Compilation failure with libsystemd

[planetmarshall]

Summary

Compilation failure with GCC 16

spdlog/include/spdlog/sinks/systemd_namespace_sink.h:78:42: error: call to non-‘constexpr’ function ‘fmt::v12::basic_string_view<Char>::basic_string_view(const Char*) [with Char = char]’ [-Wtemplate-body]

Steps to reproduce

cmake -S . -B build -DSPDLOG_BUILD_TESTS=ON
cmake --build build

System info

Arch Linux 7.0.10-arch1-1
gcc (GCC) 16.1.1 20260430
spdlog git:d5275e3343c4dca4feb9fa772199181509f2a657

[tt4g]

That source code was copied from the fmt repository.
If it is a problem with fmt, you need to report the issue to fmt so it can be fixed.
We need to determine whether the problem lies with spdlog or fmt.

The fmt::v12::basic_string_view::basic_string_view(const Char*) constructor in fmt v12.1.0 is defined as follows.

https://github.com/fmtlib/fmt/blob/12.1.0/include/fmt/base.h#L540-L549

#if FMT_GCC_VERSION
  FMT_ALWAYS_INLINE
#endif
  FMT_CONSTEXPR20 basic_string_view(const Char* s) : data_(s) {
#if FMT_HAS_BUILTIN(__builtin_strlen) || FMT_GCC_VERSION || FMT_CLANG_VERSION
    if (std::is_same<Char, char>::value && !detail::is_constant_evaluated()) {
      size_ = __builtin_strlen(detail::narrow(s));  // strlen is not constexpr.
      return;
    }
#endif

Since GCC16 should use C++20 by default, the FMT_CONSTEXPR20 macro should expand to constexpr.
I don't understand why the error call to non-‘constexpr’ function ... is being reported.

https://github.com/fmtlib/fmt/blob/12.1.0/include/fmt/base.h#L117-L144

#ifdef FMT_USE_CONSTEVAL
// Use the provided definition.
#elif !defined(__cpp_lib_is_constant_evaluated)
#  define FMT_USE_CONSTEVAL 0
#elif FMT_CPLUSPLUS < 201709L
#  define FMT_USE_CONSTEVAL 0
#elif FMT_GLIBCXX_RELEASE && FMT_GLIBCXX_RELEASE < 10
#  define FMT_USE_CONSTEVAL 0
#elif FMT_LIBCPP_VERSION && FMT_LIBCPP_VERSION < 10000
#  define FMT_USE_CONSTEVAL 0
#elif defined(__apple_build_version__) && __apple_build_version__ < 14000029L
#  define FMT_USE_CONSTEVAL 0  // consteval is broken in Apple clang < 14.
#elif FMT_MSC_VERSION && FMT_MSC_VERSION < 1929
#  define FMT_USE_CONSTEVAL 0  // consteval is broken in MSVC VS2019 < 16.10.
#elif defined(__cpp_consteval)
#  define FMT_USE_CONSTEVAL 1
#elif FMT_GCC_VERSION >= 1002 || FMT_CLANG_VERSION >= 1101
#  define FMT_USE_CONSTEVAL 1
#else
#  define FMT_USE_CONSTEVAL 0
#endif
#if FMT_USE_CONSTEVAL
#  define FMT_CONSTEVAL consteval
#  define FMT_CONSTEXPR20 constexpr
#else
#  define FMT_CONSTEVAL
#  define FMT_CONSTEXPR20
#endif

Can you check whether FMT_CONSTEXPR20 is expanded to constexpr in your build process?

[tt4g]

At least on Compiler Explorer, when using gcc16, the FMT_CONSTEXPR20 macro expanded to constexpr.

Compile Explorer

#include <fmt/core.h>
#include <type_traits>

#define TO_STR(x) TO_STR2(x)
#define TO_STR2(x) #x

FMT_CONSTEXPR20 bool cons()
{
    return std::is_constant_evaluated();
}

int main() {
    fmt::print("FMT_CONSTEXPR20: {}\n", TO_STR(FMT_CONSTEXPR20));

    constexpr auto r = cons();
    fmt::print("constant evaluated: {}\n", r);
}

Output:

FMT_CONSTEXPR20: constexpr
constant evaluated: true

[tt4g]

I was unable to reproduce this issue using the following code.

