new and malloc allocates extra 16 bytes

问题

I'm writing on c++ in VS2010 Windows 7. I try to read file of size 64 bytes. Here's the code:

    BYTE* MyReadFile(FILE *f)
{
    size_t result;
    BYTE *buffer;
    long lSize;
    if (f == NULL) 
    {
        fputs ("File error", stderr); 
        exit (1);
    }

    fseek (f, 0, SEEK_END);
    lSize = ftell (f);
    rewind (f);

    //buffer = (BYTE*) malloc (sizeof(char)*lSize);
    buffer = new BYTE[lSize];
    if (buffer == NULL) 
    {
        fputs ("Memory error", stderr); 
        exit (2);
    }

    result = fread (buffer, 1, lSize, f);
    if (result != lSize) 
    {
        fputs ("Reading error",stderr); 
        exit (3);
    }

    fclose (f);
    return buffer;
}

When I get file size it is 64, but when I allocate memory for it with new BYTE[lSize] I get 80 bytes of space and thus strange sequence ээээ««««««««оюою is added to the end of buffer. Can you please tell me how to handle this?

回答1:

There is an important difference between the number of bytes you have allocated, and the number of bytes that you see.

If lsize is 64, you have indeed allocated yourself 64 bytes. This does not mean that behind the screen the C++ run time will have asked exactly 64 bytes to Windows. In practice memory managers ask slightly more memory so they are able to do their own homework. Often these extra bytes are allocated BEFORE the pointer you get back from new/malloc so you will never see them.

However, that is not your problem. The problem is that you read 64 bytes from file using fread. There is no way that fread knows what kind of data you are reading. It could be a struct, a char buffer, a set of doubles, ... It just reads these bytes for you.

This means that if the file contains the characters "ABC" you will get exactly "ABC" back. BUT, in C, strings should be nul-terminated, so if you pass this buffer to printf, it will continue to scan memory until it finds a nul-character.

So, to solve your problem, allocate 1 byte more, and set the last byte to the nul character, like this:

buffer = new BYTE[lSize+1]; 
buffer[lSize] = '\0';

回答2:

What is behind and above is called sentinel.It is used to check if your code does not exceed boundary of allocated memory.When your program overwrite this values, CRT library will report debug messages when you release your buffer.

Look here : http://msdn.microsoft.com/en-us/library/ms220938%28v=vs.80%29.aspx

回答3:

Although this may look like a memory problem, its actually a printing problem (as @Mystical pointed out). You need to put a null termination if you are going to print out anything as a string, else memory will be wildy read till one is encountered (which is UB).

Try this instead:

buffer = new BYTE[lSize + 1];
if (buffer == NULL) 
{
    fputs ("Memory error", stderr); 
    exit (2);
}

result = fread (buffer, 1, lSize, f);
if (result != lSize) 
{
    fputs ("Reading error",stderr); 
    exit (3);
}

buffer[lSize] = '\0';

It'll ensure there is a null terminator at the end of the returned buffer.

回答4:

When memory is allocated, it is not on a per byte basis. Instead it is allocated in aligned blocks of 8 or 16 bytes (possibly with a header at the start, before the pointer). This is usually not a problem unless you create lots (many millions) of little objects. This doesn't have to be a problem in C and isn't even a major problem in Java (which doesn't support array of objects or objects allocated on the stack).

来源：https://stackoverflow.com/questions/9390940/new-and-malloc-allocates-extra-16-bytes

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