add_custom_command

Add a custom build rule to the generated build system.

There are two main signatures for add_custom_command.

Generating Files

The first signature is for adding a custom command to produce an output:

add_custom_command(OUTPUT output1 [output2 ...]
                   COMMAND command1 [ARGS] [args1...]
                   [COMMAND command2 [ARGS] [args2...] ...]
                   [MAIN_DEPENDENCY depend]
                   [DEPENDS [depends...]]
                   [BYPRODUCTS [files...]]
                   [IMPLICIT_DEPENDS <lang1> depend1
                                    [<lang2> depend2] ...]
                   [WORKING_DIRECTORY dir]
                   [COMMENT comment]
                   [DEPFILE depfile]
                   [JOB_POOL job_pool]
                   [VERBATIM] [APPEND] [USES_TERMINAL]
                   [COMMAND_EXPAND_LISTS])

This defines a command to generate specified OUTPUT file(s). A target created in the same directory (CMakeLists.txt file) that specifies any output of the custom command as a source file is given a rule to generate the file using the command at build time. Do not list the output in more than one independent target that may build in parallel or the two instances of the rule may conflict (instead use the add_custom_target() command to drive the command and make the other targets depend on that one). In makefile terms this creates a new target in the following form:

OUTPUT: MAIN_DEPENDENCY DEPENDS
        COMMAND

The options are:

APPEND

Append the COMMAND and DEPENDS option values to the custom command for the first output specified. There must have already been a previous call to this command with the same output.

If the previous call specified the output via a generator expression, the output specified by the current call must match in at least one configuration after evaluating generator expressions. In this case, the appended commands and dependencies apply to all configurations.

The COMMENT, MAIN_DEPENDENCY, and WORKING_DIRECTORY options are currently ignored when APPEND is given, but may be used in the future.

BYPRODUCTS

New in version 3.2.

Specify the files the command is expected to produce but whose modification time may or may not be newer than the dependencies. If a byproduct name is a relative path it will be interpreted relative to the build tree directory corresponding to the current source directory. Each byproduct file will be marked with the GENERATED source file property automatically.

Explicit specification of byproducts is supported by the Ninja generator to tell the ninja build tool how to regenerate byproducts when they are missing. It is also useful when other build rules (e.g. custom commands) depend on the byproducts. Ninja requires a build rule for any generated file on which another rule depends even if there are order-only dependencies to ensure the byproducts will be available before their dependents build.

The Makefile Generators will remove BYPRODUCTS and other GENERATED files during make clean.

New in version 3.20: Arguments to BYPRODUCTS may use generator expressions.

COMMAND

Specify the command-line(s) to execute at build time. If more than one COMMAND is specified they will be executed in order, but not necessarily composed into a stateful shell or batch script. (To run a full script, use the configure_file() command or the file(GENERATE) command to create it, and then specify a COMMAND to launch it.) The optional ARGS argument is for backward compatibility and will be ignored.

If COMMAND specifies an executable target name (created by the add_executable() command), it will automatically be replaced by the location of the executable created at build time if either of the following is true:

  • The target is not being cross-compiled (i.e. the CMAKE_CROSSCOMPILING variable is not set to true).

  • New in version 3.6: The target is being cross-compiled and an emulator is provided (i.e. its CROSSCOMPILING_EMULATOR target property is set). In this case, the contents of CROSSCOMPILING_EMULATOR will be prepended to the command before the location of the target executable.

If neither of the above conditions are met, it is assumed that the command name is a program to be found on the PATH at build time.

Arguments to COMMAND may use generator expressions. Use the TARGET_FILE generator expression to refer to the location of a target later in the command line (i.e. as a command argument rather than as the command to execute).

Whenever one of the following target based generator expressions are used as a command to execute or is mentioned in a command argument, a target-level dependency will be added automatically so that the mentioned target will be built before any target using this custom command (see policy CMP0112).

  • TARGET_FILE

  • TARGET_LINKER_FILE

  • TARGET_SONAME_FILE

  • TARGET_PDB_FILE

This target-level dependency does NOT add a file-level dependency that would cause the custom command to re-run whenever the executable is recompiled. List target names with the DEPENDS option to add such file-level dependencies.