Compile Explorer

#include <fmt/core.h>
#include <array>
#include <type_traits>

// https://github.com/systemd/systemd/blob/359640e02f007e2be893bed06dd47b2d069f3d7a/src/systemd/sd-daemon.h#L44-L51
#define SD_EMERG   "<0>"  /* system is unusable */
#define SD_ALERT   "<1>"  /* action must be taken immediately */
#define SD_CRIT    "<2>"  /* critical conditions */
#define SD_ERR     "<3>"  /* error conditions */
#define SD_WARNING "<4>"  /* warning conditions */
#define SD_NOTICE  "<5>"  /* normal but significant condition */
#define SD_INFO    "<6>"  /* informational */
#define SD_DEBUG   "<7>"  /* debug-level messages */

// https://github.com/gabime/spdlog/blob/d5275e3343c4dca4feb9fa772199181509f2a657/include/spdlog/common.h#L177
using string_view_t = fmt::basic_string_view<char>;

struct dummy {

    // https://github.com/gabime/spdlog/blob/d5275e3343c4dca4feb9fa772199181509f2a657/include/spdlog/sinks/systemd_namespace_sink.h#L76-L84
    using level_prefix_array = std::array<string_view_t, 7>;
    static constexpr level_prefix_array level_prefixes_ = {
        {/* spdlog::level::trace      */ SD_DEBUG,
        /* spdlog::level::debug      */ SD_DEBUG,
        /* spdlog::level::info       */ SD_INFO,
        /* spdlog::level::warn       */ SD_WARNING,
        /* spdlog::level::err        */ SD_ERR,
        /* spdlog::level::critical   */ SD_CRIT,
        /* spdlog::level::off        */ SD_INFO}};

    string_view_t lvl(int index) const {
        return level_prefixes_.at(index);
    }
};

int main() {
    constexpr dummy d;
    fmt::print("lvl: {}\n", d.lvl(6));
}

Output:

lvl: <6>

It does not appear that using GCC 16 is a prerequisite for this issue.

By the way, checking the Arch Linux packages, it seems that spdlog v1.17.0 and fmt v12.1.0 are available.
Since no issues occurred when building the Arch Linux packages, it seems unlikely that the distribution is the cause of the compilation error.

One possibility is that fmt is installed on your machine and may be affecting the local spdlog build.
Arch Linux is configured to use an external fmt, but would the problem still occur if you set the CMake variable SPDLOG_FMT_EXTERNAL=ON to use the same configuration?

[planetmarshall]

I was unable to reproduce this issue using the following code.

Thanks for looking into it. It looks like libsystemd is the culprit, the issue can be reproduced using the gcc container

podman run --rm -it gcc:16.1 /bin/bash

apt-get update
apt-get install ninja-build cmake libsystemd-dev
git clone https://github.com/gabime/spdlog.git --depth=1
cd spdlog
cmake -S . -B build -G Ninja -DSPDLOG_BUILD_TESTS=ON
cmake --build build

Ensure that libsystemd is found correctly during cmake configuration

-- Checking for module 'libsystemd'
--   Found libsystemd, version 257

[tt4g]

Since systemd_namespace_sink is a header-only file, you can use it even if libsystemd isn't present during the build—you can simply install it later.
In my opinion, since not all operating systems have libsystemd, there's no need to let the build fail just because of that library.

Although no specific compilation error is provided, isn’t the build failing at tests/test_systemd_namespace.cpp?
If so, the library can be built with SPDLOG_BUILD_TESTS=OFF.

If you have a better suggestion, please feel free to submit a PR.

[tt4g]

I was able to reproduce the issue using docker with your reproduction steps.

