Empty strings cannot be written in msgpack-cxx

[MusicScience37]

Describe the bug

msgpack-cxx 7.0.0 throws an exception when writing an empty string using fbuffer.
The exception is thrown at the following if statement.

msgpack-c/include/msgpack/v1/fbuffer.hpp

Lines 34 to 36 in 9b801f0

	if (1 != fwrite(buf, len, 1, m_file)) {
	throw std::runtime_error("fwrite() failed");
	}

To Reproduce

I ran the following using msgpack-cxx 7.0.0 and Clang 19.1.7.

#include <cstdio>

#include <msgpack.hpp>
#include <msgpack/fbuffer.hpp>

int main() {
    std::FILE* file = std::fopen("test_error_in_msgpack.data", "wb");
    if (file == nullptr) {
        std::perror("Failed to open file");
        return 1;
    }
    msgpack::fbuffer buffer(file);
    msgpack::packer<msgpack::fbuffer> packer(buffer);
    packer.pack(std::string());
    return 0;
}

Expected behavior

No exception should happen.

[redboltz]

It seems that fbuffer has a problem.

I think that a possible fix is as follows:

        if (!buf) return;
        if (len == 0) return; // this line should be added

[MusicScience37]

It worked for my case.
Thank you for quick response.

[redboltz]

Could you create a pull request to fix the issue?

[redboltz]

Thank you.