I am trying to build a simple c++ hello world program using g++ with SCons. How can I specify that I want SCons to use g++ without any dependencies on external environment variables, such as PATH?
This is what I've tried:
env = Environment(CXX = 'C:/cygwin/bin/g++')
env.Program('helloworld.c++')
This is my result:
scons: warning: No version of Visual Studio compiler found - C/C++
compilers most likely not set correctly
File "C:\Python27\Scripts\scons.py", line 201, in <module>
C:/cygwin/bin/g++ /Fohelloworld.obj /c helloworld.c++ /TP /nologo
g++: error: /Fohelloworld.obj: No such file or directory
g++: error: /c: No such file or directory
g++: error: /TP: No such file or directory
g++: error: /nologo: No such file or directory
scons: *** [helloworld.obj] Error 1
scons: building terminated because of errors.
Turns out the answer was staring me straight in the face. My first problem was that the first part of this answer:
import os
env = Environment(ENV = {'PATH' : os.environ['PATH']})
utilizes the OS PATH, which I explicitly wanted to avoid. My second problem was that I completely overlooked the answer below, which was the precise answer to my question:
The way to guarantee that the build is repeatable is to explicitly
initialize the PATH
path= ['/bin', '/usr/bin', '/path/to/other/compiler/bin']
env = Environment(ENV = {'PATH' : path})
The reason I didn't realize this was the solution is because I simply misunderstood that env['ENV']['PATH'] and os.environ['PATH'] are completely separate and distinct. Whereas os.environ['PATH'] is obviously the external OS PATH, env['ENV']['PATH'] seems to be SCons' internal equivalent. You can set is to be whatever you please.
In the end, the precise solution I chose, and the one most readable to me was:
PATH = {'PATH' : ['C:/cygwin/bin']}
env = Environment(ENV = PATH)
env['ENV'] = PATH
env['CXX'] = 'g++'
env.Program('helloworld.c++')
A slightly cleaner way
env = Environment(tools=['g++','gnulink'])
env['ENV']['PATH'] = ['C:/cygwin/bin']
env['CXX'] = 'g++'
env.Program('helloworld.c++')
Another way:
env = Environment(tools=['g++','gnulink'])
env.PrependENVPath('PATH','C:/cygwin/bin')
env.Program('helloworld.c++')
Try this:
env = Environment(tools=['ar', 'cc', 'clang', 'clangxx', 'gcc', 'g++', 'gnulink', 'link'], ENV=os.environ, toolpath=['custom_path']).
The command above will:
Create a variable env of type Environment.
Tells scons to set up requires tools.
Find these tools in system variable.
Else find in custom path(you can omit the last parameter: toolpath=['custom_path'] if you don't need it).
Where:
ar: Sets construction variables for the ar library archiver.
Sets: $AR, $ARCOM, $ARFLAGS, $LIBPREFIX, $LIBSUFFIX, $RANLIB, $RANLIBCOM, $RANLIBFLAGS.
cc: Sets construction variables for generic POSIX C compilers.
Sets: $CC, $CCCOM, $CCFLAGS, $CFILESUFFIX, $CFLAGS, $CPPDEFPREFIX, $CPPDEFSUFFIX,$FRAMEWORKPATH, $FRAMEWORKS, $INCPREFIX, $INCSUFFIX, $SHCC, $SHCCCOM, $SHCCFLAGS,$SHCFLAGS, $SHOBJSUFFIX.
clang: Set construction variables for the Clang C compiler.
Sets: $CC, $CCVERSION, $SHCCFLAGS.
clangxx: Set construction variables for the Clang C++ compiler.
Sets: $CXX, $CXXVERSION, $SHCXXFLAGS, $SHOBJSUFFIX,$STATIC_AND_SHARED_OBJECTS_ARE_THE_SAME.
g++: Set construction variables for the gXX C++ compiler.
Sets: $CXX, $CXXVERSION, $SHCXXFLAGS, $SHOBJSUFFIX.
gcc: Set construction variables for the gcc C compiler.
