Help/variable/CMAKE_ENABLE_EXPORTS.rst


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CMAKE_ENABLE_EXPORTS
--------------------

Specify whether an executable exports symbols for loadable modules.

Normally an executable does not export any symbols because it is the
final program.  It is possible for an executable to export symbols to
be used by loadable modules.  When this property is set to true CMake
will allow other targets to ``link`` to the executable with the
:command:`TARGET_LINK_LIBRARIES` command.  On all platforms a target-level
dependency on the executable is created for targets that link to it.
For DLL platforms an import library will be created for the exported
symbols and then used for linking.  All Windows-based systems
including Cygwin are DLL platforms.  For non-DLL platforms that
require all symbols to be resolved at link time, such as OS X, the
module will ``link`` to the executable using a flag like
``-bundle_loader``.  For other non-DLL platforms the link rule is simply
ignored since the dynamic loader will automatically bind symbols when
the module is loaded.

This variable is used to initialize the target property
:prop_tgt:`ENABLE_EXPORTS` for executable targets.
/*============================================================================
  CMake - Cross Platform Makefile Generator
  Copyright 2000-2009 Kitware, Inc., Insight Software Consortium

  Distributed under the OSI-approved BSD License (the "License");
  see accompanying file Copyright.txt for details.

  This software is distributed WITHOUT ANY WARRANTY; without even the
  implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
  See the License for more information.
============================================================================*/
#include "cmComputeLinkDepends.h"

#include "cmComputeComponentGraph.h"
#include "cmLocalGenerator.h"
#include "cmGlobalGenerator.h"
#include "cmMakefile.h"
#include "cmTarget.h"
#include "cmake.h"
#include "cmAlgorithms.h"

#include <assert.h>

/*

This file computes an ordered list of link items to use when linking a
single target in one configuration.  Each link item is identified by
the string naming it.  A graph of dependencies is created in which
each node corresponds to one item and directed edges lead from nodes to
those which must *follow* them on the link line.  For example, the
graph

  A -> B -> C

will lead to the link line order

  A B C

The set of items placed in the graph is formed with a breadth-first
search of the link dependencies starting from the main target.

There are two types of items: those with known direct dependencies and
those without known dependencies.  We will call the two types "known
items" and "unknown items", respectively.  Known items are those whose
names correspond to targets (built or imported) and those for which an
old-style <item>_LIB_DEPENDS variable is defined.  All other items are
unknown and we must infer dependencies for them.  For items that look
like flags (beginning with '-') we trivially infer no dependencies,
and do not include them in the dependencies of other items.

Known items have dependency lists ordered based on how the user
specified them.  We can use this order to infer potential dependencies
of unknown items.  For example, if link items A and B are unknown and
items X and Y are known, then we might have the following dependency
lists:

  X: Y A B
  Y: A B

The explicitly known dependencies form graph edges

  X -> Y  ,  X -> A  ,  X -> B  ,  Y -> A  ,  Y -> B

We can also infer the edge

  A -> B

because *every* time A appears B is seen on its right.  We do not know
whether A really needs symbols from B to link, but it *might* so we
must preserve their order.  This is the case also for the following
explicit lists:

  X: A B Y
  Y: A B

Here, A is followed by the set {B,Y} in one list, and {B} in the other
list.  The intersection of these sets is {B}, so we can infer that A
depends on at most B.  Meanwhile B is followed by the set {Y} in one
list and {} in the other.  The intersection is {} so we can infer that
B has no dependencies.

Let's make a more complex example by adding unknown item C and
considering these dependency lists:

  X: A B Y C
  Y: A C B

The explicit edges are

  X -> Y  ,  X -> A  ,  X -> B  ,  X -> C  ,  Y -> A  ,  Y -> B  ,  Y -> C

For the unknown items, we infer dependencies by looking at the
"follow" sets:

  A: intersect( {B,Y,C} , {C,B} ) = {B,C} ; infer edges  A -> B  ,  A -> C
  B: intersect( {Y,C}   , {}    ) = {}    ; infer no edges
  C: intersect( {}      , {B}   ) = {}    ; infer no edges

Note that targets are never inferred as dependees because outside
libraries should not depend on them.

------------------------------------------------------------------------------

The initial exploration of dependencies using a BFS associates an
integer index with each link item.  When the graph is built outgoing
edges are sorted by this index.

After the initial exploration of the link interface tree, any
transitive (dependent) shared libraries that were encountered and not
included in the interface are processed in their own BFS.  This BFS
follows only the dependent library lists and not the link interfaces.
They are added to the link items with a mark indicating that the are
transitive dependencies.  Then cmComputeLinkInformation deals with
them on a per-platform basis.

