Delete unix shared memory segments using a C++ program

Question

I am trying to write a C++ program to delete the shared memory segments. I know it can be done from cmd prompt using

ipcs -m | awk '{ print $2 }' | xargs ipcrm -m

But I am trying to do it using C++ so that I create a shared memory garbage collector.

The idea is to delete shared memory segments that are not attached to any process( nattach ==0) or the status == NULL

I think you will have to gain kernel mode privileges if you want to do it by-hand. You may still use existing ABI or API if available, which may be easier :) — Jaffa, Mar 01 '12 at 20:08
i want to parse the out put of this code and check if nattach==0 and then pass the args to ipcrm `ipcs -m | awk '{ print $2 }'` — venuktan, Mar 01 '12 at 20:14

score 5 · Accepted Answer · answered Apr 10 '12 at 22:10

I finally have the the answer to my own question. It can be done using shmctl flags. shmctl(0,SHM_INFO,&shm_info); gives number of segments currently present.

shmctl(i , SHM_STAT , &shm_segment) gives the segment id

it can also be accessed by shm_segment.shm_id

#include <sys/shm.h>     

int delete_segment(int seg_id){
    if ((shmctl(seg_id,IPC_RMID,0))==-1){
    std::cout<<" ERROR(C++)with shmctl(IPC_RMID): "<<strerror(errno)<<std::endl;
    return -1;
    }else//on success
        return 0;
}

void clean_segments(){

    struct shmid_ds shm_info;
    struct shmid_ds shm_segment;
    int max_id = shmctl(0,SHM_INFO,&shm_info);
    if (max_id>=0){
        for (int i=0;i<=max_id;++i) {
                int shm_id = shmctl(i , SHM_STAT , &shm_segment);
                if (shm_id<=0)
                    continue;
                else if (shm_segment.shm_nattch==0){
                    delete_segment(shm_id);
                }
        }
    }
    return result;
}

kamae · Answer 2 · 2012-03-01T23:30:06.497

2

According to a source code of the ipcrm, it calls shmctl.

shmctl(id, IPC_RMID, NULL)

edited Mar 01 '12 at 23:30

answered Mar 01 '12 at 23:18

kamae

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Yes, I know `shmctl(id, IPC_RMID, NULL)` but it only works if you have the segment ids. I need to be able to get a list of all current segment ids an check if there are any processes that are attached to it and check if they are active and delete the non active once's – venuktan Mar 03 '12 at 04:02

score 1 · Answer 3 · answered Mar 01 '12 at 23:06

I would suggest executing strace ipcrm -m <your-arguments> and see what system calls it performs. In most cases that should be suficient (at least it will point you in the right direction), if not - look at the source code of ipcrm. I'm pretty sure you do not need any special privileges (ipcrm on my system does not have SUID or SGID bits set).

score 0 · Answer 4 · answered Feb 05 '17 at 00:00

The code below use I:

    void clean_segments(int startId, int endId) {
    for (int i=startId; i<=endId; ++i) {
        struct shmid_ds shm_segment;
        int shm_id = shmctl(i, SHM_STAT, &shm_segment);
        delete_segment(shm_id);
        printf("Segment %d has been deleted\n", shm_id);
    }}

clean_segments(1146894, 6357160);// example of using the code above

Delete unix shared memory segments using a C++ program

4 Answers4