$ cmake -S . -B build -G Ninja -DSPDLOG_BUILD_TESTS=ON
$ cmake --build build
[138/148] Building CXX object tests/CMakeFiles/spdlog-utests.dir/test_systemd_namespace.cpp.o
FAILED: tests/CMakeFiles/spdlog-utests.dir/test_systemd_namespace.cpp.o
/usr/local/bin/c++ -DSPDLOG_COMPILED_LIB -I/spdlog/include -I/spdlog/build/_deps/catch2-src/src/catch2/.. -I/spdlog/build/_deps/catch2-build/generated-includes -O3 -DNDEBUG -std=c++14 -MD -MT tests/CMakeFiles/spdlog-utests.dir/test_systemd_namespace.cpp.o -MF tests/CMakeFiles/spdlog-utests.dir/test_systemd_namespace.cpp.o.d -o tests/CMakeFiles/spdlog-utests.dir/test_systemd_namespace.cpp.o -c /spdlog/tests/test_systemd_namespace.cpp

full error

```shell $ cmake --build build [138/148] Building CXX object tests/CMakeFiles/spdlog-utests.dir/test_systemd_namespace.cpp.o FAILED: tests/CMakeFiles/spdlog-utests.dir/test_systemd_namespace.cpp.o /usr/local/bin/c++ -DSPDLOG_COMPILED_LIB -I/spdlog/include -I/spdlog/build/_deps/catch2-src/src/catch2/.. -I/spdlog/build/_deps/catch2-build/generated-includes -O3 -DNDEBUG -std=c++14 -MD -MT tests/CMakeFiles/spdlog-utests.dir/test_systemd_namespace.cpp.o -MF tests/CMakeFiles/spdlog-utests.dir/test_systemd_namespace.cpp.o.d -o tests/CMakeFiles/spdlog-utests.dir/test_systemd_namespace.cpp.o -c /spdlog/tests/test_systemd_namespace.cpp In file included from /spdlog/include/spdlog/sinks/systemd_namespace_sink.h:12, from /spdlog/tests/test_systemd_namespace.cpp:2: /spdlog/include/spdlog/sinks/systemd_namespace_sink.h:78:42: error: call to non-'constexpr' function 'fmt::v12::basic_string_view::basic_string_view(const Char*) [with Char = char]' [-Wtemplate-body] 78 | {/* spdlog::level::trace */ SD_DEBUG, | ^~~~~~~~ In file included from /spdlog/include/spdlog/fmt/bundled/format.h:41, from /spdlog/include/spdlog/fmt/fmt.h:23, from /spdlog/include/spdlog/common.h:50, from /spdlog/include/spdlog/spdlog.h:12, from /spdlog/tests/includes.h:29, from /spdlog/tests/test_systemd_namespace.cpp:1: /spdlog/include/spdlog/fmt/bundled/base.h:543:19: note: 'fmt::v12::basic_string_view::basic_string_view(const Char*) [with Char = char]' declared here 543 | FMT_CONSTEXPR20 basic_string_view(const Char* s) : data_(s) { | ^~~~~~~~~~~~~~~~~ /spdlog/include/spdlog/sinks/systemd_namespace_sink.h: In constructor 'spdlog::sinks::systemd_namespace_sink::systemd_namespace_sink(std::string, std::string, bool)': /spdlog/include/spdlog/sinks/systemd_namespace_sink.h:46:15: error: '::close' has not been declared; did you mean 'pclose'? [-Wtemplate-body] 46 | ::close(stream_fd); | ^~~~~ | pclose /spdlog/include/spdlog/sinks/systemd_namespace_sink.h:55:15: error: '::close' has not been declared; did you mean 'pclose'? [-Wtemplate-body] 55 | ::close(stream_fd); | ^~~~~ | pclose /spdlog/include/spdlog/sinks/systemd_namespace_sink.h: In member function 'void spdlog::sinks::systemd_namespace_sink::sink_it_(const spdlog::details::log_msg&)': /spdlog/include/spdlog/sinks/systemd_namespace_sink.h:100:37: error: 'using spdlog::string_view_t = class fmt::v12::basic_string_view' {aka 'class fmt::v12::basic_string_view'} has no member named 'find' [-Wtemplate-body] 100 | size_t nl_pos = payload.find('\n', pos); | ^~~~ /spdlog/include/spdlog/sinks/systemd_namespace_sink.h:101:52: error: 'npos' is not a member of 'spdlog::string_view_t' {aka 'fmt::v12::basic_string_view'} [-Wtemplate-body] 101 | size_t end = (nl_pos == string_view_t::npos) ? payload.size() : nl_pos + 1; | ^~~~ /spdlog/include/spdlog/sinks/systemd_namespace_sink.h:102:49: error: 'using spdlog::string_view_t = class fmt::v12::basic_string_view' {aka 'class fmt::v12::basic_string_view'} has no member named 'substr' [-Wtemplate-body] 102 | string_view_t one_message = payload.substr(pos, end - pos); | ^~~~~~ /spdlog/include/spdlog/sinks/systemd_namespace_sink.h:111:42: error: 'npos' is not a member of 'spdlog::string_view_t' {aka 'fmt::v12::basic_string_view'} [-Wtemplate-body] 111 | if (nl_pos == string_view_t::npos) { | ^~~~ ```

