summaryrefslogtreecommitdiffstats
path: root/Help/manual/cmake-buildsystem.7.rst
blob: 07bf33f08c8fde0836618f59eb39fb0a8431e796 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
.. cmake-manual-description: CMake Buildsystem Reference

cmake-buildsystem(7)
********************

.. only:: html

   .. contents::

Introduction
============

A CMake-based buildsystem is organized as a set of high-level logical
targets.  Each target corresponds to an executable or library, or
is a custom target containing custom commands.  Dependencies between the
targets are expressed in the buildsystem to determine the build order
and the rules for regeneration in response to change.

Binary Targets
==============

Executables and libraries are defined using the :command:`add_executable`
and :command:`add_library` commands.  The resulting binary files have
appropriate prefixes, suffixes and extensions for the platform targeted.
Dependencies between binary targets are expressed using the
:command:`target_link_libraries` command:

.. code-block:: cmake

  add_library(archive archive.cpp zip.cpp lzma.cpp)
  add_executable(zipapp zipapp.cpp)
  target_link_libraries(zipapp archive)

``archive`` is defined as a static library -- an archive containing objects
compiled from ``archive.cpp``, ``zip.cpp``, and ``lzma.cpp``.  ``zipapp``
is defined as an executable formed by compiling and linking ``zipapp.cpp``.
When linking the ``zipapp`` executable, the ``archive`` static library is
linked in.

Binary Executables
------------------

The :command:`add_executable` command defines an executable target:

.. code-block:: cmake

  add_executable(mytool mytool.cpp)

Commands such as :command:`add_custom_command`, which generates rules to be
run at build time can transparently use an :prop_tgt:`EXECUTABLE <TYPE>`
target as a ``COMMAND`` executable.  The buildsystem rules will ensure that
the executable is built before attempting to run the command.

Binary Library Types
--------------------

.. _`Normal Libraries`:

Normal Libraries
^^^^^^^^^^^^^^^^

By default, the :command:`add_library` command defines a static library,
unless a type is specified.  A type may be specified when using the command:

.. code-block:: cmake

  add_library(archive SHARED archive.cpp zip.cpp lzma.cpp)

.. code-block:: cmake

  add_library(archive STATIC archive.cpp zip.cpp lzma.cpp)

The :variable:`BUILD_SHARED_LIBS` variable may be enabled to change the
behavior of :command:`add_library` to build shared libraries by default.

In the context of the buildsystem definition as a whole, it is largely
irrelevant whether particular libraries are ``SHARED`` or ``STATIC`` --
the commands, dependency specifications and other APIs work similarly
regardless of the library type.  The ``MODULE`` library type is
dissimilar in that it is generally not linked to -- it is not used in
the right-hand-side of the :command:`target_link_libraries` command.
It is a type which is loaded as a plugin using runtime techniques.
If the library does not export any unmanaged symbols (e.g. Windows
resource DLL, C++/CLI DLL), it is required that the library not be a
``SHARED`` library because CMake expects ``SHARED`` libraries to export
at least one symbol.

.. code-block:: cmake

  add_library(archive MODULE 7z.cpp)

.. _`Apple Frameworks`:

Apple Frameworks
""""""""""""""""

A ``SHARED`` library may be marked with the :prop_tgt:`FRAMEWORK`
target property to create an OS X or iOS Framework Bundle.
The ``MACOSX_FRAMEWORK_IDENTIFIER`` sets ``CFBundleIdentifier`` key
and it uniquely identifies the bundle.

.. code-block:: cmake

  add_library(MyFramework SHARED MyFramework.cpp)
  set_target_properties(MyFramework PROPERTIES
    FRAMEWORK TRUE
    FRAMEWORK_VERSION A
    MACOSX_FRAMEWORK_IDENTIFIER org.cmake.MyFramework
  )

.. _`Object Libraries`:

Object Libraries
^^^^^^^^^^^^^^^^

The ``OBJECT`` library type is also not linked to. It defines a non-archival
collection of object files resulting from compiling the given source files.
The object files collection can be used as source inputs to other targets:

.. code-block:: cmake

  add_library(archive OBJECT archive.cpp zip.cpp lzma.cpp)

  add_library(archiveExtras STATIC $<TARGET_OBJECTS:archive> extras.cpp)

  add_executable(test_exe $<TARGET_OBJECTS:archive> test.cpp)

``OBJECT`` libraries may only be used locally as sources in a buildsystem --
they may not be installed, exported, or used in the right hand side of
:command:`target_link_libraries`.  They also may not be used as the ``TARGET``
in a use of the :command:`add_custom_command(TARGET)` command signature.

Although object libraries may not be named directly in calls to
the :command:`target_link_libraries` command, they can be "linked"
indirectly by using an :ref:`Interface Library <Interface Libraries>`
whose :prop_tgt:`INTERFACE_SOURCES` target property is set to name
``$<TARGET_OBJECTS:objlib>``.

