How does mmap work in linux

An efficient implementation of mmap is actually only possible from a practical perspective because of paging and virtual memory otherwise, it would require reading the whole region into memory before the call completes. Not exactly, this ties into the next answer though, so I'll cover it there. Kind of. What's actually happening in most cases is that mmap is providing copy-on-write access to that file's data in the page cache.

As a result, the usual cache restrictions on data lifetime apply if the system needs space, pages can be dropped or flushed to disk if they're dirty from the cache and need to be faulted in again. No, because of how virtual memory works. Each process has it's own virtual address space, with it's own virtual mappings. It's worth noting that mmap doesn't just work on files, you can also do other things with it such as: Directly mapping device memory if you have sufficient privileges.

This is actually used on many embedded systems to avoid the need to write kernel mode drivers for new hardware. Map shared memory segments. Explicitly map huge pages. Allocate memory that you can then call madvise 2 on which in turn lets you do useful things like prevent data from being copied to a child process on fork 2 , or mark data for KSM Linux's memory deduplication feature. Improve this answer. For very large requests, malloc uses the mmap system call to find addressable memory space.

This process helps reduce the negative effects of memory fragmentation when large blocks of memory are freed but locked by smaller, more recently allocated blocks lying between them and the end of the allocated space. 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. Memory-mapped file objects behave like both bytearray and like file objects.

You can use mmap objects in most places where bytearray are expected; for example, you can use the re module to search through a memory-mapped file. Skip to content Android Windows Linux Apple. Active Oldest Votes. Improve this answer. Wyzard Wyzard I have a situation where I am working on an embedded system with an ARM9 dual core and I am creating two structs in a driver to hold all the data I need for two character devices which I register in a platform probe.

Long story short, mmap does not fail in the user space, but when I write to data that should be mapped, only some of it gets changed in the equivalent kernel space areas.

Care to speculate? For starters who are new to the concept of virtual memory, found this link to be a good place to start. The short answer is: it depends. Sign up or log in Sign up using Google. Sign up using Facebook. Sign up using Email and Password. Post as a guest Name. Email Required, but never shown. The Overflow Blog. If this argument is NULL, then the kernel can place the mapping anywhere it sees fit. This argument is used to control what kind of access is permitted.

The access types of read, write and execute are the permissions on the content. This argument is used to control the nature of the map. Following are some common values of the flags:. This is offset from where the file mapping started.