However, the problem was the -std=c++14 option.
I’m not sure where this option came from, but I was able to work around it by setting the CMake variable CMAKE_CXX_STANDARD=20.

And I've found two more issues:

1. unistd.h header has not been included

   The build will fail because the close() function symbol cannot be found.

   /spdlog/include/spdlog/sinks/systemd_namespace_sink.h: In constructor 'spdlog::sinks::systemd_namespace_sink<Mutex>::systemd_namespace_sink(std::string, std::string, bool)':
   /spdlog/include/spdlog/sinks/systemd_namespace_sink.h:46:15: error: '::close' has not been declared; did you mean 'pclose'? [-Wtemplate-body]
      46 |             ::close(stream_fd);
         |               ^~~~~
         |               pclose
   /spdlog/include/spdlog/sinks/systemd_namespace_sink.h:55:15: error: '::close' has not been declared; did you mean 'pclose'? [-Wtemplate-body]
      55 |             ::close(stream_fd);
         |               ^~~~~
         |               pclose
   

2. systemd_namespace_sink.h cannot be built unless the USE_STD_FORMAT=ON option

   Basically, spdlog::string_view_t is an alias for fmt::v12::basic_string_view<Char>::basic_string_view.
   Since this template class does not have find and npos, the build failed.

   $ cmake -S . -B build -G Ninja -DSPDLOG_BUILD_TESTS=ON -DCMAKE_CXX_STANDARD=20
   $ cmake --build build
   
   ...
   
   [138/148] Building CXX object tests/CMakeFiles/spdlog-utests.dir/test_systemd_namespace.cpp.o
   FAILED: tests/CMakeFiles/spdlog-utests.dir/test_systemd_namespace.cpp.o
   /usr/local/bin/c++ -DSPDLOG_COMPILED_LIB -I/spdlog/include -I/spdlog/build/_deps/catch2-src/src/catch2/.. -I/spdlog/build/_deps/catch2-build/generated-includes -O3 -DNDEBUG -std=c++14 -MD -MT tests/CMakeFiles/spdlog-utests.dir/test_systemd_namespace.cpp.o -MF tests/CMakeFiles/spdlog-utests.dir/test_systemd_namespace.cpp.o.d -o tests/CMakeFiles/spdlog-utests.dir/test_systemd_namespace.cpp.o -c /spdlog/tests/test_systemd_namespace.cpp
   In file included from /spdlog/include/spdlog/sinks/systemd_namespace_sink.h:12,
                    from /spdlog/tests/test_systemd_namespace.cpp:2:
   /spdlog/include/spdlog/sinks/systemd_namespace_sink.h:78:42: error: call to non-'constexpr' function 'fmt::v12::basic_string_view<Char>::basic_string_view(const Char*) [with Char = char]' [-Wtemplate-body]
      78 |         {/* spdlog::level::trace      */ SD_DEBUG,
         |                                          ^~~~~~~~
   In file included from /spdlog/include/spdlog/fmt/bundled/format.h:41,
                    from /spdlog/include/spdlog/fmt/fmt.h:23,
                    from /spdlog/include/spdlog/common.h:50,
                    from /spdlog/include/spdlog/spdlog.h:12,
                    from /spdlog/tests/includes.h:29,
                    from /spdlog/tests/test_systemd_namespace.cpp:1:
   /spdlog/include/spdlog/fmt/bundled/base.h:543:19: note: 'fmt::v12::basic_string_view<Char>::basic_string_view(const Char*) [with Char = char]' declared here