Build Specification and Usage Requirements
==========================================

The :command:`target_include_directories`, :command:`target_compile_definitions`
and :command:`target_compile_options` commands specify the build specifications
and the usage requirements of binary targets.  The commands populate the
:prop_tgt:`INCLUDE_DIRECTORIES`, :prop_tgt:`COMPILE_DEFINITIONS` and
:prop_tgt:`COMPILE_OPTIONS` target properties respectively, and/or the
:prop_tgt:`INTERFACE_INCLUDE_DIRECTORIES`, :prop_tgt:`INTERFACE_COMPILE_DEFINITIONS`
and :prop_tgt:`INTERFACE_COMPILE_OPTIONS` target properties.

Each of the commands has a ``PRIVATE``, ``PUBLIC`` and ``INTERFACE`` mode.  The
``PRIVATE`` mode populates only the non-``INTERFACE_`` variant of the target
property and the ``INTERFACE`` mode populates only the ``INTERFACE_`` variants.
The ``PUBLIC`` mode populates both variants of the respective target property.
Each command may be invoked with multiple uses of each keyword:

.. code-block:: cmake

  target_compile_definitions(archive
    PRIVATE BUILDING_WITH_LZMA
    INTERFACE USING_ARCHIVE_LIB
  )

Note that usage requirements are not designed as a way to make downstreams
use particular :prop_tgt:`COMPILE_OPTIONS` or
:prop_tgt:`COMPILE_DEFINITIONS` etc for convenience only.  The contents of
the properties must be **requirements**, not merely recommendations or
convenience.

See the :ref:`Creating Relocatable Packages` section of the
:manual:`cmake-packages(7)` manual for discussion of additional care
that must be taken when specifying usage requirements while creating
packages for redistribution.

Target Properties
-----------------

The contents of the :prop_tgt:`INCLUDE_DIRECTORIES`,
:prop_tgt:`COMPILE_DEFINITIONS` and :prop_tgt:`COMPILE_OPTIONS` target
properties are used appropriately when compiling the source files of a
binary target.

Entries in the :prop_tgt:`INCLUDE_DIRECTORIES` are added to the compile line
with ``-I`` or ``-isystem`` prefixes and in the order of appearance in the
property value.

Entries in the :prop_tgt:`COMPILE_DEFINITIONS` are prefixed with ``-D`` or
``/D`` and added to the compile line in an unspecified order.  The
:prop_tgt:`DEFINE_SYMBOL` target property is also added as a compile
definition as a special convenience case for ``SHARED`` and ``MODULE``
library targets.

Entries in the :prop_tgt:`COMPILE_OPTIONS` are escaped for the shell and added
in the order of appearance in the property value.  Several compile options have
special separate handling, such as :prop_tgt:`POSITION_INDEPENDENT_CODE`.

The contents of the :prop_tgt:`INTERFACE_INCLUDE_DIRECTORIES`,
:prop_tgt:`INTERFACE_COMPILE_DEFINITIONS` and
:prop_tgt:`INTERFACE_COMPILE_OPTIONS` target properties are
*Usage Requirements* -- they specify content which consumers
must use to correctly compile and link with the target they appear on.
For any binary target, the contents of each ``INTERFACE_`` property on
each target specified in a :command:`target_link_libraries` command is
consumed:

.. code-block:: cmake

  set(srcs archive.cpp zip.cpp)
  if (LZMA_FOUND)
    list(APPEND srcs lzma.cpp)
  endif()
  add_library(archive SHARED ${srcs})
  if (LZMA_FOUND)
    # The archive library sources are compiled with -DBUILDING_WITH_LZMA
    target_compile_definitions(archive PRIVATE BUILDING_WITH_LZMA)
  endif()
  target_compile_definitions(archive INTERFACE USING_ARCHIVE_LIB)

  add_executable(consumer)
  # Link consumer to archive and consume its usage requirements. The consumer
  # executable sources are compiled with -DUSING_ARCHIVE_LIB.
  target_link_libraries(consumer archive)

Because it is common to require that the source directory and corresponding
build directory are added to the :prop_tgt:`INCLUDE_DIRECTORIES`, the
:variable:`CMAKE_INCLUDE_CURRENT_DIR` variable can be enabled to conveniently
add the corresponding directories to the :prop_tgt:`INCLUDE_DIRECTORIES` of
all targets.  The variable :variable:`CMAKE_INCLUDE_CURRENT_DIR_IN_INTERFACE`
can be enabled to add the corresponding directories to the
:prop_tgt:`INTERFACE_INCLUDE_DIRECTORIES` of all targets.  This makes use of
targets in multiple different directories convenient through use of the
:command:`target_link_libraries` command.


.. _`Target Usage Requirements`:

Transitive Usage Requirements
-----------------------------

The usage requirements of a target can transitively propagate to dependents.
The :command:`target_link_libraries` command has ``PRIVATE``,
``INTERFACE`` and ``PUBLIC`` keywords to control the propagation.

.. code-block:: cmake

  add_library(archive archive.cpp)
  target_compile_definitions(archive INTERFACE USING_ARCHIVE_LIB)

  add_library(serialization serialization.cpp)
  target_compile_definitions(serialization INTERFACE USING_SERIALIZATION_LIB)

  add_library(archiveExtras extras.cpp)
  target_link_libraries(archiveExtras PUBLIC archive)
  target_link_libraries(archiveExtras PRIVATE serialization)
  # archiveExtras is compiled with -DUSING_ARCHIVE_LIB
  # and -DUSING_SERIALIZATION_LIB

  add_executable(consumer consumer.cpp)
  # consumer is compiled with -DUSING_ARCHIVE_LIB
  target_link_libraries(consumer archiveExtras)

Because ``archive`` is a ``PUBLIC`` dependency of ``archiveExtras``, the
usage requirements of it are propagated to ``consumer`` too.  Because
``serialization`` is a ``PRIVATE`` dependency of ``archive``, the usage
requirements of it are not propagated to ``consumer``.

