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]
                   [DEPENDS_EXPLICIT_ONLY])

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 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. See the Example: Generating Files for Multiple Targets below.

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.

See policy CMP0058 for the motivation behind this feature.

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 a restricted set of generator expressions. Target-dependent expressions are not permitted.

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.

New in version 3.26: Arguments to COMMENT may use generator expressions.

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. 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.

If an output file name is a relative path, its absolute path is determined by interpreting it relative to:

  1. the build directory corresponding to the current source directory (CMAKE_CURRENT_BINARY_DIR), or

  2. the current source directory (CMAKE_CURRENT_SOURCE_DIR).

The path in the build directory is preferred unless the path in the source tree is mentioned as an absolute source file path elsewhere in the current directory.

New in version 3.20: Arguments to OUTPUT may use a restricted set of generator expressions. Target-dependent expressions are not permitted.

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 depfile which holds dependencies for the custom command. It is usually emitted by the custom command itself. This keyword may only be used if the generator supports it, as detailed below.

The expected format, compatible with what is generated by gcc with the option -M, is independent of the generator or platform.

The formal syntax, as specified using BNF notation with the regular extensions, is the following:

depfile       ::=  rule*
rule          ::=  targets (':' (separator dependencies?)?)? eol
targets       ::=  target (separator target)* separator*
target        ::=  pathname
dependencies  ::=  dependency (separator dependency)* separator*
dependency    ::=  pathname
separator     ::=  (space | line_continue)+
line_continue ::=  '\' eol
space         ::=  ' ' | '\t'
pathname      ::=  character+
character     ::=  std_character | dollar | hash | whitespace
std_character ::=  <any character except '$', '#' or ' '>
dollar        ::=  '$$'
hash          ::=  '\#'
whitespace    ::=  '\ '
eol           ::=  '\r'? '\n'

Note

As part of pathname, any slash and backslash is interpreted as a directory separator.

New in version 3.7: The Ninja generator supports DEPFILE since the keyword was first added.

New in version 3.17: Added the Ninja Multi-Config generator, which included support for the DEPFILE keyword.

New in version 3.20: Added support for Makefile Generators.

Note

DEPFILE cannot be specified at the same time as the IMPLICIT_DEPENDS option for Makefile Generators.

New in version 3.21: Added support for Visual Studio Generators with VS 2012 and above, and for the Xcode generator. Support for generator expressions was also added.

Using DEPFILE with generators other than those listed above is an error.

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, which is always NEW for Makefile Generators, Visual Studio Generators, and the Xcode generator.

DEPENDS_EXPLICIT_ONLY

New in version 3.27.

Indicates that the command's DEPENDS argument represents all files required by the command and implicit dependencies are not required.

Without this option, if any target uses the output of the custom command, CMake will consider that target's dependencies as implicit dependencies for the custom command in case this custom command requires files implicitly created by those targets.

This option can be enabled on all custom commands by setting CMAKE_ADD_CUSTOM_COMMAND_DEPENDS_EXPLICIT_ONLY to ON.

Only the Ninja Generators actually use this information to remove unnecessary implicit dependencies.

See also the OPTIMIZE_DEPENDENCIES target property, which may provide another way for reducing the impact of target dependencies in some scenarios.

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.

Example: Generating Files for Multiple Targets

If multiple independent targets need the same custom command output, it must be attached to a single custom target on which they all depend. Consider the following example:

add_custom_command(
  OUTPUT table.csv
  COMMAND makeTable -i ${CMAKE_CURRENT_SOURCE_DIR}/input.dat
                    -o table.csv
  DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/input.dat
  VERBATIM)
add_custom_target(generate_table_csv DEPENDS table.csv)

add_custom_command(
  OUTPUT foo.cxx
  COMMAND genFromTable -i table.csv -case foo -o foo.cxx
  DEPENDS table.csv           # file-level dependency
          generate_table_csv  # target-level dependency
  VERBATIM)
add_library(foo foo.cxx)

add_custom_command(
  OUTPUT bar.cxx
  COMMAND genFromTable -i table.csv -case bar -o bar.cxx
  DEPENDS table.csv           # file-level dependency
          generate_table_csv  # target-level dependency
  VERBATIM)
add_library(bar bar.cxx)

Output foo.cxx is needed only by target foo and output bar.cxx is needed only by target bar, but both targets need table.csv, transitively. Since foo and bar are independent targets that may build concurrently, we prevent them from racing to generate table.csv by placing its custom command in a separate target, generate_table_csv. The custom commands generating foo.cxx and bar.cxx each specify a target-level dependency on generate_table_csv, so the targets using them, foo and bar, will not build until after target generate_table_csv is built.

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]
                   [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

This option has unique behavior for the Visual Studio Generators. When using one of the Visual Studio generators, the command will run before any other rules are executed within the target. With all other generators, this option behaves the same as PRE_LINK instead. Because of this, it is recommended to avoid using PRE_BUILD except when it is known that a Visual Studio generator is being used.

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.

Projects should always specify one of the above three keywords when using the TARGET form. For backward compatibility reasons, POST_BUILD is assumed if no such keyword is given, but projects should explicitly provide one of the keywords to make clear the behavior they expect.

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 11 2012 (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.

New in version 3.21: Support for target-dependent generator expressions.

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.

See Also