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Reply-To: <shanem AT citr DOT com DOT au>
From: "Shane Mann" <shanem AT citr DOT com DOT au>
To: "Cygwin AT Cygwin. Com" <cygwin AT cygwin DOT com>
Subject: MMap offset parameter failing with ENOMEM error
Date: Thu, 19 Sep 2002 12:19:06 +1000
Message-ID: <000b01c25f82$ee240380$8e90a8c0@citr.com.au>
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Hi,

I am having trouble with mmap using the last 'offset' parameter. I have
attached a small piece of code (and sample file) which demonstrates the
problem. Basically if the offset parameter is passed as a variable to mmap
then the call fails with an ENOMEM error. But if you pass 0 as the parameter
and then access the return address from mmap + offset - it is fine. My
question: is mmap handling the offset parameter correctly?

Usage for the program:

./a.exe <offset> <file> eg: ./a.exe 11200 index.idx

To see the address + offset work, comment out the first mmap call and return
statement and uncomment the lines below each one.

Any help appreciated.

Regards,
Shane

Shane Mann
Software Engineer          Phone: +61-7-3259-2223
LeadUp Software Pty Ltd    Fax:   +61-7-3259-2259
339 Coronation Drive,      Email: shane DOT mann AT leadup DOT com DOT au
Milton, QLD, 4064          Web:   http://www.leadup.com.au

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	name="mmap_file2.c"
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	filename="mmap_file2.c"

#include <sys/types.h>=0A=
#include <sys/stat.h>=0A=
#include <sys/mman.h>=0A=
#include <unistd.h>=0A=
#include <string.h>=0A=
#include <signal.h>=0A=
#include <stdio.h>=0A=
#include <errno.h>=0A=
#include <fcntl.h>=0A=
=0A=
#define PAGEPROT PROT_READ|PROT_WRITE=0A=
#define MAPFLAGS MAP_PRIVATE|MAP_ANONYMOUS=0A=
=0A=
static int syspagesize =3D 0, pagemask, offmask, error_cnt =3D 0;=0A=
=0A=
void sigsegv(int unused)=0A=
{=0A=
  _exit(0);=0A=
}=0A=
=0A=
caddr_t femmap(int fd, off_t foff, size_t syz, int prot, u_char *msg) {=0A=
  /* femmap() and femunmap() take care of the need to allocate=0A=
     on page boundaries and in multiples of pages. */=0A=
  off_t pfoff;  /* foff on a page boundary */=0A=
  size_t psyz; /* syz rounded up to full pages */=0A=
  int chopped;=0A=
  caddr_t rslt;=0A=
  if (syspagesize =3D=3D 0) {=0A=
    syspagesize =3D getpagesize();=0A=
    offmask =3D syspagesize - 1;=0A=
    pagemask =3D ~offmask;=0A=
    if (1) printf("--- Syspagesize %d, pagemask %X, offmask %X\n",=0A=
        syspagesize, pagemask, offmask);=0A=
  }=0A=
  printf("foff is %d\n", foff);=0A=
  printf("--- Syspagesize %d, pagemask %X, offmask %X\n",=0A=
        syspagesize, pagemask, offmask);=0A=
  pfoff =3D foff & pagemask;=0A=
  printf("pfoff is %d\n", pfoff);=0A=
  chopped =3D foff & offmask;=0A=
  psyz =3D (syz + chopped + syspagesize - 1) & pagemask;=0A=
=0A=
  printf("**** Vars just before mmap call ****\n");=0A=
  printf("psyz =3D %d\nprot =3D %d\nfd =3D %d\npfoff =3D %d\n", psyz, =
prot, fd, (int)pfoff);=0A=
=0A=
  if ((rslt =3D mmap(0, psyz, prot, MAP_SHARED, fd, pfoff))  =
=3D=3DMAP_FAILED) {=0A=
  /* if ((rslt =3D mmap(0, psyz, prot, MAP_SHARED, fd, 0))  =
=3D=3DMAP_FAILED) {=0A=
  */=0A=
    printf("perror reports errno of %d\n", errno);=0A=
    printf("   Mapping %d at %X <---> %d (%s\n", psyz, (int)rslt, =
(int)pfoff,=0A=
      msg);=0A=
    error_cnt++;=0A=
    exit(1);=0A=
  }=0A=
  if (1)=0A=
    printf("   Mapping %d at %X <---> %d (%s\n", psyz, (int)rslt, =
(int)pfoff,=0A=
      msg);=0A=
  return (rslt + chopped);=0A=
  /* return (rslt + chopped + pfoff);=0A=
  */=0A=
}=0A=
     =0A=
int main(int argc, char **argv)=0A=
{=0A=
  int fh;=0A=
  off_t foff;=0A=
  size_t syz;=0A=
  int prot;=0A=
  u_char *msg;=0A=
  int i;=0A=
  off_t size;=0A=
  u_char *result =3D NULL;=0A=
=0A=
  struct stat finfo;=0A=
=0A=
  foff =3D atoi(argv[1]);=0A=
  printf("Offset is %d\n", foff);=0A=
=0A=
  for (i=3D2; i<argc; i++)=0A=
  {=0A=
     fh =3D open(argv[i], O_RDONLY);=0A=
=0A=
     fstat(fh, &finfo);=0A=
=0A=
     size =3D finfo.st_size;=0A=
=0A=
     printf("File %s opened on descriptor %d.  It is %ld bytes long\n",=0A=
             argv[i], fh, (u_long) size);=0A=
=0A=
     result =3D (u_char *)femmap(fh, foff, (int) size, PROT_READ, =
"mapping fh");=0A=
=0A=
     printf("Result from femmap is %X\n", result);=0A=
     printf("Byte is %d\n", *result);=0A=
=0A=
     close(fh);=0A=
  }=0A=
=0A=
=0A=
  return 0;=0A=
} =0A=


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