Why would 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.

How does mmap work in C?

mmap works by manipulating your process’s page table, a data structure your CPU uses to map address spaces. The CPU will translate “virtual” addresses to “physical” ones, and does so according to the page table set up by your kernel. When you access the mapped memory for the first time, your CPU generates a page fault.

What does mmap return?

Return Value Upon successful completion, the mmap() function returns the address at which the mapping was placed; otherwise, it returns a value of MAP_FAILED, which has a value of 0, and sets errno to indicate the error.

Why is mmap faster than malloc?

Almost always, memory is much faster than disk, and malloc is not what’s costing time. The mmap code is faster because for your program, mmap has resulted in either less disk access, or more efficient disk access, than whatever reads and writes you compared against.

Does mmap use the heap?

mmap is a way of creating new memory regions, independently of malloc (and so independently of the heap).

What does mmap stand for?

MMAP

How does malloc use mmap?

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.

Does mmap affect the heap?

Additionally, malloc() itself can be made more versatile by using mmap() to allocate memory rather than simply raising the process’s break by using sbrk() . Calling sbrk() necessitates that memory may be freed only at the top of the heap (i.e., by lowering the break).