Generally, a dependency should be specified in a use of
:command:`target_link_libraries` with the ``PRIVATE`` keyword if it is used by
only the implementation of a library, and not in the header files.  If a
dependency is additionally used in the header files of a library (e.g. for
class inheritance), then it should be specified as a ``PUBLIC`` dependency.
A dependency which is not used by the implementation of a library, but only by
its headers should be specified as an ``INTERFACE`` dependency.  The
:command:`target_link_libraries` command may be invoked with multiple uses of
each keyword:

.. code-block:: cmake

  target_link_libraries(archiveExtras
    PUBLIC archive
    PRIVATE serialization
  )

Usage requirements are propagated by reading the ``INTERFACE_`` variants
of target properties from dependencies and appending the values to the
non-``INTERFACE_`` variants of the operand.  For example, the
:prop_tgt:`INTERFACE_INCLUDE_DIRECTORIES` of dependencies is read and
appended to the :prop_tgt:`INCLUDE_DIRECTORIES` of the operand.  In cases
where order is relevant and maintained, and the order resulting from the
:command:`target_link_libraries` calls does not allow correct compilation,
use of an appropriate command to set the property directly may update the
order.

For example, if the linked libraries for a target must be specified
in the order ``lib1`` ``lib2`` ``lib3`` , but the include directories must
be specified in the order ``lib3`` ``lib1`` ``lib2``:

.. code-block:: cmake

  target_link_libraries(myExe lib1 lib2 lib3)
  target_include_directories(myExe
    PRIVATE $<TARGET_PROPERTY:lib3,INTERFACE_INCLUDE_DIRECTORIES>)

Note that care must be taken when specifying usage requirements for targets
which will be exported for installation using the :command:`install(EXPORT)`
command.  See :ref:`Creating Packages` for more.

.. _`Compatible Interface Properties`:

Compatible Interface Properties
-------------------------------

Some target properties are required to be compatible between a target and
the interface of each dependency.  For example, the
:prop_tgt:`POSITION_INDEPENDENT_CODE` target property may specify a
boolean value of whether a target should be compiled as
position-independent-code, which has platform-specific consequences.
A target may also specify the usage requirement
:prop_tgt:`INTERFACE_POSITION_INDEPENDENT_CODE` to communicate that
consumers must be compiled as position-independent-code.

.. code-block:: cmake

  add_executable(exe1 exe1.cpp)
  set_property(TARGET exe1 PROPERTY POSITION_INDEPENDENT_CODE ON)

  add_library(lib1 SHARED lib1.cpp)
  set_property(TARGET lib1 PROPERTY INTERFACE_POSITION_INDEPENDENT_CODE ON)

  add_executable(exe2 exe2.cpp)
  target_link_libraries(exe2 lib1)

Here, both ``exe1`` and ``exe2`` will be compiled as position-independent-code.
``lib1`` will also be compiled as position-independent-code because that is the
default setting for ``SHARED`` libraries.  If dependencies have conflicting,
non-compatible requirements :manual:`cmake(1)` issues a diagnostic:

.. code-block:: cmake

  add_library(lib1 SHARED lib1.cpp)
  set_property(TARGET lib1 PROPERTY INTERFACE_POSITION_INDEPENDENT_CODE ON)

  add_library(lib2 SHARED lib2.cpp)
  set_property(TARGET lib2 PROPERTY INTERFACE_POSITION_INDEPENDENT_CODE OFF)

  add_executable(exe1 exe1.cpp)
  target_link_libraries(exe1 lib1)
  set_property(TARGET exe1 PROPERTY POSITION_INDEPENDENT_CODE OFF)

  add_executable(exe2 exe2.cpp)
  target_link_libraries(exe2 lib1 lib2)

The ``lib1`` requirement ``INTERFACE_POSITION_INDEPENDENT_CODE`` is not
"compatible" with the ``POSITION_INDEPENDENT_CODE`` property of the ``exe1``
target.  The library requires that consumers are built as
position-independent-code, while the executable specifies to not built as
position-independent-code, so a diagnostic is issued.

The ``lib1`` and ``lib2`` requirements are not "compatible".  One of them
requires that consumers are built as position-independent-code, while
the other requires that consumers are not built as position-independent-code.
Because ``exe2`` links to both and they are in conflict, a diagnostic is
issued.

To be "compatible", the :prop_tgt:`POSITION_INDEPENDENT_CODE` property,
if set must be either the same, in a boolean sense, as the
:prop_tgt:`INTERFACE_POSITION_INDEPENDENT_CODE` property of all transitively
specified dependencies on which that property is set.

