I’ve been writing in C for over 20 years, and it is humbling when you suddenly realise how little you know of a topic.
It reminds me of when I worked for IBM, and the company wanted a skills register. The questions were along the lines of

Rate your skills in the following areas from 0 (nothing) to 10 (expert).

• z/OS
• DB2
• CICS
• etc

Overall the skills register was found to be not useful, as the rankings were inverted. If someone put themselves down as 10 – it usually meant they knew very little, they knew enough for their day to day work. If someone put themselves down as 2 they may be an expert who realises how much they do not know, or someone who honestly realises they do not know very much.

My humbling discovery was that when I ported some existing C code to run on z/OS, the functions were not visible outside of the C program. There were two reasons for this.

• The functions were defined as static,
• The functions were not exported.

Static functions

static int hidden(int  i)
{
  return 0;
}
int visible(int i)
{
  int x = hidden(1);
  return 0; 
}

I think that using “static” in this case is the wrong word. It does not mean static. I think “internal” would be a clearer description, but I do not think that I’ll have any success changing the C language to use “internal”.

The function “hidden” can only be used within the compiled unit. It cannot be referenced from outside of the compiled object. The “visible” function can use the “hidden” function as the code shows.
The function”visible” is potentially visible to external programs. You can load the module and execute the function.

Exported functions

You have to tell the compiler to externalise functions within the compile unit.

For example

#pragma export(COLIN)
int COLIN(char * self, int args) {
   return 8;
}

or the compiler option EXPORTALL for example

cc… -Wc,EXPORTALL

What has been exported?

When you bind (linkedit) your program, the binder and report the exported functions. You need the binder parameters XREF and DYNAM=DLL

This gave output like

IMPORT/EXPORT     TYPE    SYMBOL              DLL                 DDNAME   SEQ  MEMBER 
-------------     ------  ----------------    ----------------    -------- ---  --------- 
   IMPORT         CODE64  __a2e_l             CELQV003            CELQS003  01  CELQS003 
   IMPORT         CODE64  malloc              CELQV003            CELQS003  01  CELQS003 
   IMPORT         CODE64  CSQB3BAK            CSQBLB16            MQ        01  CSQBMQ2X 
                                                                                                   
   EXPORT         DATA64  ascii_tab 
   EXPORT         CODE64  printHex 
   EXPORT         CODE64  COLIN 
   

This shows the imported symbols, and where they came from, and what was exported.

• ascii_tab is a table of data in 64 bit mode
• printHex is a function in 64 bit mode
• COLIN is a function in 64 bit mode.

How to use it.

You can use handle= dlopen(name,mode) to get the load module into storage, and functionPointer=dlsym(handle,”COLIN”) to locate the external symbol COLIN in the load module.

Why is a static function useful?

With Python external functions (written in C), it uses static functions to hide internals. For example

static PyObject * pymqe_MQCONN(
... )
...
static struct PyMethodDef pymqe_methods[] = { 
  {"MQCONN", (PyCFunction)pymqe_MQCONN,... }, 
  {"MQCONNX", (PyCFunction)pymqe_MQCONNX,... },
  ....  
}

When the external function is imported, the initialisation routine returns the pymqe_methods data to Python.

Python now knows what functions the module provides (MQCONN, MQCONNX), and the C code to be executed when the function is executed.

This means that you cannot load the module, and accidentally try to use the function pymqe_MQCONN; which I thought was good defensive programming.

As part of playing with Python on z/OS I found you can call a z/OS Unix Services load module ( a .so object) from Python. It can also use a load module in a PDSE.

What sort is it?

A load module on z/OS can be used on one of two ways.

• Load it from steplib or PATH environment variable (which uses dllopen under the covers), call the function, and pass the parameters The parameters might be a byte string ( char * in C terms), a Unicode string, integer etc. You return one value, for example an integer return code.
• As part of a package where you use the Python “import package” statement. This gets loaded from PYTHONPATH, the current directory, and other directories (but not steplib). The parameters passed in are Python Objects, and you have to use Python functions to extract the value. You can return a complex Python object, for example a character string, return code and reason code.

This article is on the first case.

In both cases, the values passed in are in ASCII. If you use printf to display data, the printf treats your data as ASCII.

There is a good article on Python ctypes , a foreign function library for Python.

My initial program was

int add_it(int i, int j)
{
   return i+j;
}

I compiled it with a bash script

wc64=”-Wc,SO,LIST(lst64),XREF,LP64,DLL,SSCOM,EXPORT”
cc -c -o add.o ${wc64} add.c
cc -o add -V -Wl,DYNAM=DLL,LP64 add.o 1>ax 2>bx

This created a load module object “add”. Note: You need the EXPORT to make the entry point(s) visible to callers.