The complete graph formed from all known and inferred dependencies may
not be acyclic, so an acyclic version must be created.
The original graph is converted to a directed acyclic graph in which
each node corresponds to a strongly connected component of the
original graph.  For example, the dependency graph

  X -> A -> B -> C -> A -> Y

contains strongly connected components {X}, {A,B,C}, and {Y}.  The
implied directed acyclic graph (DAG) is

  {X} -> {A,B,C} -> {Y}

We then compute a topological order for the DAG nodes to serve as a
reference for satisfying dependencies efficiently.  We perform the DFS
in reverse order and assign topological order indices counting down so
that the result is as close to the original BFS order as possible
without violating dependencies.

------------------------------------------------------------------------------

The final link entry order is constructed as follows.  We first walk
through and emit the *original* link line as specified by the user.
As each item is emitted, a set of pending nodes in the component DAG
is maintained.  When a pending component has been completely seen, it
is removed from the pending set and its dependencies (following edges
of the DAG) are added.  A trivial component (those with one item) is
complete as soon as its item is seen.  A non-trivial component (one
with more than one item; assumed to be static libraries) is complete
when *all* its entries have been seen *twice* (all entries seen once,
then all entries seen again, not just each entry twice).  A pending
component tracks which items have been seen and a count of how many
times the component needs to be seen (once for trivial components,
twice for non-trivial).  If at any time another component finishes and
re-adds an already pending component, the pending component is reset
so that it needs to be seen in its entirety again.  This ensures that
all dependencies of a component are satisfied no matter where it
appears.

After the original link line has been completed, we append to it the
remaining pending components and their dependencies.  This is done by
repeatedly emitting the first item from the first pending component
and following the same update rules as when traversing the original
link line.  Since the pending components are kept in topological order
they are emitted with minimal repeats (we do not want to emit a
component just to have it added again when another component is
completed later).  This process continues until no pending components
remain.  We know it will terminate because the component graph is
guaranteed to be acyclic.

The final list of items produced by this procedure consists of the
original user link line followed by minimal additional items needed to
satisfy dependencies.  The final list is then filtered to de-duplicate
items that we know the linker will re-use automatically (shared libs).

*/

//----------------------------------------------------------------------------
cmComputeLinkDepends
::cmComputeLinkDepends(const cmGeneratorTarget* target,
                       const std::string& config)
{
  // Store context information.
  this->Target = target;
  this->Makefile = this->Target->Target->GetMakefile();
  this->GlobalGenerator =
      this->Target->GetLocalGenerator()->GetGlobalGenerator();
  this->CMakeInstance = this->GlobalGenerator->GetCMakeInstance();

  // The configuration being linked.
  this->HasConfig = !config.empty();
  this->Config = (this->HasConfig)? config : std::string();
  this->LinkType = this->Target->Target->ComputeLinkType(this->Config);

  // Enable debug mode if requested.
  this->DebugMode = this->Makefile->IsOn("CMAKE_LINK_DEPENDS_DEBUG_MODE");

  // Assume no compatibility until set.
  this->OldLinkDirMode = false;

  // No computation has been done.
  this->CCG = 0;
}

//----------------------------------------------------------------------------
cmComputeLinkDepends::~cmComputeLinkDepends()
{
  cmDeleteAll(this->InferredDependSets);
  delete this->CCG;
}

//----------------------------------------------------------------------------
void cmComputeLinkDepends::SetOldLinkDirMode(bool b)
{
  this->OldLinkDirMode = b;
}

//----------------------------------------------------------------------------
std::vector<cmComputeLinkDepends::LinkEntry> const&
cmComputeLinkDepends::Compute()
{
  // Follow the link dependencies of the target to be linked.
  this->AddDirectLinkEntries();

  // Complete the breadth-first search of dependencies.
  while(!this->BFSQueue.empty())
    {
    // Get the next entry.
    BFSEntry qe = this->BFSQueue.front();
    this->BFSQueue.pop();

    // Follow the entry's dependencies.
    this->FollowLinkEntry(qe);
    }

  // Complete the search of shared library dependencies.
  while(!this->SharedDepQueue.empty())
    {
    // Handle the next entry.
    this->HandleSharedDependency(this->SharedDepQueue.front());
    this->SharedDepQueue.pop();
    }

  // Infer dependencies of targets for which they were not known.
  this->InferDependencies();

