LINUX shm_open::Bad file descriptor / (SGI Runs Well)

the following is a mistake, and is confusing you.



The problem is that  either the chmod or the ftruncate
could have changed errno.  And likely did.

Test my_memory  only *immediately* after the shm_open
if you want the perror to report anything even remotely meaningful.
And test the result codes from chmod and ftruncate (immediately
after each operation, of course)!

David Anderson

Hello,

  After changing my code to use the file "my_memory" instead of
"/var/tmp/my_memory", it compiled and ran on Linux!

  How can I change the code to use "/var/tmp/my_memory" instead of
"my_memory"?

  Can you find anything else wrong with my code?

  Thanks for reading my postings,
  Christopher Lusardi

:   After changing my code to use the file "my_memory" instead of
: "/var/tmp/my_memory", it compiled and ran on Linux!
:
:   How can I change the code to use "/var/tmp/my_memory" instead of
: "my_memory"?
:
:   Can you find anything else wrong with my code?

shm_open creates shared memory objects, not files. You should therefore not
assume that shm objects will appear in the filesystem namespace either at all or
where you might expect them to appear.

It is therefore incorrect to make a call to chmod(), you should call fchmod()
instead.

The puzzle, of course, is what is the most correct name for the shm object? The
linux manpage suggests that the portable solution is for the name to contain
only a leading slash and no other slashes, thus the name would be "/my_memory".

Unfortunately IRIX does place the SHM objects directly into the filesystem
namespace, so "/my_memory" would attempt to create the file in the root
directory of the filesystem, which is undesired.

In linux, shm objects are usually created in a separate ram-based filesystem
mounted at /dev/shm. This filesystem (for obvious reasons) does not contain a
"var/tmp/" directory, and is in fact usually very empty.

I would suggest the following code: (you should also make the corrections that
David Anderson suggests)

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

#ifdef __sgi
#define SHM_NAMESPACE "/var/tmp/"
#else
#define SHM_NAMESPACE "/"
#endif

void * initialize()
{
        int             my_size;
        void           *my_ptr;
        int             my_memory;

        my_size = 0x100000;     /* number of bytes */

        my_memory = shm_open(SHM_NAMESPACE "my_memory", (O_RDWR | O_CREAT),
(S_IRWXO | S_IRWXG | S_IRWXU));     /* r/w others group users */

        fchmod(my_memory, (S_IRWXG | S_IRWXU)); /* g/u */

        ftruncate(my_memory, my_size);  /* Truncate CREATE file! */

        if (my_memory == -1) {
                perror("shm_open:");
                exit(1);
        }

        my_ptr = (void *) mmap(NULL, my_size, (PROT_READ | PROT_WRITE),
(MAP_SHARED), my_memory, 0);    /* memory */

        if (my_ptr == MAP_FAILED) {
                perror("mmap");
                exit(1);
        }

        if (close(my_memory)) {
                perror("close");
                exit(1);
        }

        return (my_ptr);
}

/********************* Caller Routine **********************/

int main()
{
        (void) initialize();    /* check stubbed routine */
}

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Cheers - Tony 'Nicoya' Mantler :)