My python program was

import ctypes
from ctypes.util import find_library
zzmqe = ctypes.CDLL(“add”)
print(“mql”, zzmqe.add_it(2,5))

When this ran it produced

mql 7

As expected. To be consistent with Unix platforms, the load module should be called add.so, but “add” works.

Using strings is more complex

I changed the program (add2) to have strings as input

int add_one(char * a, char *b)
{
  printf("a %s\n",a);  
  printf("b %s\n",b);
  return 2 ;
}

and a Python program, passing a byte string.

import ctypes
from ctypes.util import find_library
zzmqe = ctypes.CDLL("add2")
print("mql", zzmqe.add_one(b'abc',b'aaa'))

This ran and gave output

-@abc–@aaa-mql 2

This shows that Python has converted the printf output ASCII to EBCDIC, and so the “a” and “b” in the printf statements are converted to strange characters, and the \n (new line) is treated as hex rather than a new line.

When I compiled the program with ASCII (-Wc…ASCII), the output from the Python program was

a abc
b aaa
mql 2

Displaying the data as expected.

Using a load module in a PDSE.

The JCL

//COLINC3 JOB 1,MSGCLASS=H,COND=(4,LE)
//S1 JCLLIB ORDER=CBC.SCCNPRC
// SET LOADLIB=COLIN.C.REXX.LOAD
// SET LIBPRFX=CEE
//COMPILE EXEC PROC=EDCCB,
// LIBPRFX=&LIBPRFX,
// CPARM=’OPTFILE(DD:SYSOPTF),LSEARCH(/usr/include/),RENT’,
// BPARM=’SIZE=(900K,124K),RENT,LIST,XREF,RMODE=ANY,AMODE=64‘
//COMPILE.SYSOPTF DD DISP=SHR,DSN=COLIN.C.REXX(CPARMS)
// DD *
EXPORT,LP64
/*
//COMPILE.SYSIN DD *
int add_one(int i, int j)
{
return i+j;
}
//COMPILE.SYSLIB DD
// DD
// DD DISP=SHR,DSN=COLIN.C.REXX
//BIND.SYSLMOD DD DISP=SHR,DSN=&LOADLIB.
//BIND.SYSLIB DD DISP=SHR,DSN=CEE.SCEEBND2
// DD DISP=SHR,DSN=CEE.SCEELKED
// DD DISP=SHR,DSN=CEE.SCEELIB
//BIND.OBJLIB DD DISP=SHR,DSN=COLIN.C.REXX.OBJ
//BIND.SYSIN DD *
NAME ADD3(R)
/*

In Unix services

export STEPLIB=”COLIN.C.REXX.LOAD“:$STEPLIB

The python program

import ctypes
from ctypes.util import find_library
zzmqe = ctypes.CDLL(“ADD3”)
print(zzmqe)
print(dir(zzmqe))
print(“steplib”, zzmqe.add_one(2,5))

This gave

steplib 7

Byte string and character strings parameters

I set up a C program with a function COLIN

#pragma export(COLIN)
int COLIN(char * p1, int p2) { 
  printf(" p1 %4.4s\n",p1);
  printf(" p2 %i\n",p2);
  return 8;
}

This was compiled in Unix Services, and bound using JCL into a PDSE load library as member YYYYY.

I used a shell to invoke the Python script

export LIBPATH=/u/tmp/python/usr/lpp/IBM/cyp/v3r10/pyz/lib/:$LIBPATH
export STEPLIB=COLIN.C.REXX.LOAD:$STEPLIB 
python3 dll.py

where the Python dll.py script was

import ctypes
testlib = ctypes.CDLL("YYYYY")
name =b'CSQ9'
i = 7
zz = testlib.COLIN(name,i)
print("return code",zz)

This displayed

p1 CSQ9
p2 7
return code 8

The name, a binary string CSQ9, was passed as a null terminated ASCII string (0x43535139 00).

When a string name = “CSQ9” was passed in, the data was in a Unicode string, hex values

00000043 00000053 00000051 00000039 00000000

You need to be sure to pass in the correct data (binary or Unicode), and be sure to handle the data in ASCII.

Is this how ‘import’ works?

This is different process to when a module is used via a Python import statement. If you passed in b’ABCD’ to a C extension which has been “imported” this would be passed as a Python Object, rather than the null terminated string 0x’4142434400′.

