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Subject: Segfault when accessing mmaped memory on Cygwin
Message-ID: <c26a63be-f805-7983-1893-74398ac7b60f@netcologne.de>
Date: Fri, 1 Jan 2021 22:11:01 +0100
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From: Thomas Koenig via Cygwin <cygwin AT cygwin DOT com>
Reply-To: Thomas Koenig <tkoenig AT netcologne DOT de>
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Hi,

when trying out uf a certain shared memory allocator would
work on Cygwin, I tried out the sample program below (which
works on Linunx, *BSD, AIX and Solaris) and got a suprising
falure with a segmentation fault at the line

   *i1 = 42;

mmap() had succeeded.

Is this a known issue, and would it be possible to work
around that?

Best regards

	Thomas

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

#define NAME "/random_namexxx"

int main()
{

   int fd;
   long pagesize;
   void *p1, *p2;
   volatile int *i1, *i2;
   size_t size1, size2;
   off_t offset;

   pagesize = sysconf (_SC_PAGE_SIZE);
   if (pagesize == -1)
     {
       perror ("sysconf failed");
       exit (EXIT_FAILURE);
     }

   fd = shm_open (NAME, O_CREAT | O_EXCL | O_RDWR,  S_IRUSR | S_IWUSR);
   if (fd == -1)
     {
       perror ("shm_open failed");
       exit (EXIT_FAILURE);
     }
   shm_unlink (NAME);

   offset = 0;
   size1 = pagesize;
   p1 = mmap (NULL, size1, PROT_READ | PROT_WRITE, MAP_SHARED, fd, offset);
   if (p1 == MAP_FAILED)
     {
       perror ("mmap 1 failed");
       exit (EXIT_FAILURE);
     }
   printf ("p1 = %p\n", p1);
   ftruncate (fd, size1);
   i1 = p1;
   *i1 = 42;
   size2 = 2 * size1;
   p2 = mmap (NULL, size2, PROT_READ | PROT_WRITE, MAP_SHARED, fd, offset);
   if (p2 == MAP_FAILED)
     {
       perror ("mmap 1 failed");
       exit (EXIT_FAILURE);
     }
   printf ("p2 = %p\n", p2);
   i2 = p2;
   ftruncate (fd, size2);
   printf ("%d\n", *i2);
   return 0;
}
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