COMMENT

Display the given message before the commands are executed at build time.

DEPENDS

Specify files on which the command depends. Each argument is converted to a dependency as follows:

  1. If the argument is the name of a target (created by the add_custom_target(), add_executable(), or add_library() command) a target-level dependency is created to make sure the target is built before any target using this custom command. Additionally, if the target is an executable or library, a file-level dependency is created to cause the custom command to re-run whenever the target is recompiled.

  2. If the argument is an absolute path, a file-level dependency is created on that path.

  3. If the argument is the name of a source file that has been added to a target or on which a source file property has been set, a file-level dependency is created on that source file.

  4. If the argument is a relative path and it exists in the current source directory, a file-level dependency is created on that file in the current source directory.

  5. Otherwise, a file-level dependency is created on that path relative to the current binary directory.

If any dependency is an OUTPUT of another custom command in the same directory (CMakeLists.txt file), CMake automatically brings the other custom command into the target in which this command is built.

New in version 3.16: A target-level dependency is added if any dependency is listed as BYPRODUCTS of a target or any of its build events in the same directory to ensure the byproducts will be available.

If DEPENDS is not specified, the command will run whenever the OUTPUT is missing; if the command does not actually create the OUTPUT, the rule will always run.

New in version 3.1: Arguments to DEPENDS may use generator expressions.

COMMAND_EXPAND_LISTS

New in version 3.8.

Lists in COMMAND arguments will be expanded, including those created with generator expressions, allowing COMMAND arguments such as ${CC} "-I$<JOIN:$<TARGET_PROPERTY:foo,INCLUDE_DIRECTORIES>,;-I>" foo.cc to be properly expanded.

IMPLICIT_DEPENDS

Request scanning of implicit dependencies of an input file. The language given specifies the programming language whose corresponding dependency scanner should be used. Currently only C and CXX language scanners are supported. The language has to be specified for every file in the IMPLICIT_DEPENDS list. Dependencies discovered from the scanning are added to those of the custom command at build time. Note that the IMPLICIT_DEPENDS option is currently supported only for Makefile generators and will be ignored by other generators.

Note

This option cannot be specified at the same time as DEPFILE option.

JOB_POOL

New in version 3.15.

Specify a pool for the Ninja generator. Incompatible with USES_TERMINAL, which implies the console pool. Using a pool that is not defined by JOB_POOLS causes an error by ninja at build time.

MAIN_DEPENDENCY

Specify the primary input source file to the command. This is treated just like any value given to the DEPENDS option but also suggests to Visual Studio generators where to hang the custom command. Each source file may have at most one command specifying it as its main dependency. A compile command (i.e. for a library or an executable) counts as an implicit main dependency which gets silently overwritten by a custom command specification.

OUTPUT

Specify the output files the command is expected to produce. If an output name is a relative path it will be interpreted relative to the build tree directory corresponding to the current source directory. Each output file will be marked with the GENERATED source file property automatically. If the output of the custom command is not actually created as a file on disk it should be marked with the SYMBOLIC source file property.

New in version 3.20: Arguments to OUTPUT may use generator expressions.

USES_TERMINAL

New in version 3.2.

The command will be given direct access to the terminal if possible. With the Ninja generator, this places the command in the console pool.

VERBATIM

All arguments to the commands will be escaped properly for the build tool so that the invoked command receives each argument unchanged. Note that one level of escapes is still used by the CMake language processor before add_custom_command even sees the arguments. Use of VERBATIM is recommended as it enables correct behavior. When VERBATIM is not given the behavior is platform specific because there is no protection of tool-specific special characters.

WORKING_DIRECTORY

Execute the command with the given current working directory. If it is a relative path it will be interpreted relative to the build tree directory corresponding to the current source directory.

New in version 3.13: Arguments to WORKING_DIRECTORY may use generator expressions.

DEPFILE

New in version 3.7.

Specify a .d depfile for the Ninja generator and Makefile Generators. A .d file holds dependencies usually emitted by the custom command itself. Using DEPFILE with other generators than Ninja or Makefile Generators is an error.

New in version 3.20: Added the support of Makefile Generators.