Sets: $CC, $CCVERSION, $SHCCFLAGS.
gnulink: Set construction variables for GNU linker/loader
Sets: $LDMODULEVERSIONFLAGS, $RPATHPREFIX, $RPATHSUFFIX, $SHLIBVERSIONFLAGS,$SHLINKFLAGS, $_LDMODULESONAME, $_SHLIBSONAME.
link: Sets construction variables for generic POSIX linkers. This is a "smart" linker tool which selects a compiler tocomplete the linking based on the types of source files.
Sets: $LDMODULE, $LDMODULECOM, $LDMODULEFLAGS, $LDMODULENOVERSIONSYMLINKS,$LDMODULEPREFIX, $LDMODULESUFFIX, $LDMODULEVERSION, $LDMODULEVERSIONFLAGS,$LIBDIRPREFIX, $LIBDIRSUFFIX, $LIBLINKPREFIX, $LIBLINKSUFFIX, $LINK,$LINKCOM, $LINKFLAGS, $SHLIBSUFFIX, $SHLINK, $SHLINKCOM, $SHLINKFLAGS,$__LDMODULEVERSIONFLAGS, $__SHLIBVERSIONFLAGS.Uses: $LDMODULECOMSTR, $LINKCOMSTR, $SHLINKCOMSTR
Or you can use default config tools for your system by:
env = Environment(tools=['default'], ENV=os.environ)
To quote from scons man 4.1.0
default
Sets construction variables for a default list of Tool modules. Use
default in the tools list to retain the original defaults, since the
tools parameter is treated as a literal statement of the
tools to be made available in that construction environment, not
an addition.
The list of tools selected by default is not static, but
is dependent both on the platform and on the software installed on the
platform. Some tools will not initialize if an underlying command is
not found, and some tools are selected from a list of choices on
a first-found basis. The finished tool list can be examined
by inspecting the TOOLS construction variable in the construction
environment.
On all platforms, all tools from the following list are
selected whose respective conditions are met: filesystem,wix, lex,
yacc, rpcgen, swig, jar, javac, javah, rmic, dvipdf, dvips, gs, tex,
latex, pdflatex, pdftex, tar, zip, textfile.
On Linux systems, the
default tools list selects (first-found): a C compiler from gcc,
intelc, icc, cc; a C++ compilerfrom g++, intelc, icc, cxx; an
assembler from gas, nasm, masm; a linker from gnulink, ilink; a
Fortran compilerfrom gfortran, g77, ifort, ifl, f95, f90, f77; and a
static archiver 'ar'. It also selects all found from the list m4,
rpm.
On Windows systems, the default tools list selects
(first-found): a C compiler from msvc, mingw, gcc, intelc,icl,
icc, cc, bcc32; a C++ compiler from msvc, intelc, icc, g++, cxx,
bcc32; an assembler from masm, nasm, gas,386asm; a linker from mslink,
gnulink, ilink, linkloc, ilink32; a Fortran compiler from gfortran,
g77, ifl, cvf, f95,f90, fortran; and a static archiver from mslib, ar,
tlib; It also selects all found from the list msvs, midl.
On MacOS
systems, the default tools list selects (first-found): a C compiler
from gcc, cc; a C++ compiler from g++, cxx; an assembler 'as'; a
linker from applelink, gnulink; a Fortran compiler from gfortran, f95,
f90, g77; anda static archiver ar. It also selects all found from the
list m4, rpm.
Default lists for other platforms can be found by
examining the scons source code (see SCons/Tool/init.py).
In CMake, the flags for the C++ compiler can be influenced in various ways: setting CMAKE_CXX_FLAGS manually, using add_definitions(), forcing a certain C++ standard, and so forth.
In order to compile a target in the same project with different rules (a precompiled header, in my case), I need to reproduce the exact command that is used to compile files added by a command like add_executable() in this directory.
Reading CMAKE_CXX_FLAGS only returns the value set to it explicitly, CMAKE_CXX_FLAGS_DEBUG and siblings only list default Debug/Release options. There is a special functions to retrieve the flags from add_definitions() and add_compiler_options(), but none seem to be able to return the final command line.
