Provided by: libczmq-dev_3.0.2-5_amd64 
NAME
czmq - high-level C binding for ZeroMQ
SYNOPSIS
#include <czmq.h>
cc ['flags'] 'files' -lzmq -lczmq ['libraries']
DESCRIPTION
Classes
These classes provide the main socket and message API:
• zsock(3) - working with ZeroMQ sockets (high-level)
• zstr(3) - sending and receiving strings
• zmsg(3) - working with multipart messages
• zframe(3) - working with single message frames
• zactor(3) - Actor class (socket + thread)
• zloop(3) - event-driven reactor
• zpoller(3) - trivial socket poller class
• zproxy(3) - proxy actor (like zmq_proxy_steerable)
• zmonitor(3) - monitor events on ZeroMQ sockets
These classes support authentication and encryption:
• zauth(3) - authentication actor for ZeroMQ servers
• zcert(3) - work with CURVE security certificates
• zcertstore(3) - work with CURVE security certificate stores
These classes provide generic containers:
• zhash(3) - simple generic hash container
• zhashx(3) - extended generic hash container
• zlist(3) - simple generic list container
• zlistx(3) - extended generic list container
These classes wrap-up non-portable functionality:
• zbeacon(3) - LAN discovery and presence
• zclock(3) - millisecond clocks and delays
• zdir(3) - work with file-system directories
• zdir_patch(3) - work with directory differences
• zfile(3) - work with file-system files
• zsys(3) - system-level methods
• zuuid(3) - UUID support class
• ziflist(3) - list available network interfaces
And these utility classes add value:
• zchunk(3) - work with memory chunks
• zconfig(3) - work with textual config files
• zrex(3) - work with regular expressions
• zgossip(3) - decentralized configuration management
These classes are deprecated:
• zctx(3) - working with ZeroMQ contexts
• zsocket(3) - working with ZeroMQ sockets (low-level)
• zsockopt(3) - get/set ZeroMQ socket options
• zthread(3) - working with system threads
• zauth_v2(3) - authentication for ZeroMQ servers
• zbeacon_v2(3) - LAN discovery and presence
• zmonitor_v2(3) - socket event monitor
• zproxy_v2(3) - zmq_proxy wrapper
Scope and Goals
CZMQ has these goals:
• To wrap the ØMQ core API in semantics that are natural and lead to shorter, more
readable applications.
• To hide the differences between versions of ØMQ.
• To provide a space for development of more sophisticated API semantics.
Ownership and License
CZMQ is maintained by the ZeroMQ community at github.com/zeromq. Its other authors and
contributors are listed in the AUTHORS file.
The contributors are listed in AUTHORS. This project uses the MPL v2 license, see LICENSE.
Contributing
To submit an issue use the issue tracker at http://github.com/zeromq/czmq/issues. All
discussion happens on the zeromq-dev list or #zeromq IRC channel at irc.freenode.net.
The proper way to submit patches is to clone this repository, make your changes, and use
git to create a patch or a pull request. See http://www.zeromq.org/docs:contributing. 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 CZMQ 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.
USING CZMQ
Building and Installing
CZMQ uses autotools for packaging. To build from git (all example commands are for Linux):
git clone git://github.com/zeromq/czmq.git
cd czmq
sh autogen.sh
./configure
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 CZMQ selftests:
cd src
./czmq_selftest
Linking with an Application
Include czmq.h in your application and link with CZMQ. Here is a typical gcc link command:
gcc -lczmq -lzmq myapp.c -o myapp
You should read czmq.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 -lczmq -lzmq -l myapp
The Class Model
CZMQ consists of classes, each class consisting of a .h and a .c. Classes may depend on
other classes.
czmq.h includes all classes header files, all the time. For the user, CZMQ forms one
single package. All classes start by including czmq.h. All applications that use CZMQ
start by including czmq.h. czmq.h also defines a limited number of small, useful macros
and typedefs that have proven useful for writing clearer C code.
All classes (with some exceptions) are based on a flat C class system and follow these
rules (where zclass is the class name):
• Class typedef: zclass_t
• Constructor: zclass_new
• Destructor: zclass_destroy
• Property methods: zclass_property_set, zclass_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 (with some exceptions) have a test method called zclass_test.
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.
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 (http://www.openamq.org), 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 CZMQ class):
zregexp_t *regexp = zregexp_new (regexp_string);
if (!regexp)
printf ("E: invalid regular expression: %s\n", regexp_string);
else
if (zregexp_match (regexp, input_buffer))
printf ("I: successful match for %s\n", input buffer);
zregexp_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
CZMQ 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:
zclassname_methodname
And the reader can easily parse this without needing special syntax to separate the class
name from the method name.
