How to get writes via an mmap mapped memory pointer to flush immediately?

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Copyright and license for this manual page

MMAP(2) Linux Programmer 's Manual                MMAP(2)
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Returns a byte at the current file position as an integer, and advances the file position by 1.,mmap — Memory-mapped file support,Returns a single line, starting at the current file position and up to the next newline. The file position is updated to point after the bytes that were returned.,Returns the current position of the file pointer.

import mmap

# write a simple example file
with open("hello.txt", "wb") as f:
   f.write(b "Hello Python!\n")

with open("hello.txt", "r+b") as f:
   # memory - map the file, size 0 means whole file
mm = mmap.mmap(f.fileno(), 0)
# read content via standard file methods
print(mm.readline()) # prints b "Hello Python!\n"
# read content via slice notation
print(mm[: 5]) # prints b "Hello"
# update content using slice notation;
# note that new content must have same size
mm[6: ] = b " world!\n"
#...and read again using standard file methods
mm.seek(0)
print(mm.readline()) # prints b "Hello  world!\n"
# close the map
mm.close()
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On modern operating systems, it is possible to mmap (pronounced “em-map”) a file to a region of memory. When this is done, the file can be accessed just like an array in the program. ,Note that actual writing may take place at any time. You need to use msync, described below, if it is important that other processes using conventional I/O get a consistent view of the file. ,This flag makes sure the data is actually written to disk. Normally msync only makes sure that accesses to a file with conventional I/O reflect the recent changes. ,Either address was unusable (because it is not a multiple of the applicable page size), or inconsistent flags were given.

size_t page_size = (size_t) sysconf(_SC_PAGESIZE);
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The application cannot write or store any data via the memory mapping which includes any tagged (16-byte) pointers because the pointer attribute will be lost. Some examples of tagged pointers include space pointers, system pointers, invocation pointers etc.. If the DTAMDL(*LLP64) parameter is used when compiling an ILE C program, this limitation does not apply as the pointers will be 8 byte pointers, and their pointer attribute will be preserved.,If the DTAMDL(*LLP64) parameter is used when compiling an ILE C program, this limitation does not apply as the pointers will be 8 byte pointers, and their pointer attribute will be preserved.,The following example creates two files and then produces a contiguous memory mapping of the first data page of each file using two invocations of mmap().,An exact address range specification. The system will set up the mapping at this location if the address range is valid. Any mapping in the successfully mapping range that existed prior to the mapping operation is implicitly unmapped by this operation.


 #include <sys /types.h>
    #include <sys /mman.h>

       void *mmap( void *addr,
       size_t len,
       int protection,
       int flags,
       int fildes,
       off_t off);
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File-backed mapping maps an area of the process's virtual memory to files; i.e. reading those areas of memory causes the file to be read. It is the default mapping type. ,MapViewOfFile win32 function is somewhat equivalent to mmap.,Windows MapViewOfFile win32 function is somewhat equivalent to mmap.,More example source code: SharedHashFile, An open source, shared memory hash table implemented using mmap().

#include <sys/types.h>
#include <sys/mman.h>
#include <err.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

/* This example shows how an mmap of /dev/zero is equivalent to
   using anonymous memory (MAP_ANON) not connected to any file.
   N.B. MAP_ANONYMOUS or MAP_ANON are supported by most UNIX
   versions, removing the original purpose of /dev/zero.
*/
/* Does not work on OS X or macOS, where you can't mmap over /dev/zero */
int main(void)
{
        const char str1[] = "string 1";
        const char str2[] = "string 2";
        pid_t parpid = getpid(), childpid;
        int fd = -1;
        char *anon, *zero;

        if ((fd = open("/dev/zero", O_RDWR, 0)) == -1)
                err(1, "open");

        anon = (char*)mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_ANON|MAP_SHARED, -1, 0);
        zero = (char*)mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);

        if (anon == MAP_FAILED || zero == MAP_FAILED)
                errx(1, "either mmap");

        strcpy(anon, str1);
        strcpy(zero, str1);

        printf("PID %d:\tanonymous %s, zero-backed %s\n", parpid, anon, zero);
        switch ((childpid = fork())) {
        case -1:
                err(1, "fork");
                /* NOTREACHED */
        case 0:
                childpid = getpid();
                printf("PID %d:\tanonymous %s, zero-backed %s\n", childpid, anon, zero);
                sleep(3);

                printf("PID %d:\tanonymous %s, zero-backed %s\n", childpid, anon, zero);
                munmap(anon, 4096);
                munmap(zero, 4096);
                close(fd);
                return EXIT_SUCCESS;
        }

        sleep(2);
        strcpy(anon, str2);
        strcpy(zero, str2);

        printf("PID %d:\tanonymous %s, zero-backed %s\n", parpid, anon, zero);
        munmap(anon, 4096);
        munmap(zero, 4096);
        close(fd);
        return EXIT_SUCCESS;
}
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