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Full Discussion: Loading Data in shared memory (C++)
Top Forums Programming Loading Data in shared memory (C++) Post 302467563 by Corona688 on Friday 29th of October 2010 10:20:22 PM
Old 10-29-2010
Instead of using shmat, how about mmap? This solves the storage problem -- you no longer need to explicitly write to disk, just alter the memory and the OS will follow suit. It also makes it possible to use data larger than your system's memory without excessive swapping, since it only reads in the parts you're presently using. It also makes it easier for your readers to find the file -- it's just an ordinary file, not some weird special memory object thingamabob.

It does make the file fixed length though -- or at least, cumbersome to change the length of.

Code:
#include <sys/mman.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

...

mystruct str, *shared;
int fd, n, len;

fd=open("filename", O_RDWR);
memset(&str, 0, sizeof(str));

// Create a blank file 1024 records long
for(n=0; n<1024; n++)
{
        write(fd, &str, shared);
}

// Mapped area must be a multiple of the page size
len=sizeof(str) * 1024;
if(len % getpagesize())
{
        len -= (len % getpagesize());
        len += getpagesize();
}

// Map the file into memory.
mystruct *mem=mmap(NULL, len, PROT_READ|PROT_WRITE,
        MAP_SHARED, fd, 0);
// We can now close the fd.
close(fd);

if(mem == MAP_FAILED)
{
        fprintf(stderr, "Couldn't map memory\n");
}
else
{
        fprintf(stderr, "mapped into %p\n", mem);
        // As the contents of the memory change, so will the contents on disk.
        // This I/O may just be cached in memory for the OS to write when
        // convenient, but the data shared between processes will remain
        // current and consistent.
        strcpy(mem[0].fName, "Foo");
        strcpy(mem[0].lName, "Bar");
        munmap(mem, len);
}

 

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LEARN ABOUT HPUX

shm_unlink

shm_unlink(2)							System Calls Manual						     shm_unlink(2)

NAME

       shm_unlink - unlink a shared memory object

SYNOPSIS

DESCRIPTION

       The  system  call  removes  the	name of the shared memory object named by the string pointed to by name.  If one or more references to the
       shared memory object exists when the object is unlinked, the name will be removed before returns, but the removal of the memory object con-
       tents will be postponed until all open and map references to the shared memory object have been removed.

RETURN VALUE

       returns the following values:

	      Successful completion.

	      Failure,
		   is set to indicate the error.

ERRORS

       If fails, is set to one of the following values:

	      [EACCES]	     Permission to unlink the named shared memory object is denied.

	      [ENAMETOOLONG] The  length of the name string exceeds or the length of a (pathname) component of the name string exceeds while is in
			     effect.

	      [ENOENT]	     The named shared memory object does not exist.

	      [ENOSYS]	     is not supported by the implementation.

SEE ALSO

       shm_open(2), close(2), mmap(2), munmap(2), privileges(5).

STANDARDS CONFORMANCE

																     shm_unlink(2)
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