I enjoy using Python on Linux, because it is very powerful. I thought it would be interesting to port the MQ Python interface pymqi to z/OS. This exposed many of the challenges of running Python on z/OS.

I’ll cover some of the lessons I learned in doing this work. Thanks to Steve Pitman who helped me package the extension.

• Creating files that would compile, was a challenge
• Compiling files
  • My directory tree
• Setup.py
• Setup
• Doing the compile and install for testing
• Packaging the package
  • Python build which worked
  • Failing build. This built but did not install.
• Installing the package

IBM Open Enterprise Python for z/OS, V3.8, user’s guide is a useful book.

Packaging Python programs

See Python import, packages and modules.

You need a package for Python source, and a separate package for load module(s).

Creating files that would compile was a challenge.

See here.

With experience, I’ve found an easier way -just compile the source with the right options.

Compiling files.

I copied the pymqi C code to z/OS Unix Services, and tried to compile it. This was a mistake, as it took me a long time to get the compile options right. I found that using the setup.py script was the right way to go.

My directory tree

/u/pymqi
..setup.py
..include
....sample.h
..code
....pymqi
......__init__.py        
......CMQCFC.py          
......CMQXC.py           
......CMQZC.py           
......const.py           
......CMQC.py            
......pymqe.c            

Setup.py

This script needs export _C89_CCMODE=1, otherwise you get FSUM3008 message

Specify a file with the correct suffix (.c, .i, .s, .o, .x, .p, .I, or .a), or a corresponding data set name, instead of -L

import setuptools 
from distutils.core import setup, Extension 
import os 
import sysconfig 
# 
# This script needs    export _C89_CCMODE=1 
# Otherwise you get FSUM3008  messages 
# 
import os 
os.environ['_C89_CCMODE'] = '1' 
bindings_mode = 1 
version = '1.12.0' 
setup(name = 'pymqi', 
    version = version, 
    description = 'Python...', 
    platforms='OS Independent', 
    package_dir = {'': 'code'}, 
    packages = ['pymqi'],  
    py_modules = ['pymqi.CMQC', 'pymqi.CMQCFC', 'pymqi.CMQXC', 'pymqi.CMQZC'], 
    ext_modules = [Extension('pymqi.pymqe',['code/pymqi/pymqe.c'], define_macros=[('PYQMI_BINDINGS_MODE_BUILD', 
bindings_mode)], 
    include_dirs=["//'COLIN.MQ924.SCSQC370'"], 
    extra_link_args=["//'COLIN.MQ924.SCSQDEFS.OBJ(CSQBMQ2X)'"], 
      )] 
) 
# I had extra_link_args=["-Wl,INFO,LIST,MAP",.... when setting 
# this up
# I used 
# extra_compile_args=["-Wc,LIST(c.lst),XREF"], 
# to get out a listing and cross reference.

Which says

• The package name is packages = [‘pymqi’],
• The Python files are py_modules = [‘pymqi.CMQC’….
• There is an extension .. ext_modules=.. with the source program code/pymqi/pymqe.c
• It needs “//’COLIN.MQ924.SCSQC370′” to compile and “//’COLIN.MQ924.SCSQDEFS.OBJ(CSQBMQ2X)'” at bind time. This file contains the MQ Binder input
• When I wanted the binder output – “-Wl,INFO,LIST,MAP”. This goes to the terminal. I used a ‘>’ command to pipe the output of the python3 setup build it to a file.
• and a C listing “-Wc,LIST(c.lst),XREF”. The listing goes to c.lst

You need

• import setuptools so that the setup bdist_wheel packaging works. You also need the wheel package installed.

Setup

There is a buglet in the compile set up. You need to specify

export _C89_CCMODE=1

Without it you get

FSUM3008 Specify a file with the correct suffix (.c, .i, .s, .o, .x, .p, .I, or .a), or a corresponding data set name, instead of -obuild/lib.os390-27.00-1090-3.8/pymqi/pymqe.so.

You also need the binder input in a data set with the correct suffix. For example .OBJ

“//’COLIN.MQ924.SCSQDEFS.OBJ(CSQBMQ2X)'”

If you do not have the correct suffix you get

FSUM3218 xlc: File //’COLIN.MQ924.SCSQDEFS(CSQBMQ2X)’ contains an incorrect file suffix.

