fwrite after fread. It's different

Question

I read a text file and it works very well.

std::string str_buf = "";
FILE *file = fopen("/home/pi/log_2019-03-07.txt", "r");
if (file != NULL)
{
    while (true)
    {
        char buffer[MAX_BUFFER_SIZE] = { 0x00, };
        size_t rSize = fread(buffer, MAX_BUFFER_SIZE, sizeof(char), file);

        str_buf += buffer;

        if (rSize == 0)
            break;
    }

    printf("%s", str_buf.data());
    fclose(file);
}

Then I try to write it to same path, another name. This is the code:

FILE *writefile = fopen("/home/pi/WriteTest.txt", "wb");
if (writefile != NULL)
{
    int offset = 0;
    while (true)
    {
        size_t wSize = fwrite(&str_buf.data()[offset], sizeof(char), strlen(str_buf.data()) - 1, writefile);

        offset += (int)wSize;

        if (offset >= strlen(str_buf.data()))
            break;
    }
    fclose(writefile);
}

If I try to execute this code, it works. I open WriteTest.txt , it has same string. It's perfectly same.
But I found WriteTest.txt 's volume is almost 2 twice read text.

Why it happens?

Answer 1

size_t wSize = fwrite(&str_buf.data()[offset], sizeof(char), strlen(str_buf.data()) - 1, writefile);

you start writing the text at offset &str_buf.data()[offset] but you write the length of a string starting at position 0. You are writing offset bytes too much. You should

size_t wSize = fwrite(&str_buf.data()[offset], sizeof(char), 
                      strlen(str_buf.data()) - offset, writefile);

Also, you don't write string length nor a NUL terminator. So you'd have trouble figuring out how much to read unless, like in your simple example, it is at file's end.

And last, it's better to use str_buf.length() rather than strlen. It's faster and works with strings that have NUL in the middle.