This property of "compatible interface requirement" may be extended to other
properties by specifying the property in the content of the
:prop_tgt:`COMPATIBLE_INTERFACE_BOOL` target property.  Each specified property
must be compatible between the consuming target and the corresponding property
with an ``INTERFACE_`` prefix from each dependency:

.. code-block:: cmake

  add_library(lib1Version2 SHARED lib1_v2.cpp)
  set_property(TARGET lib1Version2 PROPERTY INTERFACE_CUSTOM_PROP ON)
  set_property(TARGET lib1Version2 APPEND PROPERTY
    COMPATIBLE_INTERFACE_BOOL CUSTOM_PROP
  )

  add_library(lib1Version3 SHARED lib1_v3.cpp)
  set_property(TARGET lib1Version3 PROPERTY INTERFACE_CUSTOM_PROP OFF)

  add_executable(exe1 exe1.cpp)
  target_link_libraries(exe1 lib1Version2) # CUSTOM_PROP will be ON

  add_executable(exe2 exe2.cpp)
  target_link_libraries(exe2 lib1Version2 lib1Version3) # Diagnostic

Non-boolean properties may also participate in "compatible interface"
computations.  Properties specified in the
:prop_tgt:`COMPATIBLE_INTERFACE_STRING`
property must be either unspecified or compare to the same string among
all transitively specified dependencies. This can be useful to ensure
that multiple incompatible versions of a library are not linked together
through transitive requirements of a target:

.. code-block:: cmake

  add_library(lib1Version2 SHARED lib1_v2.cpp)
  set_property(TARGET lib1Version2 PROPERTY INTERFACE_LIB_VERSION 2)
  set_property(TARGET lib1Version2 APPEND PROPERTY
    COMPATIBLE_INTERFACE_STRING LIB_VERSION
  )

  add_library(lib1Version3 SHARED lib1_v3.cpp)
  set_property(TARGET lib1Version3 PROPERTY INTERFACE_LIB_VERSION 3)

  add_executable(exe1 exe1.cpp)
  target_link_libraries(exe1 lib1Version2) # LIB_VERSION will be "2"

  add_executable(exe2 exe2.cpp)
  target_link_libraries(exe2 lib1Version2 lib1Version3) # Diagnostic

The :prop_tgt:`COMPATIBLE_INTERFACE_NUMBER_MAX` target property specifies
that content will be evaluated numerically and the maximum number among all
specified will be calculated:

.. code-block:: cmake

  add_library(lib1Version2 SHARED lib1_v2.cpp)
  set_property(TARGET lib1Version2 PROPERTY INTERFACE_CONTAINER_SIZE_REQUIRED 200)
  set_property(TARGET lib1Version2 APPEND PROPERTY
    COMPATIBLE_INTERFACE_NUMBER_MAX CONTAINER_SIZE_REQUIRED
  )

  add_library(lib1Version3 SHARED lib1_v3.cpp)
  set_property(TARGET lib1Version3 PROPERTY INTERFACE_CONTAINER_SIZE_REQUIRED 1000)

  add_executable(exe1 exe1.cpp)
  # CONTAINER_SIZE_REQUIRED will be "200"
  target_link_libraries(exe1 lib1Version2)

  add_executable(exe2 exe2.cpp)
  # CONTAINER_SIZE_REQUIRED will be "1000"
  target_link_libraries(exe2 lib1Version2 lib1Version3)

Similarly, the :prop_tgt:`COMPATIBLE_INTERFACE_NUMBER_MIN` may be used to
calculate the numeric minimum value for a property from dependencies.

Each calculated "compatible" property value may be read in the consumer at
generate-time using generator expressions.

Note that for each dependee, the set of properties specified in each
compatible interface property must not intersect with the set specified in
any of the other properties.

Property Origin Debugging
-------------------------

Because build specifications can be determined by dependencies, the lack of
locality of code which creates a target and code which is responsible for
setting build specifications may make the code more difficult to reason about.
:manual:`cmake(1)` provides a debugging facility to print the origin of the
contents of properties which may be determined by dependencies.  The properties
which can be debugged are listed in the
:variable:`CMAKE_DEBUG_TARGET_PROPERTIES` variable documentation:

.. code-block:: cmake

  set(CMAKE_DEBUG_TARGET_PROPERTIES
    INCLUDE_DIRECTORIES
    COMPILE_DEFINITIONS
    POSITION_INDEPENDENT_CODE
    CONTAINER_SIZE_REQUIRED
    LIB_VERSION
  )
  add_executable(exe1 exe1.cpp)

In the case of properties listed in :prop_tgt:`COMPATIBLE_INTERFACE_BOOL` or
:prop_tgt:`COMPATIBLE_INTERFACE_STRING`, the debug output shows which target
was responsible for setting the property, and which other dependencies also
defined the property.  In the case of
:prop_tgt:`COMPATIBLE_INTERFACE_NUMBER_MAX` and
:prop_tgt:`COMPATIBLE_INTERFACE_NUMBER_MIN`, the debug output shows the
value of the property from each dependency, and whether the value determines
the new extreme.

