11-26-2008
Quote:
Originally Posted by
hurricane86
Hi everbody, i have a problem with shared memory and child-processes in C (unix).
I have a server that do forks to create (N) child processes. This processes work with a shared "stuct" thanks to shared memory and a semaphore. The problem is when a child modify the shared memory and the others can't see its changes...but if i done it in the main process it work fine.
finally, the question is:
can a child-process "realloc" a shared memory and the others can see it?
Sorry for my bad english, thanks a lot bye.
Once new processes fork off, their heap and stack spaces are not shared. In fact, neither are their memory mappings -- changing the
mapping of one process won't change the mapping in the others, so you can't enlarge the memory in one and expect the others to mirror it. So somehow, they all have to have the right mapping in the first place.
One way this could work is to
mmap() a file into memory with the MAP_SHARED flag, and give the mapping a much longer length than the length of the file. It will let you do this, it'll just give you a
bus error signal when you use memory outside the bounds of the file. You can extend the size of the file with
truncate() when you need to. You'll need to remember which bits of file/memory are being used by yourself.
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LEARN ABOUT POSIX
shm_overview
SHM_OVERVIEW(7) Linux Programmer's Manual SHM_OVERVIEW(7)
NAME
shm_overview - overview of POSIX shared memory
DESCRIPTION
The POSIX shared memory API allows processes to communicate information by sharing a region of memory.
The interfaces employed in the API are:
shm_open(3) Create and open a new object, or open an existing object. This is analogous to open(2). The call returns a file descriptor
for use by the other interfaces listed below.
ftruncate(2) Set the size of the shared memory object. (A newly created shared memory object has a length of zero.)
mmap(2) Map the shared memory object into the virtual address space of the calling process.
munmap(2) Unmap the shared memory object from the virtual address space of the calling process.
shm_unlink(3) Remove a shared memory object name.
close(2) Close the file descriptor allocated by shm_open(3) when it is no longer needed.
fstat(2) Obtain a stat structure that describes the shared memory object. Among the information returned by this call are the
object's size (st_size), permissions (st_mode), owner (st_uid), and group (st_gid).
fchown(2) To change the ownership of a shared memory object.
fchmod(2) To change the permissions of a shared memory object.
Versions
POSIX shared memory is supported since Linux 2.4 and glibc 2.2.
Persistence
POSIX shared memory objects have kernel persistence: a shared memory object will exist until the system is shut down, or until all pro-
cesses have unmapped the object and it has been deleted with shm_unlink(3)
Linking
Programs using the POSIX shared memory API must be compiled with cc -lrt to link against the real-time library, librt.
Accessing shared memory objects via the filesystem
On Linux, shared memory objects are created in a (tmpfs(5)) virtual filesystem, normally mounted under /dev/shm. Since kernel 2.6.19,
Linux supports the use of access control lists (ACLs) to control the permissions of objects in the virtual filesystem.
NOTES
Typically, processes must synchronize their access to a shared memory object, using, for example, POSIX semaphores.
System V shared memory (shmget(2), shmop(2), etc.) is an older shared memory API. POSIX shared memory provides a simpler, and better
designed interface; on the other hand POSIX shared memory is somewhat less widely available (especially on older systems) than System V
shared memory.
SEE ALSO
fchmod(2), fchown(2), fstat(2), ftruncate(2), mmap(2), mprotect(2), munmap(2), shmget(2), shmop(2), shm_open(3), shm_unlink(3), sem_over-
view(7)
COLOPHON
This page is part of release 4.15 of the Linux man-pages project. A description of the project, information about reporting bugs, and the
latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.
Linux 2016-12-12 SHM_OVERVIEW(7)