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From: Stanislav Kascak <stanislav.kascak@gmail.com>
Date: Fri, 3 May 2019 13:33:43 +0200
Message-ID: <CALLhcm4QGY+eP0_CRiSbJwQ12kOetvTK=6-AtC17x7d+QhGKTw@mail.gmail.com>
Subject: possible problem with memory allocation using calloc/mmap/munmap
To: cygwin@cygwin.com
Content-Type: text/plain; charset="UTF-8"

Hello cygwin team,

I came across a problem with memory allocation/deallocation when
trying to compile and run tests of openldap under cygwin.

I created a test program to simulate sequence of actions. First a
bigger chunk of memory (>~262kB) is allocated using calloc(), then
mmap() is called with requested memory size much bigger than actual
file size. Subsequently,  munmap() to free previous mmap(), and at the
end an attempt to access memory allocated with calloc(). That last
memory access is causing Segmentation fault.

It seems that when mmap() is called with length argument exceeding
size of file, only memory to fit that file is allocated. munmap()
however frees the full specified length. Since (at least on my
computer) big chunk of memory allocated by calloc() is located after
mmap() allocation, munmap() frees even memory of that calloc().

I'm not sure if this is only a problem of my environment, or there is
something I'm missing.

Below is the test (notice that memory allocated by mmap() is only 64k
bytes in front of the memory allocated by calloc(), not 32MB as
requested).

My system is Windows 7 64 bit, cygwin64 3.0.7.

Best regards,

Stanislav Kascak

$ cat a.c
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <errno.h>

int main(int argc, char** argv){
  char* x;
  char* y;
  int fd;
  size_t s1 = 0x40000;
  size_t s2 = 0x2000000;
  x = (char*)calloc(1, s1);
  if(x == NULL){
    printf("calloc failed\n");
  }
  x[0] = 'a';
  fd = open("testfile", O_RDONLY);
  if(fd == -1){
    printf("open failed: %d\n", errno);
  }
  y = mmap(NULL, s2, PROT_READ, MAP_SHARED, fd, 0);
  if(y == MAP_FAILED){
    printf("mmap failed: %d\n", errno);
  }
  close(fd);
  printf("%lx -> %c\n", x, x[0]);
  printf("%lx\n", y);
  if(munmap(y, s2)){
    printf("munmap failed: %d\n", errno);
  }
  printf("%lx -> %c\n", x, x[0]);
  free(x);
}

$ gcc a.c

$ echo "testdata" > testfile

$ ./a.exe
6fffffb0010 -> a
6fffffa0000
Segmentation fault (core dumped)

$ uname -a
CYGWIN_NT-6.1 myhost 3.0.7(0.338/5/3) 2019-04-30 18:08 x86_64 Cygwin

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