Build Specification with Generator Expressions
----------------------------------------------

Build specifications may use
:manual:`generator expressions <cmake-generator-expressions(7)>` containing
content which may be conditional or known only at generate-time.  For example,
the calculated "compatible" value of a property may be read with the
``TARGET_PROPERTY`` expression:

.. code-block:: cmake

  add_library(lib1Version2 SHARED lib1_v2.cpp)
  set_property(TARGET lib1Version2 PROPERTY
    INTERFACE_CONTAINER_SIZE_REQUIRED 200)
  set_property(TARGET lib1Version2 APPEND PROPERTY
    COMPATIBLE_INTERFACE_NUMBER_MAX CONTAINER_SIZE_REQUIRED
  )

  add_executable(exe1 exe1.cpp)
  target_link_libraries(exe1 lib1Version2)
  target_compile_definitions(exe1 PRIVATE
      CONTAINER_SIZE=$<TARGET_PROPERTY:CONTAINER_SIZE_REQUIRED>
  )

In this case, the ``exe1`` source files will be compiled with
``-DCONTAINER_SIZE=200``.

Configuration determined build specifications may be conveniently set using
the ``CONFIG`` generator expression.

.. code-block:: cmake

  target_compile_definitions(exe1 PRIVATE
      $<$<CONFIG:Debug>:DEBUG_BUILD>
  )

The ``CONFIG`` parameter is compared case-insensitively with the configuration
being built.  In the presence of :prop_tgt:`IMPORTED` targets, the content of
:prop_tgt:`MAP_IMPORTED_CONFIG_DEBUG <MAP_IMPORTED_CONFIG_<CONFIG>>` is also
accounted for by this expression.

Some buildsystems generated by :manual:`cmake(1)` have a predetermined
build-configuration set in the :variable:`CMAKE_BUILD_TYPE` variable.  The
buildsystem for the IDEs such as Visual Studio and Xcode are generated
independent of the build-configuration, and the actual build configuration
is not known until build-time.  Therefore, code such as

.. code-block:: cmake

  string(TOLOWER ${CMAKE_BUILD_TYPE} _type)
  if (_type STREQUAL debug)
    target_compile_definitions(exe1 PRIVATE DEBUG_BUILD)
  endif()

may appear to work for ``Makefile`` based and ``Ninja`` generators, but is not
portable to IDE generators.  Additionally, the :prop_tgt:`IMPORTED`
configuration-mappings are not accounted for with code like this, so it should
be avoided.

The unary ``TARGET_PROPERTY`` generator expression and the ``TARGET_POLICY``
generator expression are evaluated with the consuming target context.  This
means that a usage requirement specification may be evaluated differently based
on the consumer:

.. code-block:: cmake

  add_library(lib1 lib1.cpp)
  target_compile_definitions(lib1 INTERFACE
    $<$<STREQUAL:$<TARGET_PROPERTY:TYPE>,EXECUTABLE>:LIB1_WITH_EXE>
    $<$<STREQUAL:$<TARGET_PROPERTY:TYPE>,SHARED_LIBRARY>:LIB1_WITH_SHARED_LIB>
    $<$<TARGET_POLICY:CMP0041>:CONSUMER_CMP0041_NEW>
  )

  add_executable(exe1 exe1.cpp)
  target_link_libraries(exe1 lib1)

  cmake_policy(SET CMP0041 NEW)

  add_library(shared_lib shared_lib.cpp)
  target_link_libraries(shared_lib lib1)

The ``exe1`` executable will be compiled with ``-DLIB1_WITH_EXE``, while the
``shared_lib`` shared library will be compiled with ``-DLIB1_WITH_SHARED_LIB``
and ``-DCONSUMER_CMP0041_NEW``, because policy :policy:`CMP0041` is
``NEW`` at the point where the ``shared_lib`` target is created.

The ``BUILD_INTERFACE`` expression wraps requirements which are only used when
consumed from a target in the same buildsystem, or when consumed from a target
exported to the build directory using the :command:`export` command.  The
``INSTALL_INTERFACE`` expression wraps requirements which are only used when
consumed from a target which has been installed and exported with the
:command:`install(EXPORT)` command:

.. code-block:: cmake

  add_library(ClimbingStats climbingstats.cpp)
  target_compile_definitions(ClimbingStats INTERFACE
    $<BUILD_INTERFACE:ClimbingStats_FROM_BUILD_LOCATION>
    $<INSTALL_INTERFACE:ClimbingStats_FROM_INSTALLED_LOCATION>
  )
  install(TARGETS ClimbingStats EXPORT libExport ${InstallArgs})
  install(EXPORT libExport NAMESPACE Upstream::
          DESTINATION lib/cmake/ClimbingStats)
  export(EXPORT libExport NAMESPACE Upstream::)

  add_executable(exe1 exe1.cpp)
  target_link_libraries(exe1 ClimbingStats)

In this case, the ``exe1`` executable will be compiled with
``-DClimbingStats_FROM_BUILD_LOCATION``.  The exporting commands generate
:prop_tgt:`IMPORTED` targets with either the ``INSTALL_INTERFACE`` or the
``BUILD_INTERFACE`` omitted, and the ``*_INTERFACE`` marker stripped away.
A separate project consuming the ``ClimbingStats`` package would contain:

.. code-block:: cmake

  find_package(ClimbingStats REQUIRED)

  add_executable(Downstream main.cpp)
  target_link_libraries(Downstream Upstream::ClimbingStats)

Depending on whether the ``ClimbingStats`` package was used from the build
location or the install location, the ``Downstream`` target would be compiled
with either ``-DClimbingStats_FROM_BUILD_LOCATION`` or
``-DClimbingStats_FROM_INSTALL_LOCATION``.  For more about packages and
exporting see the :manual:`cmake-packages(7)` manual.

