StackOverflow2013

Note that there are some explanatory texts on larger screens.

plurals

POSharing Multiple Variables Via sys/shm.h
primarykey
Id
2414376
data
AcceptedAnswerId
2414502
AnswerCount
2
ClosedDate
CommentCount
0
CommunityOwnedDate
CreationDate
2010-03-10T03:43:58.647
FavoriteCount
1
LastActivityDate
2011-01-29T02:10:23.557
LastEditDate
LastEditorUserId
0
OwnerUserId
257064
ParentId
0
PostTypeId
1
Score
2
ViewCount
2135
LastEditorDisplayName
text
Body
I am trying to share two different using one shared memory block using the shm.h library. I wrote the following example, where one shared memory block is created and is big enough to hold two integers. I then attach two integers to it and create two processes. The first process increments the first integer. The second process then prints out the value of two integers. But what happens is that both of the integers get incremented. What am I doing wrong? I just started learning how to use shm library. This is the code: <pre><code>#include <sys/sem.h> #include <sys/ipc.h> #include <sys/types.h> #include <sys/shm.h> #include <stdio.h> #include <unistd.h> int main() { // Declare variables int shmID; int *data1; int *data2; // Create a shared memory segment if((shmID=shmget(IPC_PRIVATE, 2*sizeof(int), 0666 | IPC_CREAT))<0) { fprintf(stderr,"Problem initializing shared memory\n"); perror("main"); return -1; } if((data1=shmat(shmID,NULL,0))==(int *)-1) { fprintf(stderr,"Problem attaching memory 1\n"); perror("main"); return -1; } if((data2=shmat(shmID,NULL,0))==(int *)-1) { fprintf(stderr,"Problem attaching memory 2\n"); perror("main"); return -1; } printf("%p %p\n",data1,data2); (*data1)=0; (*data2)=0; if(fork()) { // Process 1 will be the incrementer for(int i=0;i<100;i++) { (*data1)++; printf("IN: %d\n",(*data1)); sleep(1); } printf("IN DONE\n"); } else { while((*data1)<50) { printf("OUT: %d %d\n",(*data1),(*data2)); sleep(1); } printf("OUT DONE\n"); } } </code></pre> And this is the output: <pre><code>0x7fcd42a97000 0x7fcd42a96000 IN: 1 OUT: 1 1 IN: 2 OUT: 2 2 IN: 3 OUT: 3 3 </code></pre> I am running this on Gentoo Linux.
Tags
<linux><gcc><shared-memory>
Title
Sharing Multiple Variables Via sys/shm.h
singulars
PostAcceptedAnswerId
1. PO
 singulars
 PostTypePostTypeId
 PTAnswer
PostParentId
1. This table or related slice is empty.
PostTypePostTypeId
1. PTQuestion
UserLastEditorUserId
1. This table or related slice is empty.
UserOwnerUserId
1. USkkrizka
plurals
PostLinksPostIdRelatedPostId
1. This table or related slice is empty.
PostLinksRelatedPostIdPostId
1. This table or related slice is empty.
PostsAcceptedAnswerId
1. This table or related slice is empty.
PostsParentIdCreationDate
1. PO
 singulars
 PostTypePostTypeId
 PTAnswer
2. PO
 singulars
 PostTypePostTypeId
 PTAnswer
VotesPostIdCreationDate
1. VO
 singulars
 PostPostId
 POSharing Multiple Variables Via sys/shm.h
 UserUserId
 This table or related slice is empty.
 VoteTypeVoteTypeId
 VTUpMod
2. VO
 singulars
 PostPostId
 POSharing Multiple Variables Via sys/shm.h
 UserUserId
 This table or related slice is empty.
 VoteTypeVoteTypeId
 VTUpMod
3. VO
 singulars
 PostPostId
 POSharing Multiple Variables Via sys/shm.h
 UserUserId
 USsbditto85
 VoteTypeVoteTypeId
 VTFavorite
CommentsPostId
1. This table or related slice is empty.

Querying!

Guidance

A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.