  // Cleanup the constraint graph.
  this->CleanConstraintGraph();

  // Display the constraint graph.
  if(this->DebugMode)
    {
    fprintf(stderr,
            "---------------------------------------"
            "---------------------------------------\n");
    fprintf(stderr, "Link dependency analysis for target %s, config %s\n",
            this->Target->GetName().c_str(),
            this->HasConfig?this->Config.c_str():"noconfig");
    this->DisplayConstraintGraph();
    }

  // Compute the final ordering.
  this->OrderLinkEntires();

  // Compute the final set of link entries.
  // Iterate in reverse order so we can keep only the last occurrence
  // of a shared library.
  std::set<int> emmitted;
  for(std::vector<int>::const_reverse_iterator
        li = this->FinalLinkOrder.rbegin(),
        le = this->FinalLinkOrder.rend();
      li != le; ++li)
    {
    int i = *li;
    LinkEntry const& e = this->EntryList[i];
    cmTarget const* t = e.Target;
    // Entries that we know the linker will re-use do not need to be repeated.
    bool uniquify = t && t->GetType() == cmTarget::SHARED_LIBRARY;
    if(!uniquify || emmitted.insert(i).second)
      {
      this->FinalLinkEntries.push_back(e);
      }
    }
  // Reverse the resulting order since we iterated in reverse.
  std::reverse(this->FinalLinkEntries.begin(), this->FinalLinkEntries.end());

  // Display the final set.
  if(this->DebugMode)
    {
    this->DisplayFinalEntries();
    }

  return this->FinalLinkEntries;
}

//----------------------------------------------------------------------------
std::map<std::string, int>::iterator
cmComputeLinkDepends::AllocateLinkEntry(std::string const& item)
{
  std::map<std::string, int>::value_type
    index_entry(item, static_cast<int>(this->EntryList.size()));
  std::map<std::string, int>::iterator
    lei = this->LinkEntryIndex.insert(index_entry).first;
  this->EntryList.push_back(LinkEntry());
  this->InferredDependSets.push_back(0);
  this->EntryConstraintGraph.push_back(EdgeList());
  return lei;
}

//----------------------------------------------------------------------------
int cmComputeLinkDepends::AddLinkEntry(cmLinkItem const& item)
{
  // Check if the item entry has already been added.
  std::map<std::string, int>::iterator lei = this->LinkEntryIndex.find(item);
  if(lei != this->LinkEntryIndex.end())
    {
    // Yes.  We do not need to follow the item's dependencies again.
    return lei->second;
    }

  // Allocate a spot for the item entry.
  lei = this->AllocateLinkEntry(item);

  // Initialize the item entry.
  int index = lei->second;
  LinkEntry& entry = this->EntryList[index];
  entry.Item = item;
  entry.Target = item.Target;
  entry.IsFlag = (!entry.Target && item[0] == '-' && item[1] != 'l' &&
                  item.substr(0, 10) != "-framework");

  // If the item has dependencies queue it to follow them.
  if(entry.Target)
    {
    // Target dependencies are always known.  Follow them.
    BFSEntry qe = {index, 0};
    this->BFSQueue.push(qe);
    }
  else
    {
    // Look for an old-style <item>_LIB_DEPENDS variable.
    std::string var = entry.Item;
    var += "_LIB_DEPENDS";
    if(const char* val = this->Makefile->GetDefinition(var))
      {
      // The item dependencies are known.  Follow them.
      BFSEntry qe = {index, val};
      this->BFSQueue.push(qe);
      }
    else if(!entry.IsFlag)
      {
      // The item dependencies are not known.  We need to infer them.
      this->InferredDependSets[index] = new DependSetList;
      }
    }

  return index;
}

//----------------------------------------------------------------------------
void cmComputeLinkDepends::FollowLinkEntry(BFSEntry const& qe)
{
  // Get this entry representation.
  int depender_index = qe.Index;
  LinkEntry const& entry = this->EntryList[depender_index];

  // Follow the item's dependencies.
  if(entry.Target)
    {
    cmGeneratorTarget* gtgt =
        this->GlobalGenerator->GetGeneratorTarget(entry.Target);
    // Follow the target dependencies.
    if(cmLinkInterface const* iface =
       gtgt->GetLinkInterface(this->Config, this->Target->Target))
      {
      const bool isIface =
                      entry.Target->GetType() == cmTarget::INTERFACE_LIBRARY;
      // This target provides its own link interface information.
      this->AddLinkEntries(depender_index, iface->Libraries);

      if (isIface)
        {
        return;
        }