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mmap & msync

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# 1  
mmap & msync

I really can't understand why this programm doesn't work (on linux); it open mapped memory, changes one byte and write back to disk:

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

int main() {
int fd, err;
void * mm;
fd = open("math.dat", O_RDWR);
mm = (char*)mmap(0, 10, PROT_READ|PROT_WRITE, MAP_PRIVATE, fd, 0);
if ((int)mm == -1) perror("mmap");

sprintf((char*) mm, "%d\n", 100);
err = msync(mm, 10, MS_ASYNC);
if (err == -1) perror("msync");
err = munmap(mm, 10);
if (err == -1) perror("munmap");

No errors, just no modification on disk.
# 2  
# 3  
Thanks, it works now... And as I reread the manpage, I see it was stated there...
# 4  
i have the same problem and MAP_SHARED does not work. can anyone help?

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