Containers
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 zlist and zhash, 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.
Portability
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.
czmq acts as a portability layer, similar to but thinner than libraries like the [Apache
Portable Runtime](http://apr.apache.org) (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 czmq_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 czmq_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 ();
#else
pid = 0;
#endif
CZMQ uses the GNU autotools system, so non-portable code can use the macros this defines.
It can also use macros defined by the czmq_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 z, 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 czmq_selftest application does this.
• Memory management: CZMQ 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 valgrind.
UNDER THE HOOD
Adding a New Class
If you define a new CZMQ class myclass you need to:
• Write the zmyclass.c and zmyclass.h source files, in src and include respectively.
• Add`#include <zmyclass.h>` to include/czmq.h.
• Add the myclass header and test call to src/czmq_selftest.c.
• Add a reference documentation to doc/zmyclass.txt.
• Add myclass to 'src/Makefile.am` and doc/Makefile.am.
The bin/newclass.sh shell script will automate these steps for you.
Coding Style
In general the zctx class defines the style for the whole library. The overriding rules
for coding style are consistency, clarity, and ease of maintenance. We use the C99
standard for syntax including principally:
• The // comment style.
• Variables definitions placed in or before the code that uses them.
So while ANSI C code might say:
zblob_t *file_buffer; /* Buffer for our file */
... (100 lines of code)
file_buffer = zblob_new ();
...
The style in CZMQ would be:
zblob_t *file_buffer = zblob_new ();
Assertions
We use assertions heavily to catch bad argument values. The CZMQ classes do not attempt to
validate arguments and report errors; bad arguments are treated as fatal application
programming errors.
We also use assertions heavily on calls to system functions that are never supposed to
fail, where failure is to be treated as a fatal non-recoverable error (e.g. running out of
memory).
Assertion code should always take this form:
int rc = some_function (arguments);
assert (rc == 0);
Rather than the side-effect form:
assert (some_function (arguments) == 0);
Since assertions may be removed by an optimizing compiler.
Documentation
Man pages are generated from the class header and source files via the doc/mkman tool, and
similar functionality in the gitdown tool (http://github.com/imatix/gitdown). The header
file for a class must wrap its interface as follows (example is from include/zclock.h):
// @interface
// Sleep for a number of milliseconds
void
zclock_sleep (int msecs);
// Return current system clock as milliseconds
int64_t
zclock_time (void);
// Self test of this class
int
zclock_test (Bool verbose);
// @end
The source file for a class must provide documentation as follows:
/*
@header
...short explanation of class...
@discuss
...longer discussion of how it works...
@end
*/
The source file for a class then provides the self test example as follows:
// @selftest
int64_t start = zclock_time ();
zclock_sleep (10);
assert ((zclock_time () - start) >= 10);
// @end
The template for man pages is in doc/mkman.
Development
CZMQ is developed through a test-driven process that guarantees no memory violations or
leaks in the code:
• Modify a class or method.
• Update the test method for that class.
• Run the selftest script, which uses the Valgrind memcheck tool.
• Repeat until perfect.
Porting CZMQ
When you try CZMQ on an OS that it’s not been used on (ever, or for a while), you will hit
code that does not compile. In some cases the patches are trivial, in other cases (usually
when porting to Windows), the work needed to build equivalent functionality may be quite
heavy. In any case, the benefit is that once ported, the functionality is available to all
applications.
Before attempting to patch code for portability, please read the czmq_prelude.h header
file. There are several typical types of changes you may need to make to get functionality
working on a specific operating system:
• Defining typedefs which are missing on that specific compiler: do this in
czmq_prelude.h.
• Defining macros that rename exotic library functions to more conventional names: do
this in czmq_prelude.h.
• Reimplementing specific methods to use a non-standard API: this is typically needed on
Windows. Do this in the relevant class, using #ifdefs to properly differentiate code
for different platforms.
The canonical standard operating system for all CZMQ code is Linux, gcc, POSIX.
AUTHORS
The czmq manual was written by the authors in the AUTHORS file.
RESOURCES
Main web site:
Report bugs to the email <zeromq-dev@lists.zeromq.org[1]>
COPYRIGHT
Copyright (c) 1991-2012 iMatix Corporation -- http://www.imatix.com Copyright other
contributors as noted in the AUTHORS file. This file is part of CZMQ, the high-level C
binding for 0MQ: http://czmq.zeromq.org This Source Code Form is subject to the terms of
the Mozilla Public License, v. 2.0. If a copy of the MPL was not distributed with this
file, You can obtain one at http://mozilla.org/MPL/2.0/. LICENSE included with the czmq
distribution.
NOTES
1. zeromq-dev@lists.zeromq.org
mailto:zeromq-dev@lists.zeromq.org