unistd.h 中的 pwrite() 未将数据写入 C++ 中的文件

Question

Here is my code snippet:

void
    DirectIO::writeFileUnix(const std::string &file_path,
                            unsigned long long int index,
                            size_t size,
                            std::string &bytes_data) {
        WRITE_COUNT++;

        const int file_descriptor = open(file_path.c_str(),
                                         O_WRONLY | O_DIRECT | O_SYNC | O_CREAT, S_IRUSR | S_IWUSR);
        if (file_descriptor == -1 || size % SECTOR_SIZE != 0) {
            throw std::exception();
        }

        char *buffer = &bytes_data[0];
        ssize_t number_of_bytes_written = pwrite(file_descriptor, buffer, size, index * size);

        if (number_of_bytes_written == -1 || number_of_bytes_written < size) {
            throw std::exception();
        }

        close(file_descriptor);
    }

The function seems correct logically. But number_of_bytes_written is always -1. The file may get created if not exists. I don't understand why it isn't working.

Update 1

Okay, so I got the problem. char* buffer is limited by null character. Hence, the line char *buffer = &bytes_data[0]; will point to a only a small part of string bytes_data .

When I came to know this, I updated that part of my code to

void *buffer = bytes_data.data();
        ssize_t number_of_bytes_written = pwrite(file_descriptor, buffer, size, index * size);

        if (number_of_bytes_written == -1 || number_of_bytes_written < size) {
            throw std::exception();
        }

And data() function of std::string doesn't care about null characters according to its C++ reference page as well as this SO post

But still, it isn't working out.

If anyone wants to try out the code, I am posting here my sample data to try on.

bytes_data - "\000\000\000\000\005\000\000\000\000\000\000\000\000\000\377\377\377\377\377\377\377\377\377\377\377\377\377\377\377\377\000....\000" (octal representation as supported by clion)
index = 0
size = 4096 (length of bytes_data)
file_path = "abc.binary"

Update 2

I tried writing a string will no null character and still it throws an error. Hence, the problem isn't will null character. This seems some memory alignment issue as the disk alignment is already taken care of (I might be wrong because of my limited knowledge in C++).

Answer 1

As, updated in the question at last, the issue wasn't due to null character. Well it was at some point, but the modified code in last edit made sure that null character caused no issues.

With the help of @IanAbbott, I found out that the issue was with (virtual) memory alignment. Until then, I always thought that only file offsets are needed to be aligned. But by looking at man page of open() , I found that virtual memory address space also needs to be aligned with SECTOR_SIZE.

By reading a few more articles and documents, I found that mmap is made available for this task. The address returned by mmap (a memory space mapped to file) is always aligned. Hence, finally, I used mmap for doing the task and the code ran without any errors.