Doing the compile and test install

I used a shell script to do the compiles and install

touch code/pymqi/*.c
rm a b c d
export _C89_CCMODE=1
#python3 setup.py clean
python3 setup.py build 1>a 2>b
python3 setup.py install 1>c 2>d

I captured the output from the setup.py jobs using 1>a etc because I could not see how to direct the binder output to a file. It comes out on the terminal – and there was a lot of it!.

Packaging the package

Python build which worked

I had to install wheel package. See How to install software in an isolated environment, or just use python3 -m pip install wheel if your z/OS image is connected to the network.

The command I used was

python3 -m pip install –user –no-cache-dir /u/tmp/py/wheel-0.37.1-py2.py3-none-any.whl-f /u/tmp/py/wheel-0.37.1-py2.py3-none

I had to add import setuptools to my setup.py file (at the top). (This converted the install package from a dist-utils to a setuptools packaging)

python3 setup.py bdist_wheel

This created a file “/u/pymqi/dist/pymqi-1.12.0-cp310-cp310-os390_27_00_1090.whl“

This file is specific to python 3.10

For a wheel package, you’ll need to build it for all major versions and cannot just use one. Note that there were some issues in 3.8/3.9 with wheels that have been resolved in 3.10, so it’s recommended you to use 3.10.

Steven Pitman

This means you need to have multiple levels of Python installed, and build for each one!

This also has the operating system level (os390_27_00 – this may be constant across machines) and the hardware 1090. For this to work on other hardware, one solution would be to manually rename the file to pretend it is for a different machine, but this both not supported nor recommended, and has no guarantee to work. So it is hard to know the best thing to do. I do not have every 390 machine from IBM to do a build on !

Failing build. This built but did not install.

python3 setup.py bdist –format=tar

It built the package and create a file

./dist/pymqi-1.12.0.os390-27.00-1090.tar

This tar file is not completely readable by the z/OS tar command.

When I used tar -tf ….tar it gave

FSUMF371 Value 1641318408.0 is not valid for keyword mtime. Keyword not set.

It uses a Python tar command, not the operating system tar command.

You can display the contents using a Python program like

import tarfile
tar = tarfile.open("dist/pymqi-1.12.0.os390-27.00-1090.tar.gz")
# tar.extractall() 
for x in tar:
    print(x)

This gave output like

<TarInfo ‘.’ at 0x5008ad3880>
<TarInfo ‘./usr’ at 0x5008ad3dc0>
<TarInfo ‘./usr/lpp’ at 0x5008ad3a00>

…

Installing the package

From an authorised user in OMVS,

python3 -m pip install –no-cache-dir /u/pymqi/dist/pymqi-1.12.0-cp310-cp310-os390_27_00_1090.whl /u/pymqi/dist/pymqi-1.12.0-cp310-cp310-os390_27_00_1090.whl
Processing /u/pymqi/dist/pymqi-1.12.0-cp310-cp310-os390_27_00_1090.whl
Installing collected packages: pymqi
Successfully installed pymqi-1.12.0

If you do not use –no-cache-dir, you may get

-[33]WARNING: The directory ‘/u/.cache/pip’ or its parent directory is not owned or is not writable by the current user. The cache has been disabled. Check the permissions and owner of that directory. If executing pip with sudo, you should use sudo’s -H flag.-[0]

The compile options

The following text is the compile and bind options used for my code. Some of the options are pymqi specific.

/bin/xlc -DNDEBUG -O3 -qarch=10 -qlanglvl=extc99 -q64
-Wc,DLL
-D_XOPEN_SOURCE_EXTENDED
-D_UNIX03_THREADS
-D_POSIX_THREADS
-D_OPEN_SYS_FILE_EXT
-qexportall -qascii -qstrict -qnocsect
-Wa,asa,goff -Wa,xplink
-qgonumber -qenum=int
-DPYQMI_BINDINGS_MODE_BUILD=1 -I//’COLIN.MQ924.SCSQC370′
-I/u/tmp/python/usr/lpp/IBM/cyp/v3r10/pyz/include/python3.10
-c code/pymqi/cpmqe.c
-o build/temp.os390-27.00-1090-3.10/code/pymqi/cpmqe.o

/bin/xlc build/temp.os390-27.00-1090-3.10/code/pymqi/cpmqe.o -L.
-o build/lib.os390-27.00-1090-3.10/pymqi/cpmqe.cpython-310.so -Wl,INFO,LIST,MAP,DLL //’COLIN.MQ924.SCSQDEFS.OBJ(CSQBMQ2X)’
-Wl,dll
/u/tmp/python/usr/lpp/IBM/cyp/v3r10/pyz/lib/python3.10/config-3.10/libpython3.10.x
-q64