msync

Name

msync - synchronizes a file with a memory map

Library

libc.lib

Synopsis

#include <sys/mman.h>

int msync (void *addr, size_t len, int flags);

Return values

Upon successful completion, msync() shall return 0; otherwise, it shall return -1 and set errno to indicate the error.

Detailed description

The msync system call writes any modified pages back to the file system. if len is non-zero, only those pages containing addr and len-1 succeeding locations will be examined. The flags argument may be specified as follows:

`MS_ASYNC`
	This flag is currently not supported.
`MS_SYNC`
	Perform synchronous writes
`MS_INVALIDATE`
	Invalidate all cached data

Examples

/*
* Detailed description: Example to sync changes on mapped memory to file.
* Precondition: None
*               
*/
#include <unistd.h>
#include <stdio.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <string.h>
int main(void)
{
    int fd = -1;
    char* mapaddr;
    int len = getpagesize();
    int prot = PROT_WRITE | PROT_READ;
    if((fd = open("C:\\Test.txt", O_RDWR | O_CREAT, 0666) ) < 0)
    {
         printf("File open failed");
    }
    mapaddr = (char*)mmap((void*)0, len, prot, MAP_SHARED, fd, 0);
    if(mapaddr == MAP_FAILED)
    {
         printf("mmap on file failed");
    }
    strcpy(mapaddr, "This is a write through mapped memory ");
    if(-1 == msync(mapaddr, len, MS_SYNC))
    {
        printf("Sync on mapped memory to file failed");
    }
    printf("Sync on mapped memory to file succeeded");
}

Errors

The msync system call will fail if:

[`EBUSY`]
	Some or all of the pages in the specified region are locked and `MS_INVALIDATE` is specified. (This errno is currently not supported)
[`EINVAL`]
	The addr argument is not a multiple of the hardware page size.
[`EINVAL`]
	The len argument is too large or negative.
[`EINVAL`]
	The flags argument was both MS_ASYNC and MS_INVALIDATE. Only one of these flags is allowed or the flags are invalid.
[`ENOMEM`]
	The addresses in the range starting at addr and continuing for len bytes are outside the range allowed for the address space of a process or specify one or more pages that are not mapped.