How can I get all flags passed to the compiler into a CMake variable?
To answer my own question: It seems like the only way of getting all compiler flags is to reconstruct them from the various sources. The code I'm working with now is the following (for GCC):
macro (GET_COMPILER_FLAGS TARGET VAR)
if (CMAKE_COMPILER_IS_GNUCXX)
set(COMPILER_FLAGS "")
# Get flags form add_definitions, re-escape quotes
get_target_property(TARGET_DEFS ${TARGET} COMPILE_DEFINITIONS)
get_directory_property(DIRECTORY_DEFS COMPILE_DEFINITIONS)
foreach (DEF ${TARGET_DEFS} ${DIRECTORY_DEFS})
if (DEF)
string(REPLACE "\"" "\\\"" DEF "${DEF}")
list(APPEND COMPILER_FLAGS "-D${DEF}")
endif ()
endforeach ()
# Get flags form include_directories()
get_target_property(TARGET_INCLUDEDIRS ${TARGET} INCLUDE_DIRECTORIES)
foreach (DIR ${TARGET_INCLUDEDIRS})
if (DIR)
list(APPEND COMPILER_FLAGS "-I${DIR}")
endif ()
endforeach ()
# Get build-type specific flags
string(TOUPPER ${CMAKE_BUILD_TYPE} BUILD_TYPE_SUFFIX)
separate_arguments(GLOBAL_FLAGS UNIX_COMMAND
"${CMAKE_CXX_FLAGS} ${CMAKE_CXX_FLAGS_${BUILD_TYPE_SUFFIX}}")
list(APPEND COMPILER_FLAGS ${GLOBAL_FLAGS})
# Add -std= flag if appropriate
get_target_property(STANDARD ${TARGET} CXX_STANDARD)
if ((NOT "${STANDARD}" STREQUAL NOTFOUND) AND (NOT "${STANDARD}" STREQUAL ""))
list(APPEND COMPILER_FLAGS "-std=gnu++${STANDARD}")
endif ()
endif ()
set(${VAR} "${COMPILER_FLAGS}")
endmacro ()
This could be extended to also include options induced by add_compiler_options() and more.
Easiest way is to use make VERBOSE=1 when compiling.
cd my-build-dir
cmake path-to-my-sources
make VERBOSE=1
This will do a single-threaded build, and make will print every shell command it runs just before it runs it. So you'll see output like:
[ 0%] Building CXX object Whatever.cpp.o
<huge scary build command it used to build Whatever.cpp>
There actually is a fairly clean way to do this at compile time using CXX_COMPILER_LAUNCHER:
If you have a script print_args.py
#!/usr/bin/env python
import sys
import argparse
print(" ".join(sys.argv[1:]))
# we need to produce an output file so that the link step does not fail
p = argparse.ArgumentParser()
p.add_argument("-o")
args, _ = p.parse_known_args()
with open(args.o, "w") as f:
f.write("")
You can set the target's properties as follows:
add_library(${TARGET_NAME} ${SOURCES})
set_target_properties(${TARGET_NAME} PROPERTIES
CXX_COMPILER_LAUNCHER
${CMAKE_CURRENT_SOURCE_DIR}/print_args.py
)
# this tells the linker to not actually link. Which would fail because output file is empty
set_target_properties(${TARGET_NAME} PROPERTIES
LINK_FLAGS
-E
)
This will print the exact compilation command at compile time.
Short answer
It's not possible to assign final value of compiler command line to variable in CMake script, working in all use cases.
Long answer
Unfortunately, even solution accepted as answer still not gets all compiler flags. As gets noted in comments, there are Transitive Usage Requirements. It's a modern and proper way to write CMake files, getting more and more popular. Also you may have some compile options defined using generator expressions (they look like variable references but will not expand when needed).
Consider having following example:
add_executable(myexe ...);
target_compile_definitions(myexe PRIVATE "PLATFORM_$<PLATFORM_ID>");
add_library(mylib ...);
target_compile_definitions(mylib INTERFACE USING_MY_LIB);
target_link_libraries(myexe PUBLIC mylib);
If you try to call proposed GET_COMPILER_FLAGS macro with myexe target, you will get resulting output -DPLATFORM_$<PLATFORM_ID> instead of expected -DPLATFORM_Linux -DUSING_MY_LIB.
