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ZFL - ØMQ Function Library


OverviewScope and GoalsOwnership and LicenseContributing

Design IdeologyThe Problem with CA Simple Class ModelNaming StyleContainersInheritancePortabilityTechnical Aspects

Using ZFLBuilding and installingUsing ZFLClass ModelZFL Classes

Under the HoodAdding a New ClassMemory leak testingThis Document


Scope and Goals

ZFL is the ØMQ Function Library, a thin portability & function library for ZeroMQ applications in C/C++. It is written as clear readable C classes, portable to all ØMQ platforms, and licensed under the LGPL.

The main goal is to allow the construction of industrial-scale ØMQ services and devices that integrate properly with the operating system, while staying 100% portable. ZFL acts as a primary abstraction layer on top of the ØMQ API, mainly for C applications but also for other languages. ZFL is inspired by the iMatix Standard Function Library (SFL), and borrows liberally from it. See

ZFL is meant to be lightweight, consistent, class-based, minimalistic, highly efficient, and aimed at making it faster and easier to develop realistic, secure, and portable ØMQ devices and applications.

Ownership and License

ZFL is maintained by Pieter Hintjens and Martin Hurton. Its other authors and contributors are listed in the AUTHORS file. It is held by the ZeroMQ organization at

The authors of ZFL grant you free use of this software under the terms of the GNU Lesser General Public License (LGPL). For details see the files COPYING and COPYING.LESSER in this directory.


To submit an issue use the issue tracker. All discussion happens on the zeromq-dev list or #zeromq IRC channel at

The proper way to submit patches is to clone this repository, make your changes, and use git to create a patch. See All contributors are listed in AUTHORS.

All classes are maintained by a single person, who is the responsible editor for that class and who is named in the header as such. This is usually the originator of the class. When several people collaborate on a class, one single person is always the lead maintainer and the one to blame when it breaks.

The general rule is, if you contribute code to ZFL you must be willing to maintain it as long as there are users of it. Code with no active maintainer will in general be deprecated and/or removed.

Design Ideology

The Problem with C

C has the significant advantage of being a small language that, if we take a little care with formatting and naming, can be easily interchanged between developers. Every C developer will use much the same 90% of the language. Larger languages like C++ provide powerful abstractions like STL containers but at the cost of interchange. Every C++ developer will use a different 20% of the language.

The huge problem with C is that any realistic application needs packages of functionality to bring the language up to the levels we expect for the 21st century. Much can be done by using external libraries but every additional library is a dependency that makes the resulting applications harder to build and port. While C itself is a highly portable language (and can be made more so by careful use of the preprocessor), most C libraries consider themselves part of the operating system, and as such do not attempt to be portable.

The answer to this, as we learned from building enterprise-level C applications at iMatix from 1995-2005, is to create our own fully portable, high-quality libraries of pre-packaged functionality, in C. Doing this right solves both the requirements of richness of the language, and of portability of the final applications.

A Simple Class Model

C has no standard API style. It is one thing to write a useful component, but something else to provide an API that is consistent and obvious across many components. We learned from building OpenAMQ, a messaging client and server of 0.5M LoC, that a consistent model for extending C makes life for the application developer much easier.

The general model is that of a class (the source package) that provides objects (in fact C structures). The application creates objects and then works with them. When done, the application destroys the object. In C, we tend to use the same name for the object as the class, when we can, and it looks like this (to take a fictitious ZFL class):

zfl_regexp_t *regexp = zfl_regexp_new (regexp_string);
if (!regexp)
    printf ("E: invalid regular expression: %s\n", regexp_string);
if (zfl_regexp_match (regexp, input_buffer))
    printf ("I: successful match for %s\n", input buffer);
zfl_regexp_destroy (&regexp);

As far as the C program is concerned, the object is a reference to a structure (not a void pointer). We pass the object reference to all methods, since this is still C. We could do weird stuff like put method addresses into the structure so that we can emulate a C++ syntax but it's not worthwhile. The goal is not to emulate an OO system, it's simply to gain consistency. The constructor returns an object reference, or NULL if it fails. The destructor nullifies the class pointer, and is idempotent.

