What is mmap () used for?

The mmap() function is used for mapping between a process address space and either files or devices. When a file is mapped to a process address space, the file can be accessed like an array in the program.

Is mmap Atomic?

It’s not only not atomic, it’s permitted to unmap all the pages first and then start mapping the new ones in any order or otherwise operate in any way it wants to so long as it doesn’t break pages not altered by the operation.

What is mmap memory?

From Wikipedia, the free encyclopedia. In computing, mmap(2) is a POSIX-compliant Unix system call that maps files or devices into memory. It is a method of memory-mapped file I/O. It implements demand paging because file contents are not read from disk directly and initially do not use physical RAM at all.

What is Map_shared?

This behavior is determined by including exactly one of the following values in flags: MAP_SHARED Share this mapping. Updates to the mapping are visible to other processes mapping the same region, and (in the case of file-backed mappings) are carried through to the underlying file.

How do I allocate more memory with mmap?

You can use mmap to allocate an area of private memory by setting MAP_ANONYMOUS in the flags parameter and setting the file descriptor fd to -1 . This is similar to allocating memory from the heap using malloc , except that the memory is page-aligned and in multiples of pages.

What are the advantages of using mmap ()?

mmap has the advantage when you have random access on big files. Another advantage is that you access it with memory operations (memcpy, pointer arithmetic), without bothering with the buffering. Normal I/O can sometimes be quite difficult when using buffers when you have structures bigger than your buffer.

What causes mmap to fail?

The mmap() function will fail if: [EACCES] The fildes argument is not open for read, regardless of the protection specified, or fildes is not open for write and PROT_WRITE was specified for a MAP_SHARED type mapping. The fildes argument is not a valid open file descriptor.

Is mmap shared memory?

mmap / shm_open is the new POSIX way to do shared memory and is easier to use.

When should you use mmap?

6 Answers. mmap is great if you have multiple processes accessing data in a read only fashion from the same file, which is common in the kind of server systems I write. mmap allows all those processes to share the same physical memory pages, saving a lot of memory.

Are atomic property accesses good for performance?

Here’s the interesting part: Performance using atomic property accesses in uncontested (e.g. single-threaded) cases can be really very fast in some cases. In less than ideal cases, use of atomic accesses can cost more than 20 times the overhead of nonatomic.

What are atomic operations in threads?

Any time two threads operate on a shared variable concurrently, and one of those operations performs a write, both threads must use atomic operations.

What is @atomic in Java?

Atomic is the default: if you don’t type anything, your property is atomic. An atomic property is guaranteed that if you try to read from it, you will get back a valid value. It does not make any guarantees about what that value might be, but you will get back good data, not just junk memory.

What is a non-atomic memory operation?

A memory operation can be non-atomic because it uses multiple CPU instructions, non-atomic even when using a single CPU instruction, or non-atomic because you’re writing portable code and you simply can’t make the assumption. Let’s look at a few examples. Suppose you have a 64-bit global variable, initially zero.