This is because there are two stages between invoking CMake and getting build system generated:
Processing. At this stage CMake reads and executes commands from cmake script(s), particularly, variable values getting evaluated and assigned. At this moment CMake just collecting all required info and being prepared to generate build system (makefiles).
Generating. CMake uses values of special variables and properties, being left at end of processed scripts to finally decide and form generated output. This is where it constructs final command line for compiler according to its internal algorithm, not avaliable for scripting.
Target properties which might be retrieved at processing stage with get_target_property(...) or get_property(... TARGET ...) aren't complete (even when invoked at the end of script). At generating stage CMake walks through each target dependency tree (recursively) and appends properties values according to transitive usage requirements (PUBLIC and INTERFACE tagged values gets propagated).
Although, there are workarounds, depending on what final result you aiming to achieve. This is possible by applying generator expressions, which allows use final values of properties of any target (defined at processing stage)... but later!
Two general possibilites are avaliable:
Generate any output file based on template, which content contains variable references and/or generator expressions, and defined as either string variable value, or input file. It's not flexible due to very limited support of conditional logic (i.e. you cannot use complex concatenations available only with nested foreach() loops), but has advantages, that no further actions required and content described in platform-independent way. Use file(GENERATE ...) command variant. Note, that it behaves differently from file (WRITE ...) variant.
Add custom target (and/or custom command) which implements further usage of expanded value. It's platform dependent and requires user to additionally invoke make (either with some special target, or include to all target), but has advantage, that it's flexible enough because you may implement shell script (but without executable bit).
Example demonstrating solution with combining these options:
set(target_name "myexe")
file(GENERATE OUTPUT script.sh CONTENT "#!/bin/sh\n echo \"${target_name} compile definitions: $<TARGET_PROPERTY:${target_name},COMPILE_DEFINITIONS>\"")
add_custom_target(mycustomtarget
COMMAND echo "\"Platform: $<PLATFORM_ID>\""
COMMAND /bin/sh -s < script.sh
)
After calling CMake build directory will contain file script.sh and invoking make mycustomtarget will print to console:
Platform: Linux
myexe compile definitions: PLATFORM_Linux USING_MY_LIB
Use
set(CMAKE_EXPORT_COMPILE_COMMANDS true)
and get compile_commands.json
I cannot understand what I'm doing wrong.
I'm always getting the string "$<TARGET_FILE:tgt1>" instead of the path to the library.
I've created the dummy project.
Here is my root CMakeLists.txt
cmake_minimum_required (VERSION 3.0) # also tried 2.8 with the same result
set(PROJECT_NAME CMP0026)
add_subdirectory(src)
set(TGT_PATH $<TARGET_FILE:tgt1>)
message(STATUS "${TGT_PATH}")
Here is my src/CMakeLists.txt
add_library(tgt1 a.c)
File a.c is created and is empty
I've tried the following generators: Visual Studio 2013 Win64, Ninja and MingW Makefile. I've used Android toolchain for the last two, downloaded from here
I expect that the last message(STATUS command would print full path to the library. However, all variants print the string $<TARGET_FILE:tgt1>.
Generator expressions are not evaluated at configure time (when CMake is parsing CMakeLists, executing commands like add_target() or message() etc.). At this time, a generator expression is just a literal string - the character $ followed by <, then T, then ...
Evaluation of generator expressions happens at generate time (that's why they are called "generator expressions"). Generate time occurs after all CMake code is parsed and processed, and CMake is starting to act on the data therein to produce buildsystem files. Only then does it have all the information necessary to evaluate generator expressions.
So you can only really use generator expressions for things which occur at generate time or later (such as build time). A contrived example would be this:
add_custom_target(
GenexDemo
COMMAND ${CMAKE_COMMAND} -E echo "$<TARGET_FILE:tgt1>"
VERBATIM
)
At configure time, CMake will record the literal string $<TARGET_FILE:tgt1> as the argument of COMMAND. Then at generate time (when the location of tgt1 is known for each configuration and guaranteed not to change any more), it will substitute it for the generator expression.
