What are the usage differences between size_t and off_t?

Question

Other than the size of the values that each type can hold, what are the main differences in usage between size_t and off_t? Is it just a convention that size_t types are used for absolute sizes and off_t types are used for offsets? Or does it go deeper than that?

I am writing a wrapper class to enable the writing of large files using mmap and I want to know what the best types are to use for their arguments. Given that I want to write to files > 4GB, I'm tempted to use size_t for everything, but is that the best practice? (or should I be using some off64_t types for certain functions?)

For example, should my writeAt function be declared as:

MMapWriter::writeAt(off64_t offset, const void* src, size_t size)

or

MMapWriter::writeAt(size_t offset, const void* src, size_t size)

Answer 1

size_t is for objects, off_t is for files.

mmap merges the two concepts, pretty much by definition. Personally I think I'd use size_t, since no matter what else it is, a mapped file is also an array in (virtual) memory.

size_t is standard C++, off_t is Posix, and off64_t is a GNU extension that goes with the functions fopen64, ftello64, etc. I think it should always be the same type as off_t on 64 bit GNU systems, but don't bet your company on that without checking.

Should it be relevant, off_t is signed whereas size_t is unsigned. But the signed counterpart to size_t is ptrdiff_t, so when you need a signed type it doesn't automatically mean you should use off_t or off64_t.

Answer 2

size_t is part of the C++ (and C) standards, and refers to the type of a sizeof expression. off_t is defined by the Posix standard, and refers to the size of a file.