File Mapping and IPC
File mapping enables a process to treat the content of a file as if it were a block of
memory in the process's address space. Therefore, instead of using file input and
output (I/O) operations, the process can use simple pointer operations to
examine and modify the contents of the file.
The Win32 API enables two or more processes to access the same
file-mapping object. Each process receives a pointer to memory in its own address space. With
this pointer, the process can read or modify the contents of the file.
There are three ways an application can share a file-mapping object created in
one process with another process:
- Inheritance. The first process creates the file-mapping object and then allows
the handle of the object to be inherited by a child process.
- Named file mapping. The first process creates the file-mapping object with a
well-known name (which can be different from the filename). The second process
opens the file-mapping object by specifying the well-known name. Alternatively,
the first process can create a file-mapping object with a unique name and
communicate that name to the second process through some other IPC mechanism (named
pipe, mailslot, and so on).
- Handle duplication. The first process creates the file-mapping object and then
passes the handle of the object to the second process. The second process then
duplicates the handle to gain access to the shared memory. The original
process can communicate the file-mapping handle to the second process through one of
the other IPC mechanisms described in this topic (named pipe, mailslot, and so
on). For more information about duplicating handles, see Synchronization.
When two or more processes have read-write access to a shared memory block,
they must use some sort of synchronization object, such as a semaphore, to
prevent data corruption in a multitasking environment.
File mapping is quite efficient and also provides operating-system
supported security attributes that can help prevent unauthorized data
corruption. File mapping can be used only between processes on a local computer; it
cannot be used over a network. An application can, however, create a file-mapping
object to a file on a remotely mounted volume. For example, if a remote server
is mounted as the F drive, an application can create a file-mapping object to a
file on that volume. However, a process running on a remote server cannot
share a file-mapping object with a process running on a local computer.
Key Point File mapping is an efficient way for two or more processes on the same
computer to share data, but the developer must provide synchronization between the
processes. For more information, see
File Mapping, and
Synchronization.
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