Create custom targets to build projects in external trees

The ExternalProject_Add function creates a custom target to drive download, update/patch, configure, build, install and test steps of an external project:

ExternalProject_Add(<name>    # Name for custom target
  [DEPENDS projects...]       # Targets on which the project depends
  [PREFIX dir]                # Root dir for entire project
  [LIST_SEPARATOR sep]        # Sep to be replaced by ; in cmd lines
  [TMP_DIR dir]               # Directory to store temporary files
  [STAMP_DIR dir]             # Directory to store step timestamps
 #--Download step--------------
  [DOWNLOAD_NAME fname]       # File name to store (if not end of URL)
  [DOWNLOAD_DIR dir]          # Directory to store downloaded files
  [DOWNLOAD_COMMAND cmd...]   # Command to download source tree
  [CVS_REPOSITORY cvsroot]    # CVSROOT of CVS repository
  [CVS_MODULE mod]            # Module to checkout from CVS repo
  [CVS_TAG tag]               # Tag to checkout from CVS repo
  [SVN_REPOSITORY url]        # URL of Subversion repo
  [SVN_REVISION rev]          # Revision to checkout from Subversion repo
  [SVN_USERNAME john ]        # Username for Subversion checkout and update
  [SVN_PASSWORD doe ]         # Password for Subversion checkout and update
  [SVN_TRUST_CERT 1 ]         # Trust the Subversion server site certificate
  [GIT_REPOSITORY url]        # URL of git repo
  [GIT_TAG tag]               # Git branch name, commit id or tag
  [GIT_SUBMODULES modules...] # Git submodules that shall be updated, all if empty
  [HG_REPOSITORY url]         # URL of mercurial repo
  [HG_TAG tag]                # Mercurial branch name, commit id or tag
  [URL /.../src.tgz]          # Full path or URL of source
  [URL_HASH ALGO=value]       # Hash of file at URL
  [URL_MD5 md5]               # Equivalent to URL_HASH MD5=md5
  [TLS_VERIFY bool]           # Should certificate for https be checked
  [TLS_CAINFO file]           # Path to a certificate authority file
  [TIMEOUT seconds]           # Time allowed for file download operations
 #--Update/Patch step----------
  [UPDATE_COMMAND cmd...]     # Source work-tree update command
  [PATCH_COMMAND cmd...]      # Command to patch downloaded source
 #--Configure step-------------
  [SOURCE_DIR dir]            # Source dir to be used for build
  [CONFIGURE_COMMAND cmd...]  # Build tree configuration command
  [CMAKE_COMMAND /.../cmake]  # Specify alternative cmake executable
  [CMAKE_GENERATOR gen]       # Specify generator for native build
  [CMAKE_GENERATOR_TOOLSET t] # Generator-specific toolset name
  [CMAKE_ARGS args...]        # Arguments to CMake command line
  [CMAKE_CACHE_ARGS args...]  # Initial cache arguments, of the form -Dvar:string=on
 #--Build step-----------------
  [BINARY_DIR dir]            # Specify build dir location
  [BUILD_COMMAND cmd...]      # Command to drive the native build
  [BUILD_IN_SOURCE 1]         # Use source dir for build dir
 #--Install step---------------
  [INSTALL_DIR dir]           # Installation prefix
  [INSTALL_COMMAND cmd...]    # Command to drive install after build
 #--Test step------------------
  [TEST_BEFORE_INSTALL 1]     # Add test step executed before install step
  [TEST_AFTER_INSTALL 1]      # Add test step executed after install step
  [TEST_COMMAND cmd...]       # Command to drive test
 #--Output logging-------------
  [LOG_DOWNLOAD 1]            # Wrap download in script to log output
  [LOG_UPDATE 1]              # Wrap update in script to log output
  [LOG_CONFIGURE 1]           # Wrap configure in script to log output
  [LOG_BUILD 1]               # Wrap build in script to log output
  [LOG_TEST 1]                # Wrap test in script to log output
  [LOG_INSTALL 1]             # Wrap install in script to log output
 #--Custom targets-------------
  [STEP_TARGETS st1 st2 ...]  # Generate custom targets for these steps