What we aim at here is the simplest possible consistent syntax.

No model is fully consistent, and classes can define their own rules if it helps make a better result. For example:

  • Some classes may not be opaque. For example, we have cases of generated serialization classes that encode and decode structures to/from binary buffers. It feels clumsy to have to use methods to access the properties of these classes.

  • While every class has a _new method that is the formal constructor, some methods may also act as constructors. For example, a "dup" method might take one object and return a second object.

  • While every class has a _destroy method that is the formal destructor, some methods may also act as destructors. For example, a method that sends an object may also destroy the object (so that ownership of any buffers can passed to background threads). Such methods take the same "pointer to a reference" argument as the _destroy method.

Naming Style

ZFL aims for short, consistent names, following the theory that names we use most often should be shortest. Classes get one-word names, unless they are part of a family of classes in which case they may be two words, the first being the family name. Methods, similarly, get one-word names and we aim for consistency across classes (so a method that does something semantically similar in two classes will get the same name in both). So the canonical name for any method is:


And the reader can easily parse this without needing special syntax to separate the class name from the method name.


After a long experiment with containers, we've decided that we need exactly two containers:

  • A singly-linked list.
  • A hash table using text keys.

These are zfl_list and zfl_hash, respectively. Both store void pointers, with no attempt to manage the details of contained objects. You can use these containers to create lists of lists, hashes of lists, hashes of hashes, etc.

We assume that at some point we'll need to switch to a doubly-linked list.


ZFL provides two ways to do inheritance from base classes to higher level classes. First, by code copying. You may laugh but it works. The zfl_base class defines a basic syntactic structure. If we decide to change some of the ground rules shared by all classes, we modify the zfl_base class and then we manually make the same modifications in all other ZFL classes. Obviously as the number of classes in ZFL grows this becomes progressively harder, which is good: we don't want the basics to change more than they need to.

The second way is by straight encapsulation. For example if I want to make a specialized container that has some intelligence about the objects it contains, I can take the list or hash class, wrap that in a new class and add the necessary code on top. There is no attempt, nor need, to export methods or properties automatically. If I want this, I do it by hand.

Writing such code by hand may seem laborious but when we work with ruthlessly consistent style and semantics, it is easy, safe, and often the shortest path from problem to solution.


Creating a portable C application can be rewarding in terms of maintaining a single code base across many platforms, and keeping (expensive) system-specific knowledge separate from application developers. In most projects (like ØMQ core), there is no portability layer and application code does conditional compilation for all mixes of platforms. This leads to quite messy code.

ZFL is explicitly meant to become a portability layer, similar to but thinner than libraries like the Apache Portable Runtime (APR).

These are the places a C application is subject to arbitrary system differences:

  • Different compilers may offer slightly different variants of the C language, often lacking specific types or using neat non-portable names. Windows is a big culprit here. We solve this by 'patching' the language in zfl_prelude.h, e.g. defining int64_t on Windows.
  • System header files are inconsistent, i.e. you need to include different files depending on the OS type and version. We solve this by pulling in all necessary header files in zfl_prelude.h. This is a proven brute-force approach that increases recompilation times but eliminates a major source of pain.
  • System libraries are inconsistent, i.e. you need to link with different libraries depending on the OS type and version. We solve this with an external compilation tool, 'C', which detects the OS type and version (at runtime) and builds the necessary link commands.
  • System functions are inconsistent, i.e. you need to call different functions depending, again, on OS type and version. We solve this by building small abstract classes that handle specific areas of functionality, and doing conditional compilation in these.

An example of the last:

#if (defined (__UNIX__))
    pid = GetCurrentProcessId();
#elif (defined (__WINDOWS__))
    pid = getpid ();
    pid = 0;

ZFL uses the GNU autotools system, so non-portable code can use the macros this defines. It can also use macros defined by the zfl_prelude.h header file.