.. _`Include Directories and Usage Requirements`:

Include Directories and Usage Requirements
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Include directories require some special consideration when specified as usage
requirements and when used with generator expressions.  The
:command:`target_include_directories` command accepts both relative and
absolute include directories:

.. code-block:: cmake

  add_library(lib1 lib1.cpp)
  target_include_directories(lib1 PRIVATE
    /absolute/path
    relative/path
  )

Relative paths are interpreted relative to the source directory where the
command appears.  Relative paths are not allowed in the
:prop_tgt:`INTERFACE_INCLUDE_DIRECTORIES` of :prop_tgt:`IMPORTED` targets.

In cases where a non-trivial generator expression is used, the
``INSTALL_PREFIX`` expression may be used within the argument of an
``INSTALL_INTERFACE`` expression.  It is a replacement marker which
expands to the installation prefix when imported by a consuming project.

Include directories usage requirements commonly differ between the build-tree
and the install-tree.  The ``BUILD_INTERFACE`` and ``INSTALL_INTERFACE``
generator expressions can be used to describe separate usage requirements
based on the usage location.  Relative paths are allowed within the
``INSTALL_INTERFACE`` expression and are interpreted relative to the
installation prefix.  For example:

.. code-block:: cmake

  add_library(ClimbingStats climbingstats.cpp)
  target_include_directories(ClimbingStats INTERFACE
    $<BUILD_INTERFACE:${CMAKE_CURRENT_BINARY_DIR}/generated>
    $<INSTALL_INTERFACE:/absolute/path>
    $<INSTALL_INTERFACE:relative/path>
    $<INSTALL_INTERFACE:$<INSTALL_PREFIX>/$<CONFIG>/generated>
  )

Two convenience APIs are provided relating to include directories usage
requirements.  The :variable:`CMAKE_INCLUDE_CURRENT_DIR_IN_INTERFACE` variable
may be enabled, with an equivalent effect to:

.. code-block:: cmake

  set_property(TARGET tgt APPEND PROPERTY INTERFACE_INCLUDE_DIRECTORIES
    $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR};${CMAKE_CURRENT_BINARY_DIR}>
  )

for each target affected.  The convenience for installed targets is
an ``INCLUDES DESTINATION`` component with the :command:`install(TARGETS)`
command:

.. code-block:: cmake

  install(TARGETS foo bar bat EXPORT tgts ${dest_args}
    INCLUDES DESTINATION include
  )
  install(EXPORT tgts ${other_args})
  install(FILES ${headers} DESTINATION include)

This is equivalent to appending ``${CMAKE_INSTALL_PREFIX}/include`` to the
:prop_tgt:`INTERFACE_INCLUDE_DIRECTORIES` of each of the installed
:prop_tgt:`IMPORTED` targets when generated by :command:`install(EXPORT)`.

When the :prop_tgt:`INTERFACE_INCLUDE_DIRECTORIES` of an
:ref:`imported target <Imported targets>` is consumed, the entries in the
property are treated as ``SYSTEM`` include directories, as if they were
listed in the :prop_tgt:`INTERFACE_SYSTEM_INCLUDE_DIRECTORIES` of the
dependency. This can result in omission of compiler warnings for headers
found in those directories.  This behavior for :ref:`imported targets` may
be controlled with the :prop_tgt:`NO_SYSTEM_FROM_IMPORTED` target property.

If a binary target is linked transitively to a Mac OX framework, the
``Headers`` directory of the framework is also treated as a usage requirement.
This has the same effect as passing the framework directory as an include
directory.

Link Libraries and Generator Expressions
----------------------------------------

Like build specifications, :prop_tgt:`link libraries <LINK_LIBRARIES>` may be
specified with generator expression conditions.  However, as consumption of
usage requirements is based on collection from linked dependencies, there is
an additional limitation that the link dependencies must form a "directed
acyclic graph".  That is, if linking to a target is dependent on the value of
a target property, that target property may not be dependent on the linked
dependencies:

.. code-block:: cmake

  add_library(lib1 lib1.cpp)
  add_library(lib2 lib2.cpp)
  target_link_libraries(lib1 PUBLIC
    $<$<TARGET_PROPERTY:POSITION_INDEPENDENT_CODE>:lib2>
  )
  add_library(lib3 lib3.cpp)
  set_property(TARGET lib3 PROPERTY INTERFACE_POSITION_INDEPENDENT_CODE ON)

  add_executable(exe1 exe1.cpp)
  target_link_libraries(exe1 lib1 lib3)

As the value of the :prop_tgt:`POSITION_INDEPENDENT_CODE` property of
the ``exe1`` target is dependent on the linked libraries (``lib3``), and the
edge of linking ``exe1`` is determined by the same
:prop_tgt:`POSITION_INDEPENDENT_CODE` property, the dependency graph above
contains a cycle.  :manual:`cmake(1)` issues a diagnostic in this case.

