Dealing with lots and lots of escaped characters in add_custom_command

Question

I have a file that contains a bunch of data. I want to turn it into a C++ string literal, because I need to compile this data into the binary - I cannot read it from disk.

One way of doing this is to just generate a C++ source file that declares a string literal with a known name. The CMake code to do this is straightforward, if somewhat awful:

function(make_literal_from_file dest_file source_file literal_name)              
    add_custom_command(                                                          
        OUTPUT ${dest_file}                                                      
        COMMAND printf \'char const* ${literal_name} = R\"\#\(\' > ${dest_file}  
        COMMAND cat ${source_file} >> ${dest_file}                               
        COMMAND printf \'\)\#\"\;\' >> ${dest_file}                              
        DEPENDS ${source_file})                                                  
endfunction()

This works and does what I want (printf is necessary to avoid a new line after the raw string introducer). However, the amount of escaping going on here makes it very difficult to see what's going on. Is there a way to write this function such that it's actually readable?

Note that I cannot use a file(READ ...)/configure_file(...) combo here because source_file could be something that is generated by CMake at build time and so may not be present at configuration time.

Sorry to be picky, but could you make the title more descriptive? Can't imagine this is very searchable... — Dan Mašek, Aug 23 '18 at 15:14
Hmm, could you perhaps use `CONFIGURE_FILE`, along with `FILE(READ ...)` to load the contents of source_file to a variable? — Dan Mašek, Aug 23 '18 at 15:25
@DanMašek Ah, meant to add that to the question and forgot. Added now. Not an option unfortunately. — Barry, Aug 23 '18 at 15:27
Right. A possible way to work around it is to put the file/configure_file into a separate .cmake script that you invoke from a custom command. I use similar setup to run various code generators at compile time. Although, I admit, it makes the implementation more complex that what you have right now. — Dan Mašek, Aug 23 '18 at 15:32

Justin · Answer 1 · 2018-08-23T17:47:50.473

I would recommend writing a script to do this. You could write it in CMake, but I personally prefer a better language such as Python:

# Untested, just to show roughly how to do it
import sys

dest_file, source_file, literal_name = sys.argv[1:]

with open(dest_file) as dest, open(source_file) as source:
    literal_contents = source.read()
    dest.write(f'char const* {literal_name} = R"({literal_contents})";\n')

Corresponding CMake code:

# String interpolation came in Python 3.6, thus the requirement on 3.6.
# If using CMake < 3.12, use find_package(PythonInterp) instead.
find_package(Python3 3.6 COMPONENTS Interpreter)

# Make sure this resolves correctly. ${CMAKE_CURRENT_LIST_DIR} is helpful;
# it's the directory containing the current file (this cmake file)
set(make_literal_from_file_script "path/to/make_literal_from_file.py")

function(make_literal_from_file dest_file source_file literal_name)              
    add_custom_command(                                                          
        OUTPUT "${dest_file}"
        COMMAND 
            "${Python3_EXECUTABLE}" "${make_literal_from_file_script}"
            "${dest_file}"
            "${source_file}"
            "${literal_name}"
        DEPENDS "${source_file}")                                                  
endfunction()

If you don't want the dependency on Python, you could use C++ (only the CMake code shown):

add_executable(make_literal_from_file_exe
    path/to/cpp/file.cpp
)

function(make_literal_from_file dest_file source_file literal_name)              
    add_custom_command(                                                          
        OUTPUT "${dest_file}"
        COMMAND 
            make_literal_from_file_exe
            "${dest_file}"
            "${source_file}"
            "${literal_name}"
        DEPENDS "${source_file}")
endfunction()

This works (+1)... however, it's quite a bit longer and I think I'd prefer the three really ugly CMake lines to having another external thing. I would [strongly] prefer a solution that just made it easier to deal with the escaping. — Barry, Aug 23 '18 at 18:38
@Barry That's reasonable. Do note that the original code isn't cross platform AFAIK (`printf` isn't on Windows IIRC). — Justin, Aug 23 '18 at 18:43
@Barry Have you tried using [`VERBATIM`](https://cmake.org/cmake/help/latest/command/add_custom_command.html?highlight=VERBATIM)? I think `COMMAND printf 'char const* ${literal_name} = R\"#(' > ${dest_file}` would work if you did so, maybe requiring wrapping in quotes (something like `printf "'char const* ${literal_name} = R\"#('"` or maybe `printf "char const* ${literal_name} = R\"#("`). — Justin, Aug 23 '18 at 18:45
Don't care about cross platform. I could not come up with a permutation of VERBATIM that worked (which does not mean that there isn't one that works - I just coudn't figure it out). — Barry, Aug 23 '18 at 18:50
@Barry I've run into similar problems every time I've tried to have CMake execute commands that begin to have any complexity at all, unless I break it out into a separate script. I doubt I'd be able to figure it out either... — Justin, Aug 23 '18 at 18:54

Dealing with lots and lots of escaped characters in add_custom_command

1 Answers1