If the DEPFILE argument is relative, it should be relative to CMAKE_CURRENT_BINARY_DIR, and any relative paths inside the DEPFILE should also be relative to CMAKE_CURRENT_BINARY_DIR (see policy CMP0116. This policy is always NEW for Makefile Generators).

Note

For Makefile Generators, this option cannot be specified at the same time as IMPLICIT_DEPENDS option.

Examples: Generating Files

Custom commands may be used to generate source files. For example, the code:

add_custom_command(
  OUTPUT out.c
  COMMAND someTool -i ${CMAKE_CURRENT_SOURCE_DIR}/in.txt
                   -o out.c
  DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/in.txt
  VERBATIM)
add_library(myLib out.c)

adds a custom command to run someTool to generate out.c and then compile the generated source as part of a library. The generation rule will re-run whenever in.txt changes.

New in version 3.20: One may use generator expressions to specify per-configuration outputs. For example, the code:

add_custom_command(
  OUTPUT "out-$<CONFIG>.c"
  COMMAND someTool -i ${CMAKE_CURRENT_SOURCE_DIR}/in.txt
                   -o "out-$<CONFIG>.c"
                   -c "$<CONFIG>"
  DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/in.txt
  VERBATIM)
add_library(myLib "out-$<CONFIG>.c")

adds a custom command to run someTool to generate out-<config>.c, where <config> is the build configuration, and then compile the generated source as part of a library.

Build Events

The second signature adds a custom command to a target such as a library or executable. This is useful for performing an operation before or after building the target. The command becomes part of the target and will only execute when the target itself is built. If the target is already built, the command will not execute.

add_custom_command(TARGET <target>
                   PRE_BUILD | PRE_LINK | POST_BUILD
                   COMMAND command1 [ARGS] [args1...]
                   [COMMAND command2 [ARGS] [args2...] ...]
                   [BYPRODUCTS [files...]]
                   [WORKING_DIRECTORY dir]
                   [COMMENT comment]
                   [VERBATIM] [USES_TERMINAL]
                   [COMMAND_EXPAND_LISTS])

This defines a new command that will be associated with building the specified <target>. The <target> must be defined in the current directory; targets defined in other directories may not be specified.

When the command will happen is determined by which of the following is specified:

PRE_BUILD

On Visual Studio Generators, run before any other rules are executed within the target. On other generators, run just before PRE_LINK commands.

PRE_LINK

Run after sources have been compiled but before linking the binary or running the librarian or archiver tool of a static library. This is not defined for targets created by the add_custom_target() command.

POST_BUILD

Run after all other rules within the target have been executed.

Note

Because generator expressions can be used in custom commands, it is possible to define COMMAND lines or whole custom commands which evaluate to empty strings for certain configurations. For Visual Studio 2010 (and newer) generators these command lines or custom commands will be omitted for the specific configuration and no “empty-string-command” will be added.

This allows to add individual build events for every configuration.

Examples: Build Events

A POST_BUILD event may be used to post-process a binary after linking. For example, the code:

add_executable(myExe myExe.c)
add_custom_command(
  TARGET myExe POST_BUILD
  COMMAND someHasher -i "$<TARGET_FILE:myExe>"
                     -o "$<TARGET_FILE:myExe>.hash"
  VERBATIM)

will run someHasher to produce a .hash file next to the executable after linking.

New in version 3.20: One may use generator expressions to specify per-configuration byproducts. For example, the code:

add_library(myPlugin MODULE myPlugin.c)
add_custom_command(
  TARGET myPlugin POST_BUILD
  COMMAND someHasher -i "$<TARGET_FILE:myPlugin>"
                     --as-code "myPlugin-hash-$<CONFIG>.c"
  BYPRODUCTS "myPlugin-hash-$<CONFIG>.c"
  VERBATIM)
add_executable(myExe myExe.c "myPlugin-hash-$<CONFIG>.c")

will run someHasher after linking myPlugin, e.g. to produce a .c file containing code to check the hash of myPlugin that the myExe executable can use to verify it before loading.

Ninja Multi-Config

New in version 3.20: add_custom_command supports the Ninja Multi-Config generator’s cross-config capabilities. See the generator documentation for more information.