![]() ( let* ( ( fd ( file-open "example-mmap.scm" ( + open/rdonly open/nonblock))) fstat(2) the file descriptor fd to determine its size ( use memory-mapped-files) ( use posix lolevel) open a file using the posix module, so we have the file descriptor. only to demonstrate how the mmap functions work together. This example does no error checking or cleanup, and serves Memory Mapped I/O Example example-mmap.scm The optional argument LEN specifies the length of the section to be unmapped and defaults to the complete length given when the file was mapped. MMAP should be a mapped file as returned by the procedure map-file-to-memory. Unmaps the section of a file mapped to memory using the C function munmap(). ![]() unmap-file-from-memory (unmap-file-from-memory MMAP ) Returns a machine pointer to the start of the memory region to which the file is mapped. memory-mapped-file-pointer (memory-mapped-file-pointer MMAP) The procedure move-memory! can be used to access the mapped memory. This procedure returns an object representing the mapped file section. The optional argument OFFSET gives the offset of the section of the file to be mapped and defaults to 0. ADDRESS should be a foreign pointer object or #f LEN specifies the size of the section to be mapped PROTECTION should be one or more of the flags prot/read, prot/write, prot/exec or prot/none bitwise-iored together FLAG should be one or more of the flags map/fixed, map/shared, map/private, map/anonymous or map/file FILENO should be the file-descriptor of the mapped file. ![]() Maps a section of a file to memory using the C function mmap(). ![]() map-file-to-memory (map-file-to-memory ADDRESS LEN PROTECTION FLAG FILENO ) Returns #t, if X is an object representing a memory mapped file, or #f otherwise. (require-extension memory-mapped-files) Programming interface memory-mapped-file? (memory-mapped-file? X) In some circumstances, other system calls on Linux will provide better performance see for example sendfile, copy_file_range, and splice.Interface to memory-mapped files for POSIX and Windows systems. mmap avoids some copying, but whether it’s better overall than read/ write depends on the details of what the caller is doing. This is already discussed in detail in the SO question you linked to. There is no buffer involved behind the scenes, at least as long as the process doesn’t write to the pages (and the behaviour then depends on the mmap flags).ĭoes mmap() have an advantage over read()/write(), as far as data transfer path is concerned? When the mapping is file-backed, the data is read into the page cache, and the kernel allows the mmapping process to access the corresponding physical pages in memory directly (through a separate page map). When accessing virtual memory addresses returned by mmap(), what is the path for data transfer? Is there any buffer in kernel space involved in the middle behind the scene? So the kernel reads whatever data is required to fulfill the read (if any - the data might already be in the page cache, or in a pipe buffer, or …), and then copies the requested data into the user space-provided buffer. Yes, and it has to: read reads into a user space-provided buffer, with no alignment constraints, and the data is supposed to be completely dissociated from the file descriptor once it’s been read. Is it correct that read() copies data from file to a buffer in kernel space, then from the kernel buffer to a buffer in user space? This refers to memory mapped in the kernel’s address space. Mmap doesn't require a copy of the file data from kernel to user-space. Is there any difference between the data transfer paths using read()/write() and using mmap() on a file?
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