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A memory-mapped file is a segment of virtual memory [1] that has been assigned a direct byte-for-byte correlation with some portion of a file or file-like resource. This resource is typically a file that is physically present on disk, but can also be a device, shared memory object, or other resource that an operating system can reference through a file descriptor.
In contrast, if the mapping is private (the MAP_PRIVATE flag is set), the changes will neither be seen by other processes nor written to the file. A process reading from, or writing to, the underlying file will not always see the same data as a different process that has mapped the file, since segments of the file are copied into RAM and only ...
Memory-mapped I/O is preferred in IA-32 and x86-64 based architectures because the instructions that perform port-based I/O are limited to one register: EAX, AX, and AL are the only registers that data can be moved into or out of, and either a byte-sized immediate value in the instruction or a value in register DX determines which port is the source or destination port of the transfer.
Sending that file by using mmap of file data and a cycle of write calls, reduces the context switches to 2 per write call and avoids those previous 2 extra user data copies. Sending the same file via zero copy reduces the context switches to 2 per sendfile call and eliminates all CPU extra data copies (both in user and in kernel space). [1] [2 ...
The command displays the process identifiers (PIDs) of processes using the specified files or file systems. In the default display mode, each PID is followed by a letter denoting the type of access: c current directory. e executable being run. f open file. F open file for writing. r root directory. m mmap'ed file or shared library
All write operations to /dev/zero succeed with no other effects. However, /dev/null is more commonly used for this purpose. When /dev/zero is memory-mapped, e.g., with mmap , to the virtual address space, it is equivalent to using anonymous memory; i.e. memory not connected to any file.
In computer science, a memory map is a structure of data (which usually resides in memory itself) that indicates how memory is laid out. The term "memory map" has different meanings in different contexts.
Copy-on-write (COW), also called implicit sharing [1] or shadowing, [2] is a resource-management technique [3] used in programming to manage shared data efficiently. Instead of copying data right away when multiple programs use it, the same data is shared between programs until one tries to modify it.