ofstream 以二进制模式在文件上写入空字符

Question

We are facing an issue with device reboot. We are running our application in linux os on a raspberry pi board. We are maintaining a log file to which we are appending the records every 10sec with below code. One write can have one or more records in the pBuffer.

bool FileOP::Append(const std::string & PathName, const char * pBuffer, uint64_t Size)
{
    bool AppendSuccessful = false;
    std::ofstream File;

    try
    {
        File.exceptions(std::ofstream::badbit | std::ofstream::failbit);
        File.open(PathName.c_str(), std::ofstream::out | std::ofstream::binary | std::ofstream::app);
        File.write(pBuffer, Size);
        File.close();
        AppendSuccessful = true;
    }
    catch (std::exception & e)
    {
        std::cout << "Error when appending string to file: " << PathName
                  << std::strerror(errno) << " Exception : " << e.what() << std::endl;
    }

    return AppendSuccessful;
}

We have observed that when we write the data and exactly on that time if we reboot the board(remove power), we are getting a record with complete NULL characters. File size will be increased based on the record size, for example if we write 100 bytes file size will be header size(100) + old data size (100) + new data(100) = 300bytes. When we try to read the file we are getting last 100 bytes full of NULL characters.

1. If the record is not written completely how the file size is increasing?
2. How exactly the record is filled with NULL? we have verified that every new record written does not contain NULL characters.

Answer 1

This will depend on the filesystem in use, but what is likely happening here is that the filesystem is committing the change to the file metadata (in this case, its length) before all the data is written. If you require the file to be consistent even in case of a crash, and are using ext4, try mounting with the data=journal option. Note that this has performance impacts due to disabling delayed allocation.

Answer 2

1. If the record is not written completely how the file size is increasing?

If null bytes are written, those increase file size just as much as any other byte.

2. How exactly the record is filled with NULL?

It can potentially happens on the following line:

 File.write(pBuffer, Size);

If pBuffer contains null chars, then those null chars are written to the file.