     543 |   FMT_CONSTEXPR20 basic_string_view(const Char* s) : data_(s) {
         |                   ^~~~~~~~~~~~~~~~~
   /spdlog/include/spdlog/sinks/systemd_namespace_sink.h: In constructor 'spdlog::sinks::systemd_namespace_sink<Mutex>::systemd_namespace_sink(std::string, std::string, bool)':
   /spdlog/include/spdlog/sinks/systemd_namespace_sink.h:46:15: error: '::close' has not been declared; did you mean 'pclose'? [-Wtemplate-body]
      46 |             ::close(stream_fd);
         |               ^~~~~
         |               pclose
   /spdlog/include/spdlog/sinks/systemd_namespace_sink.h:55:15: error: '::close' has not been declared; did you mean 'pclose'? [-Wtemplate-body]
      55 |             ::close(stream_fd);
         |               ^~~~~
         |               pclose
   /spdlog/include/spdlog/sinks/systemd_namespace_sink.h: In member function 'void spdlog::sinks::systemd_namespace_sink<Mutex>::sink_it_(const spdlog::details::log_msg&)':
   /spdlog/include/spdlog/sinks/systemd_namespace_sink.h:100:37: error: 'using spdlog::string_view_t = class fmt::v12::basic_string_view<char>' {aka 'class fmt::v12::basic_string_view<char>'} has no member named 'find' [-Wtemplate-body]
     100 |             size_t nl_pos = payload.find('\n', pos);
         |                                     ^~~~
   /spdlog/include/spdlog/sinks/systemd_namespace_sink.h:101:52: error: 'npos' is not a member of 'spdlog::string_view_t' {aka 'fmt::v12::basic_string_view<char>'} [-Wtemplate-body]
     101 |             size_t end = (nl_pos == string_view_t::npos) ? payload.size() : nl_pos + 1;
         |                                                    ^~~~
   /spdlog/include/spdlog/sinks/systemd_namespace_sink.h:102:49: error: 'using spdlog::string_view_t = class fmt::v12::basic_string_view<char>' {aka 'class fmt::v12::basic_string_view<char>'} has no member named 'substr' [-Wtemplate-body]
     102 |             string_view_t one_message = payload.substr(pos, end - pos);
         |                                                 ^~~~~~
   /spdlog/include/spdlog/sinks/systemd_namespace_sink.h:111:42: error: 'npos' is not a member of 'spdlog::string_view_t' {aka 'fmt::v12::basic_string_view<char>'} [-Wtemplate-body]
     111 |             if (nl_pos == string_view_t::npos) {

   Specifying the SPDLOG_USE_STD_FORMAT=ON option will ensure that the build succeeds.

   $ cmake -S . -B build -G Ninja -DSPDLOG_BUILD_TESTS=ON -DCMAKE_CXX_STANDARD=20 -DSPDLOG_USE_STD_FORMAT=ON
   $ cmake --build build

[tt4g]

I have created a pull request for the unistd.h issue: #3621

The second issue is a limitation in the systemd_namespace_sink implementation, so for now, please work around it by specifying the A option.

[tt4g]

Ensure that libsystemd is found correctly during cmake configuration

By the way, I had already configured the tests so that systemd_namespace_sink.h would not be used if libsystemd was not found during testing.

spdlog/tests/CMakeLists.txt

Lines 11 to 14 in 13680fa

	find_package(PkgConfig)
	if(PkgConfig_FOUND)
	pkg_check_modules(systemd libsystemd)
	endif()

spdlog/tests/CMakeLists.txt

Lines 61 to 67 in 13680fa

	if(systemd_FOUND)
	list(APPEND SPDLOG_UTESTS_SOURCES test_systemd.cpp)
	# systemd v256 added support for namespace stream API
	if(systemd_VERSION VERSION_GREATER_EQUAL "256")
	list(APPEND SPDLOG_UTESTS_SOURCES test_systemd_namespace.cpp)
	endif()
	endif()