Note: This is my first time using CMake. I don't know much about it, so I'm just posting a bunch of information to see if anyone can see my problem.
I would like the ability to automatically determine which c++11 flag is appropriate, given my compiler. There are many examples of this line. Here is my CMakeLists.txt following such an example:
cmake_minimum_required (VERSION 2.8)
#Add the c++11 flag, whatever it is
include(CheckCXXCompilerFlag)
CHECK_CXX_COMPILER_FLAG(-std=c++11 COMPILER_SUPPORTS_CXX11)
CHECK_CXX_COMPILER_FLAG(-std=c++0x COMPILER_SUPPORTS_CXX0X)
if(COMPILER_SUPPORTS_CXX11)
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
elseif(COMPILER_SUPPORTS_CXX0X)
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++0x")
else()
message(STATUS "The compiler ${CMAKE_CXX_COMPILER} has no C++11 support. Please use a different C++ compiler.")
endif()
project(AnalyzeGames)
set(AnalyzeGames_SRCS AnalyzeGames.cpp)
add_executable(AnalyzeGames ${AnalyzeGames_SRCS})
Here is my cmake output when trying to use this file: http://pastebin.com/3AUwqffD
Here is CMakeError.log: http://pastebin.com/EbNKvGt8
Here is CMakeOutput.log: http://pastebin.com/kVJ0enJC
echo $CC: /usr/bin/gcc
echo $CXX: /usr/bin/g++
I can compile a simple test executable with g++ using either flag manually.
cmake --version: cmake version 2.8.12.2
For some reason CMake is not recognizing that my compiler does support both of those flags.
The cmake output tells you that it does not recognize the '.cxx' extension because it doesn't know that your project is a C++ project. To fix this, you should enable C++ in the project command. Try to change the following line:
project(AnalyzeGames)
to:
project(AnalyzeGames CXX)
and then move it to the 2nd line of the CMakeLists.txt, right under cmake_minimum_required. The configuration should work as expected after this.
TLDR
Compiler checks are only performed in the variable passed is not previously defined, which includes in the cache from previous failed attempts. Use unset(my_var CACHE) to force checking to always occur, or just be aware of this behaviour and clear the cache manually when needed.
Detail
I too had this problem (with cmake 2.8.12.2) and I had to turn on trace output, and step through the code to get a similar toy build to work I had make sure the variables I used (COMPILER_SUPPORTS_CXX11_*) in these calls:
CHECK_CXX_COMPILER_FLAG(-std=c++11 COMPILER_SUPPORTS_CXX11)
CHECK_CXX_COMPILER_FLAG(-std=c++0x COMPILER_SUPPORTS_CXX0X)
Were set such that they named themselves:
set(COMPILER_SUPPORTS_CXX11 "COMPILER_SUPPORTS_CXX11")
The other posters solution didn't work for me, it mainly just seemed to limit the detecting of compilers to just CXX and ignored the C compiler.
The issue appears to be with this line of code in the cmake module:
if("${VAR}" MATCHES "^${VAR}$")
Which in the trace output is:
/usr/share/cmake/Modules/CheckCXXSourceCompiles.cmake(30): if(COMPILER_SUPPORTS_CXX0X MATCHES ^COMPILER_SUPPORTS_CXX0X$ )
It looks as if the expression on the left of the MATCHES is replaced with the variables value, but the expression on the right is assumed to be plain text.
If the MATCH fails then the main part of the macro is skipped and according the to the log the check fails.
Looking at later versions of this macro online it looks as if this line has changed to only perform the compile check if the variable is undefined.
It as at this point that I realise that this is the intent / hack of the original code; if the X is undefined then "X" MATCHES "^X$" will be true, but then the compile check can be performed, fail for some other reason and then never be performed again.
So the solution is either force unset of variable in cache before calling the macro using:
unset(COMPILER_SUPPORTS_CXX0X CACHE)
Or clear the cache manually and be prepared for this behaviour.