The *_DIR options specify directories for the project, with default directories computed as follows. If the PREFIX option is given to ExternalProject_Add() or the EP_PREFIX directory property is set, then an external project is built and installed under the specified prefix:

TMP_DIR      = <prefix>/tmp
STAMP_DIR    = <prefix>/src/<name>-stamp
DOWNLOAD_DIR = <prefix>/src
SOURCE_DIR   = <prefix>/src/<name>
BINARY_DIR   = <prefix>/src/<name>-build
INSTALL_DIR  = <prefix>

Otherwise, if the EP_BASE directory property is set then components of an external project are stored under the specified base:

TMP_DIR      = <base>/tmp/<name>
STAMP_DIR    = <base>/Stamp/<name>
DOWNLOAD_DIR = <base>/Download/<name>
SOURCE_DIR   = <base>/Source/<name>
BINARY_DIR   = <base>/Build/<name>
INSTALL_DIR  = <base>/Install/<name>

If no PREFIX, EP_PREFIX, or EP_BASE is specified then the default is to set PREFIX to <name>-prefix. Relative paths are interpreted with respect to the build directory corresponding to the source directory in which ExternalProject_Add is invoked.

If SOURCE_DIR is explicitly set to an existing directory the project will be built from it. Otherwise a download step must be specified using one of the DOWNLOAD_COMMAND, CVS_*, SVN_*, or URL options. The URL option may refer locally to a directory or source tarball, or refer to a remote tarball (e.g. http://.../src.tgz).

The ExternalProject_Add_Step function adds a custom step to an external project:

ExternalProject_Add_Step(<name> <step> # Names of project and custom step
  [COMMAND cmd...]        # Command line invoked by this step
  [COMMENT "text..."]     # Text printed when step executes
  [DEPENDEES steps...]    # Steps on which this step depends
  [DEPENDERS steps...]    # Steps that depend on this step
  [DEPENDS files...]      # Files on which this step depends
  [ALWAYS 1]              # No stamp file, step always runs
  [WORKING_DIRECTORY dir] # Working directory for command
  [LOG 1]                 # Wrap step in script to log output

The command line, comment, and working directory of every standard and custom step is processed to replace tokens <SOURCE_DIR>, <BINARY_DIR>, <INSTALL_DIR>, and <TMP_DIR> with corresponding property values.

Any builtin step that specifies a <step>_COMMAND cmd... or custom step that specifies a COMMAND cmd... may specify additional command lines using the form COMMAND cmd.... At build time the commands will be executed in order and aborted if any one fails. For example:

... BUILD_COMMAND make COMMAND echo done ...

specifies to run make and then echo done during the build step. Whether the current working directory is preserved between commands is not defined. Behavior of shell operators like && is not defined.

The ExternalProject_Get_Property function retrieves external project target properties:

ExternalProject_Get_Property(<name> [prop1 [prop2 [...]]])

It stores property values in variables of the same name. Property names correspond to the keyword argument names of ExternalProject_Add.

The ExternalProject_Add_StepTargets function generates custom targets for the steps listed:

ExternalProject_Add_StepTargets(<name> [step1 [step2 [...]]])

If STEP_TARGETS is set then ExternalProject_Add_StepTargets is automatically called at the end of matching calls to ExternalProject_Add_Step. Pass STEP_TARGETS explicitly to individual ExternalProject_Add calls, or implicitly to all ExternalProject_Add calls by setting the directory property EP_STEP_TARGETS.

If STEP_TARGETS is not set, clients may still manually call ExternalProject_Add_StepTargets after calling ExternalProject_Add or ExternalProject_Add_Step.

This functionality is provided to make it easy to drive the steps independently of each other by specifying targets on build command lines. For example, you may be submitting to a sub-project based dashboard, where you want to drive the configure portion of the build, then submit to the dashboard, followed by the build portion, followed by tests. If you invoke a custom target that depends on a step halfway through the step dependency chain, then all the previous steps will also run to ensure everything is up to date.

For example, to drive configure, build and test steps independently for each ExternalProject_Add call in your project, write the following line prior to any ExternalProject_Add calls in your CMakeLists.txt file:

set_property(DIRECTORY PROPERTY EP_STEP_TARGETS configure build test)