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msync(2) [linux man page]

MSYNC(2)						     Linux Programmer's Manual							  MSYNC(2)

msync - synchronize a file with a memory map SYNOPSIS
#include <sys/mman.h> int msync(void *addr, size_t length, int flags); DESCRIPTION
msync() flushes changes made to the in-core copy of a file that was mapped into memory using mmap(2) back to disk. Without use of this call there is no guarantee that changes are written back before munmap(2) is called. To be more precise, the part of the file that corre- sponds to the memory area starting at addr and having length length is updated. The flags argument may have the bits MS_ASYNC, MS_SYNC, and MS_INVALIDATE set, but not both MS_ASYNC and MS_SYNC. MS_ASYNC specifies that an update be scheduled, but the call returns immediately. MS_SYNC asks for an update and waits for it to complete. MS_INVALIDATE asks to invalidate other mappings of the same file (so that they can be updated with the fresh values just written). RETURN VALUE
On success, zero is returned. On error, -1 is returned, and errno is set appropriately. ERRORS
EBUSY MS_INVALIDATE was specified in flags, and a memory lock exists for the specified address range. EINVAL addr is not a multiple of PAGESIZE; or any bit other than MS_ASYNC | MS_INVALIDATE | MS_SYNC is set in flags; or both MS_SYNC and MS_ASYNC are set in flags. ENOMEM The indicated memory (or part of it) was not mapped. CONFORMING TO
POSIX.1-2001. This call was introduced in Linux 1.3.21, and then used EFAULT instead of ENOMEM. In Linux 2.4.19 this was changed to the POSIX value ENOMEM. AVAILABILITY
On POSIX systems on which msync() is available, both _POSIX_MAPPED_FILES and _POSIX_SYNCHRONIZED_IO are defined in <unistd.h> to a value greater than 0. (See also sysconf(3).) SEE ALSO
mmap(2) B.O. Gallmeister, POSIX.4, O'Reilly, pp. 128-129 and 389-391. COLOPHON
This page is part of release 3.27 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at Linux 2008-04-22 MSYNC(2)

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msync(3C)						   Standard C Library Functions 						 msync(3C)

msync - synchronize memory with physical storage SYNOPSIS
#include <sys/mman.h> int msync(void *addr, size_t len, int flags); DESCRIPTION
The msync() function writes all modified copies of pages over the range [addr, addr + len) to the underlying hardware, or invalidates any copies so that further references to the pages will be obtained by the system from their permanent storage locations. The permanent storage for a modified MAP_SHARED mapping is the file the page is mapped to; the permanent storage for a modified MAP_PRIVATE mapping is its swap area. The flags argument is a bit pattern built from the following values: MS_ASYNC perform asynchronous writes MS_SYNC perform synchronous writes MS_INVALIDATE invalidate mappings If flags is MS_ASYNC or MS_SYNC, the function synchronizes the file contents to match the current contents of the memory region. o All write references to the memory region made prior to the call are visible by subsequent read operations on the file. o All writes to the same portion of the file prior to the call may or may not be visible by read references to the memory region. o Unmodified pages in the specified range are not written to the underlying hardware. If flags is MS_ASYNC, the function may return immediately once all write operations are scheduled; if flags is MS_SYNC, the function does not return until all write operations are completed. If flags is MS_INVALIDATE, the function synchronizes the contents of the memory region to match the current file contents. o All writes to the mapped portion of the file made prior to the call are visible by subsequent read references to the mapped memory region. o All write references prior to the call, by any process, to memory regions mapped to the same portion of the file using MAP_SHARED, are visible by read references to the region. If msync() causes any write to the file, then the file's st_ctime and st_mtime fields are marked for update. RETURN VALUES
Upon successful completion, msync() returns 0; otherwise, it returns -1 and sets errno to indicate the error. ERRORS
The msync() function will fail if: EBUSY Some or all of the addresses in the range [addr, addr + len) are locked and MS_SYNC with the MS_INVALIDATE option is speci- fied. EAGAIN Some or all pages in the range [addr, addr + len) are locked for I/O. EINVAL The addr argument is not a multiple of the page size as returned by sysconf(3C). The flags argument is not some combination of MS_ASYNC and MS_INVALIDATE. EIO An I/O error occurred while reading from or writing to the file system. ENOMEM Addresses in the range [addr, addr + len) are outside the valid range for the address space of a process, or specify one or more pages that are not mapped. EPERM MS_INVALIDATE was specified and one or more of the pages is locked in memory. USAGE
The msync() function should be used by programs that require a memory object to be in a known state, for example in building transaction facilities. Normal system activity can cause pages to be written to disk. Therefore, there are no guarantees that msync() is the only control over when pages are or are not written to disk. ATTRIBUTES
See attributes(5) for descriptions of the following attributes: +-----------------------------+-----------------------------+ | ATTRIBUTE TYPE | ATTRIBUTE VALUE | +-----------------------------+-----------------------------+ |Interface Stability |Standard | +-----------------------------+-----------------------------+ |MT-Level |MT-Safe | +-----------------------------+-----------------------------+ SEE ALSO
memcntl(2), mmap(2), sysconf(3C), attributes(5), standards(5) SunOS 5.10 24 Jul 2002 msync(3C)

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