Technical Aspects

  • Thread safety: the use of opaque structures is thread safe, though ØMQ applications should not share state between threads in any case.
  • Name spaces: we prefix class names with zfl_, which ensures that all exported functions are globally safe.
  • Library versioning: we don't make any attempt to version the library at this stage. Classes are in our experience highly stable once they are built and tested, the only changes typically being added methods.
  • Performance: for critical path processing, you may want to avoid creating and destroying classes. However on modern Linux systems the heap allocator is very fast. Individual classes can choose whether or not to nullify their data on allocation.
  • Self-testing: every class has a selftest method that runs through the methods of the class. In theory, calling all selftest functions of all classes does a full unit test of the library. The zfl_selftest application does this.
  • Memory management: ZFL classes do not use any special memory management techiques to detect leaks. We've done this in the past but it makes the code relatively complex. Instead, we do memory leak testing using tools like mtrace:

    gcc -g -o zfl_selftest zfl*.c -lzmq export MALLOC_TRACE=mtrace.txt zfl_selftest mtrace zfl_selftest mtrace.txt

Using ZFL

Building and installing

ZFL uses autotools for packaging. To build from git (all example commands are for Linux):

git clone git://
cd zfl
make all
sudo make install
sudo ldconfig

You will need the pkg-config, libtool, and autoreconf packages. Set the LD_LIBRARY_PATH to /usr/local/libs unless you install elsewhere.

After building, you can run the ZFL selftests:

cd src

Using ZFL

Include zfl.h in your application and link with libzfl. Here is a typical gcc link command:

gcc -lzfl -lzmq myapp.c -o myapp

You should read zfl.h. This file includes zmq.h and the system header files that typical ØMQ applications will need. The provided 'c' shell script lets you write simple portable build scripts:

c -lzfl -lzmq -l myapp

Class Model

ZFL consists of classes, each class consisting of a .h and a .c. Classes may depend on other classes.

zfl.h includes all classes header files, all the time. For the user, ZFL forms one single package. All classes start by including zfl.h. All applications that use ZFL start by including zfl.h. zfl.h also defines a limited number of small, useful macros and typedefs that have proven useful for writing clearer C code.

The canonical example for ZFL style is the zfl_base class, which defines the template for all other classes. The nomenclature for all classes is consistent. We use zfl_base as an example:

  • Source files: zfl_base.c, zfl_base.h
  • Methods: zfl_base_test, zfl_base_print, ...

All classes are based on a flat C class system and follow these rules:

  • Class typedef: zfl_base_t
  • Constructor: zfl_base_new
  • Destructor: zfl_base_destroy
  • Property methods: zfl_base_property_set, zfl_base_property
  • Class structures are private (defined in the .c source but not the .h)
  • Properties are accessed only via methods named as described above.
  • In the class source code the object is always called self.
  • The constructor may take arbitrary arguments, and returns NULL on failure, or a new object.
  • The destructor takes a pointer to an object reference and nullifies it.

Return values for methods are:

  • For methods that return an object reference, either the reference, or NULL on failure.
  • For methods that signal success/failure, a return value of 0 means sucess, -1 failure.

Private/static functions in a class are named s_functionname and are not exported via the header file.

All classes have a test method called zfl_classname_test.

ZFL Classes

These are the existing ZFL classes:

Under the Hood

Adding a New Class

If you define a new ZFL class myclass you need to:

  • Write the zfl_myclass.c and zfl_myclass.h source files, in zfl/src and zfl/include respectively.
  • Add#include <zfl_myclass.h> to zfl/include/zfl.h.
  • Add the myclass header and test call to src/zfl_selftest.c.
  • Add a reference documentation to 'doc/zfl_myclass.txt'.
  • Add myclass to 'src/Makefile.amanddoc/`.

Memory leak testing

To test against memory leaks we use the mtrace tool under Linux. The zfl_selftest.c program calls MALLOC_TRACE, which zfl_prelude.h sets to mtrace() under Linux. This is how we build and run the selftests with mtrace:

#  Run selftests and check memory
gcc -g -o zfl_selftest zfl*.c -lzmq
export MALLOC_TRACE=mtrace.txt
zfl_selftest -v
mtrace zfl_selftest mtrace.txt

Note that mtrace is not threadsafe and will not work consistently in multithreaded applications or test cases. All test cases should therefore be single-threaded.

This Document

This document is originally at README.txt and is built using gitdown.

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