.. _`Output Artifacts`:

Output Artifacts
----------------

The buildsystem targets created by the :command:`add_library` and
:command:`add_executable` commands create rules to create binary outputs.
The exact output location of the binaries can only be determined at
generate-time because it can depend on the build-configuration and the
link-language of linked dependencies etc.  ``TARGET_FILE``,
``TARGET_LINKER_FILE`` and related expressions can be used to access the
name and location of generated binaries.  These expressions do not work
for ``OBJECT`` libraries however, as there is no single file generated
by such libraries which is relevant to the expressions.

There are three kinds of output artifacts that may be build by targets
as detailed in the following sections.  Their classification differs
between DLL platforms and non-DLL platforms.  All Windows-based
systems including Cygwin are DLL platforms.

.. _`Runtime Output Artifacts`:

Runtime Output Artifacts
^^^^^^^^^^^^^^^^^^^^^^^^

A *runtime* output artifact of a buildsystem target may be:

* The executable file (e.g. ``.exe``) of an executable target
  created by the :command:`add_executable` command.

* On DLL platforms: the executable file (e.g. ``.dll``) of a shared
  library target created by the :command:`add_library` command
  with the ``SHARED`` option.

The :prop_tgt:`RUNTIME_OUTPUT_DIRECTORY` and :prop_tgt:`RUNTIME_OUTPUT_NAME`
target properties may be used to control runtime output artifact locations
and names in the build tree.

.. _`Library Output Artifacts`:

Library Output Artifacts
^^^^^^^^^^^^^^^^^^^^^^^^

A *library* output artifact of a buildsystem target may be:

* The loadable module file (e.g. ``.dll`` or ``.so``) of a module
  library target created by the :command:`add_library` command
  with the ``MODULE`` option.

* On non-DLL platforms: the shared library file (e.g. ``.so`` or ``.dylib``)
  of a shared shared library target created by the :command:`add_library`
  command with the ``SHARED`` option.

The :prop_tgt:`LIBRARY_OUTPUT_DIRECTORY` and :prop_tgt:`LIBRARY_OUTPUT_NAME`
target properties may be used to control library output artifact locations
and names in the build tree.

.. _`Archive Output Artifacts`:

Archive Output Artifacts
^^^^^^^^^^^^^^^^^^^^^^^^

An *archive* output artifact of a buildsystem target may be:

* The static library file (e.g. ``.lib`` or ``.a``) of a static
  library target created by the :command:`add_library` command
  with the ``STATIC`` option.

* On DLL platforms: the import library file (e.g. ``.lib``) of a shared
  library target created by the :command:`add_library` command
  with the ``SHARED`` option.  This file is only guaranteed to exist if
  the library exports at least one unmanaged symbol.

* On DLL platforms: the import library file (e.g. ``.lib``) of an
  executable target created by the :command:`add_executable` command
  when its :prop_tgt:`ENABLE_EXPORTS` target property is set.

The :prop_tgt:`ARCHIVE_OUTPUT_DIRECTORY` and :prop_tgt:`ARCHIVE_OUTPUT_NAME`
target properties may be used to control archive output artifact locations
and names in the build tree.

Directory-Scoped Commands
-------------------------

The :command:`target_include_directories`,
:command:`target_compile_definitions` and
:command:`target_compile_options` commands have an effect on only one
target at a time.  The commands :command:`add_definitions`,
:command:`add_compile_options` and :command:`include_directories` have
a similar function, but operate at directory scope instead of target
scope for convenience.

Pseudo Targets
==============

Some target types do not represent outputs of the buildsystem, but only inputs
such as external dependencies, aliases or other non-build artifacts.  Pseudo
targets are not represented in the generated buildsystem.

.. _`Imported Targets`:

Imported Targets
----------------

An :prop_tgt:`IMPORTED` target represents a pre-existing dependency.  Usually
such targets are defined by an upstream package and should be treated as
immutable.  It is not possible to use an :prop_tgt:`IMPORTED` target in the
left-hand-side of the :command:`target_compile_definitions`,
:command:`target_include_directories`, :command:`target_compile_options` or
:command:`target_link_libraries` commands, as that would be an attempt to
modify it.  :prop_tgt:`IMPORTED` targets are designed to be used only in the
right-hand-side of those commands.

:prop_tgt:`IMPORTED` targets may have the same usage requirement properties
populated as binary targets, such as
:prop_tgt:`INTERFACE_INCLUDE_DIRECTORIES`,
:prop_tgt:`INTERFACE_COMPILE_DEFINITIONS`,
:prop_tgt:`INTERFACE_COMPILE_OPTIONS`,
:prop_tgt:`INTERFACE_LINK_LIBRARIES`, and
:prop_tgt:`INTERFACE_POSITION_INDEPENDENT_CODE`.

The :prop_tgt:`LOCATION` may also be read from an IMPORTED target, though there
is rarely reason to do so.  Commands such as :command:`add_custom_command` can
transparently use an :prop_tgt:`IMPORTED` :prop_tgt:`EXECUTABLE <TYPE>` target
as a ``COMMAND`` executable.

The scope of the definition of an :prop_tgt:`IMPORTED` target is the directory
where it was defined.  It may be accessed and used from subdirectories, but
not from parent directories or sibling directories.  The scope is similar to
the scope of a cmake variable.

