-Wimplicit-function-declaration]
       if (setresgid(egid,-1,egid) || setresuid(euid,-1,euid))
                                      ^
    slurm-spank-x11-plug.c: In function ‘slurm_spank_init’:
    slurm-spank-x11-plug.c:183:13: warning: implicit declaration of function ‘_spank_x11_get_mode’ [-Wimplicit-function-declaration]
      x11_mode = _spank_x11_get_mode(sp,ac,av);
                 ^
    slurm-spank-x11-plug.c: In function ‘slurm_spank_local_user_init’:
    slurm-spank-x11-plug.c:249:11: warning: implicit declaration of function ‘_x11_connect_nodes’ [-Wimplicit-function-declaration]
      status = _x11_connect_nodes(job_ptr->nodes,jobid,stepid);
               ^
    slurm-spank-x11-plug.c: In function ‘_x11_init_remote_batch’:
    slurm-spank-x11-plug.c:368:17: warning: implicit declaration of function ‘error’ [-Wimplicit-function-declaration]
                     error("x11: unable to get username for uid=%u : %s",job_ptr->user_id,
                     ^

The resulting SPANK plugin dies with segmentation fault.

Here is data about compiler and system packages::

    $ gcc --version
    gcc (Ubuntu 5.4.0-6ubuntu1~16.04.9) 5.4.0 20160609
    Copyright (C) 2015 Free Software Foundation, Inc.
    This is free software; see the source for copying conditions.  There is NO
    warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

    $ dpkg-query -l 'slurm*'
    Desired=Unknown/Install/Remove/Purge/Hold
    | Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
    |/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
    ||/ Name                                  Version                 Architecture            Description
    +++-=====================================-=======================-=======================-===============================================================================
    ii  slurm-client                          15.08.7-1build1         amd64                   SLURM client side commands
    ii  slurm-drmaa-dev                       1.0.7-1build3           amd64                   PSNC DRMAA for SLURM - devel
    ii  slurm-drmaa1                          1.0.7-1build3           amd64                   PSNC DRMAA for SLURM - runtime
    ii  slurm-wlm-basic-plugins               15.08.7-1build1         amd64                   SLURM basic plugins
    ii  slurm-wlm-torque                      15.08.7-1build1         all                     Torque compatibility wrappers for SLURM
    ii  slurmctld                             15.08.7-1build1         amd64                   SLURM central management daemon
    ii  slurmdbd                              15.08.7-1build1         amd64                   Secure enterprise-wide interface to a database for SLURM