automatic copy-on-write memory slices for C
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Automatic copy-on-write semantic memory slices library for use in C and C++.


See include/cow.h for documentation on each function. See include/cow.hpp for the C++ wrapper API class.


Each function, macro, and type definition in the header will be prefixed with cow_ or COW_. Internal and/or non-prototpyed items use the namespace _cow_ or _COW_.

C++ wrapper API

The C++ interface defines the type Cow, a reference-counted wrapper over cow_t instances that supports cloning through its subtype, Cow::Fake, and automatically ensures the originally created cow_t is not destroyed until all its clones are, as well as the namespace _cow_util which contains memory accessor helpers Span<T> and Slice<T> (aka Span<T>::Slice).

There are also the following:

  • cow/area.hpp (namespace _cow_util) - The Area type is a copy-constructable wrapper around both Cow and Cow::Fake, allowing for implicit cloning.
  • cow/slice.hpp (namespace _cow_util) - Contains the definitions for Span<T> and Slice<T>. Included automatically by cow.hpp (see above).


Run make to build to build the release (optimised) target of the library. It will create four files: libcow-release.a,, libcow.a, and (wish SONAME versioned symlinks). The latter two are hardlinked to the former two.

Run make debug to build the debug target, which disables optimisations and includes trace messages. It will create two files: libcow-debug.a and The debug target .so does not include a SONAME, nor does it produce the versioning symlinks (unless you manually set LDFLAGS="-Wl,-soname,<version>" and create the symlinks afterwards.)

Each target compiles both a static and dynamic library. You may need to run make clean before switching build targets. To build both targets, run make all.

To disable default target-specific (e.g. optimisation) flags, set TARGET_SPEC_FLAGS=no when running make.

Run sudo make install to install the libraries (static and dynamic) and header files (C and C++). Run sudo make uninstall to remove the libraries and header files.

By default, the install target is /usr/local/. Set the PREFIX variable when running make install / make uninstall to specify a different path.


To build and install with the default configuration.

$ make && sudo make install

Will build with the default optimisations enabled and install the following files/directories:

  • /usr/local/lib/libcow.a
  • /usr/local/lib/ (with SONAME versioned symlinks)
  • /usr/local/include/cow.h
  • /usr/local/include/cow.hpp
  • /usr/local/include/cow/


  • The release target specifies -march=native by default. This may be undesirable, if so, set TARGET_CPU="" when running make.
  • Many optimisation flags for the release configuration are specific to GCC (with graphite enabled by default), if builds on other compilers (or non-graphite enabled GCC builds) complain, either set the OPT_FLAGS env var or remove the problem flags from the Makefile.
  • release builds are stripped by default. run make STRIP=: release to prevent stripping.
  • The targets are all built with -fno-strict-aliasing, but functions in the header file are still annotated with restrict needed. This is just to inform users that the function will assume the pointer is not aliased. (When included in C++, where restrict is not a keyword, we temporarily define it to be __restrict__, which is the GCC equivalent for C++).
  • The debug target .so does not include a SONAME, nor does it produce the output symlinks expected of a SONAME. The release target does. The version is specified in the Makefile.


Link to either libcow.a or (or the debug target libraries), and include the header include/cow.h to your project to use this library. The header should work in C++ projects as well, but there is a C++-specific wrapper API in include/cow.hpp which you can use instead for automatic handling of resources (see above).


Relying on the memfd_create() syscall, Linux >=3.17 and glibc >=2.27 (or equivalent) are required for build. Makefile is tuned towards gcc but with some small modifications should work with gcc-similar compilers such as clang, although this is unintended.

The code itself uses GCC extensions and is targeted at the gnu11 (C11 with GNU extensions) standard. I have no plan on making this portable at all, either for non-Linux platforms or for ISO C compilers. (GNU C is superior to ISO C and thankfully the most widely used compilers accept this).


Non-propagation from clones to the origin:

#include <cow.h>

#define SIZE 4096

int main()
	cow_t* origin = cow_create(SIZE);
	strcpy(cow_ptr(origin), "Hello world");
	cow_t* fake = cow_clone(origin);
	printf("Fake (pre write): %s\n", cow_ptr_of(const char, fake));
	strcpy(cow_ptr(fake), "Hello fake!");

	printf("Real: %s\n", cow_ptr_of(const char, origin));
	printf("Fake: %s\n", cow_ptr_of(const char, fake));

	return 0;

Will print:

$ ./test
Fake (pre write): Hello world
Real: Hello world
Fake: Hello fake!

Notice the first read of fake contains the data written to origin. And that the write of Hello fake! caused only fake to be updated, not origin.

C++ API example

#include <cow.hpp>

#include <cstring>
#include <cstdio>

void write_cow(Cow& to, const char* string)
  strncpy(to.area_as<char>(), string, to.size()-1);

void read_cow(const Cow& from)
  printf("Cow says: %s\n", from.area_as<char>());

int main()
  Cow area(1024);
  write_cow(area, "Initial state");
  Cow::Fake clone = area;
  write_cow(clone, "Cloned state");

  return 0;

Will print:

$ ./test
Cow says: Initial state
Cow says: Cloned state
Cow says: Initial state

The Cow class and its subclass Cow::Fake handles freeing resources automatically. Alternatively, there is the Area class which can act as both (see cow/area.hpp).

What is happening here?

The cloned slice, fake, which is created from origin with the cow_clone() function will contain all the information within origin. The cloned slice can be written to, however, those writes will only be visible to that specific instance of cow_t, even if that cow_t* is again cow_clone()d.

Original or cloned ('fake') slices can be cloned to produce the same effect of a memory slice that starts out containing whatever data is in the original slice created with cow_create(), and can be modified to produce a automatically and lazily copied slice when written to, to which that instance only will have the results of the write operation.

Each fake slice's data will first appear as the original slice that it comes from. Writing to an original slice will propagate the write to all future clones of that slice, or clones of clones of the slice. The function cow_is_fake() can be used to determine if a slice will not propagate its writes to its children.



(this code is not valuable enough to be GPL'd).