It is also possible to define a ``GLOBAL`` :prop_tgt:`IMPORTED` target which is
accessible globally in the buildsystem.

See the :manual:`cmake-packages(7)` manual for more on creating packages
with :prop_tgt:`IMPORTED` targets.

.. _`Alias Targets`:

Alias Targets
-------------

An ``ALIAS`` target is a name which may be used interchangably with
a binary target name in read-only contexts.  A primary use-case for ``ALIAS``
targets is for example or unit test executables accompanying a library, which
may be part of the same buildsystem or built separately based on user
configuration.

.. code-block:: cmake

  add_library(lib1 lib1.cpp)
  install(TARGETS lib1 EXPORT lib1Export ${dest_args})
  install(EXPORT lib1Export NAMESPACE Upstream:: ${other_args})

  add_library(Upstream::lib1 ALIAS lib1)

In another directory, we can link unconditionally to the ``Upstream::lib1``
target, which may be an :prop_tgt:`IMPORTED` target from a package, or an
``ALIAS`` target if built as part of the same buildsystem.

.. code-block:: cmake

  if (NOT TARGET Upstream::lib1)
    find_package(lib1 REQUIRED)
  endif()
  add_executable(exe1 exe1.cpp)
  target_link_libraries(exe1 Upstream::lib1)

``ALIAS`` targets are not mutable, installable or exportable.  They are
entirely local to the buildsystem description.  A name can be tested for
whether it is an ``ALIAS`` name by reading the :prop_tgt:`ALIASED_TARGET`
property from it:

.. code-block:: cmake

  get_target_property(_aliased Upstream::lib1 ALIASED_TARGET)
  if(_aliased)
    message(STATUS "The name Upstream::lib1 is an ALIAS for ${_aliased}.")
  endif()

.. _`Interface Libraries`:

Interface Libraries
-------------------

An ``INTERFACE`` target has no :prop_tgt:`LOCATION` and is mutable, but is
otherwise similar to an :prop_tgt:`IMPORTED` target.

It may specify usage requirements such as
:prop_tgt:`INTERFACE_INCLUDE_DIRECTORIES`,
:prop_tgt:`INTERFACE_COMPILE_DEFINITIONS`,
:prop_tgt:`INTERFACE_COMPILE_OPTIONS`,
:prop_tgt:`INTERFACE_LINK_LIBRARIES`,
:prop_tgt:`INTERFACE_SOURCES`,
and :prop_tgt:`INTERFACE_POSITION_INDEPENDENT_CODE`.
Only the ``INTERFACE`` modes of the :command:`target_include_directories`,
:command:`target_compile_definitions`, :command:`target_compile_options`,
:command:`target_sources`, and :command:`target_link_libraries` commands
may be used with ``INTERFACE`` libraries.

A primary use-case for ``INTERFACE`` libraries is header-only libraries.

.. code-block:: cmake

  add_library(Eigen INTERFACE)
  target_include_directories(Eigen INTERFACE
    $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/src>
    $<INSTALL_INTERFACE:include/Eigen>
  )

  add_executable(exe1 exe1.cpp)
  target_link_libraries(exe1 Eigen)

Here, the usage requirements from the ``Eigen`` target are consumed and used
when compiling, but it has no effect on linking.

Another use-case is to employ an entirely target-focussed design for usage
requirements:

.. code-block:: cmake

  add_library(pic_on INTERFACE)
  set_property(TARGET pic_on PROPERTY INTERFACE_POSITION_INDEPENDENT_CODE ON)
  add_library(pic_off INTERFACE)
  set_property(TARGET pic_off PROPERTY INTERFACE_POSITION_INDEPENDENT_CODE OFF)

  add_library(enable_rtti INTERFACE)
  target_compile_options(enable_rtti INTERFACE
    $<$<OR:$<COMPILER_ID:GNU>,$<COMPILER_ID:Clang>>:-rtti>
  )

  add_executable(exe1 exe1.cpp)
  target_link_libraries(exe1 pic_on enable_rtti)

This way, the build specification of ``exe1`` is expressed entirely as linked
targets, and the complexity of compiler-specific flags is encapsulated in an
``INTERFACE`` library target.

The properties permitted to be set on or read from an ``INTERFACE`` library
are:

* Properties matching ``INTERFACE_*``
* Built-in properties matching ``COMPATIBLE_INTERFACE_*``
* ``EXPORT_NAME``
* ``IMPORTED``
* ``NAME``
* Properties matching ``MAP_IMPORTED_CONFIG_*``

``INTERFACE`` libraries may be installed and exported.  Any content they refer
to must be installed separately:

.. code-block:: cmake

  add_library(Eigen INTERFACE)
  target_include_directories(Eigen INTERFACE
    $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/src>
    $<INSTALL_INTERFACE:include/Eigen>
  )

  install(TARGETS Eigen EXPORT eigenExport)
  install(EXPORT eigenExport NAMESPACE Upstream::
    DESTINATION lib/cmake/Eigen
  )
  install(FILES
      ${CMAKE_CURRENT_SOURCE_DIR}/src/eigen.h
      ${CMAKE_CURRENT_SOURCE_DIR}/src/vector.h
      ${CMAKE_CURRENT_SOURCE_DIR}/src/matrix.h
    DESTINATION include/Eigen
  )