Interface status:

externallyDefinedApi

SHM_RDONLY 010000

Description

Attach read-only (else read-write)

Interface status:

externallyDefinedApi

SHM_RND 020000

Description

Round attach address to SHMLBA

Interface status:

externallyDefinedApi

SHMLBA PAGE_SIZE

Description

Segment low boundary address multiple

Interface status:

externallyDefinedApi

IMPORT_C int shmget(key_t key, int size, int shmflg);

Description

 SHM_R Read access for user.
 SHM_W Write access for user.
 ( SHM_R>>3 )
  Read access for group.
 ( SHM_W>>3 )
  Write access for group.
 ( SHM_R>>6 )
  Read access for other.
 ( SHM_W>>6 )
  Write access for other.

Based on the values of key and shmflg, shmget returns the identifier of a newly created or previously existing shared memory segment. The key is analogous to a filename: it provides a handle that names an IPC object. There are three ways to specify a key: IPC_PRIVATE may be specified, in which case a new IPC object will be created. An integer constant may be specified. If no IPC object corresponding to key is specified and the IPC_CREAT bit is set in shmflg, a new one will be created. The ftok may be used to generate a key from a pathname.

 The mode of a newly created IPC object is determined by OR 'ing the following constants into the shmflg argument: SHM_R Read access for user. SHM_W Write access for user. ( SHM_R>>3 )  Read access for group. ( SHM_W>>3 )  Write access for group. ( SHM_R>>6 )  Read access for other. ( SHM_W>>6 )  Write access for other.

When creating a new shared memory segment, size indicates the desired size of the new segment in bytes. The size of the segment may be rounded up to a multiple convenient to the kernel (i.e., the page size).

Examples:

#include <sys/ipc.h>
#include <sys/shm.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>

#define SHM_SEG_SIZE 1024

int main(void)
{
    int shm_id;
    int perm;
    /*
 Create a shared memory segment
     */
    perm = SHM_R | SHM_W;
    if ((shm_id = shmget(IPC_PRIVATE, SHM_SEG_SIZE,
                         IPC_CREAT | IPC_EXCL | perm))
         == -1) {
        printf("Shared memory create failed with errno %d", errno);
        return -1;
    }
    return 0;
}

Parameters

key_tkey_t key int size int shmflg

Return value

int Upon successful completion, shmget returns the positive integer identifier of a shared memory segment. Otherwise, -1 is returned and errno set to indicate the error.

Interface status:

externallyDefinedApi

IMPORT_C void* shmat(int shmid, const void *shmaddr, int shmflg);

Description

The shmat system call attaches the shared memory segment identified by shmid to the calling process's address space. The address where the segment is attached is determined as follows: If shmaddr is 0, the segment is attached at an address selected by the kernel. If shmaddr is nonzero and SHM_RND is not specified in shmflg, the segment is attached the specified address. (a nonzero addr is not supported) If addr is specified and SHM_RND is specified, addr is rounded down to the nearest multiple of SHMLBA.(a nonzero addr is not supported)

The shmdt system call detaches the shared memory segment at the address specified by shmaddr from the calling process's address space.

Examples:

#include <sys/ipc.h>
#include <sys/shm.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>

#define SHM_SEG_SIZE 1024

int main(void)
{
    int shm_id;
    char *shm_addr;
    /*
 Create a shared memory segment
     */
    if ((shm_id = shmget(IPC_PRIVATE, SHM_SEG_SIZE,
                         IPC_CREAT | IPC_EXCL | 0666))
         == -1) {
        printf("Shared memory create failed with errno %d", errno);
        return -1;
    }
    /*
 Attach the shared memory segment to the
 process address space
     */
    if((shm_addr = (char *)shmat(shm_id, NULL, 0)) == (void *)-1) {
       printf("Shared memory attach failed with errno %d", errno);
       return -1;
    }
    /*
 Copy data to shared memory segment
     */
    strcpy(shm_addr, "some_random_data");
    /*
 Detach the shared memory segment
     */
    if(shmdt(shm_addr) == -1) {
       printf("Shared memory detach failed with errno %d", errno);
    }
    /*
 Remove the shared memory segment
     */
    if(shmctl(shm_id, IPC_RMID, NULL) == -1) {
       printf("Shared memory destroy failed with errno %d", errno);
    }
    return 0;
}

Parameters

int shmid const void *shmaddr int shmflg Note: This description also covers the following functions - shmdt(const void *)shmdt(const void *)

Interface status:

externallyDefinedApi

IMPORT_C int shmdt(const void *shmaddr);

Description

Parameters

const void *shmaddr Refer to shmat(int,const void *,int)shmat(int,const void *,int) for the documentation

Return value

int

Interface status:

externallyDefinedApi

IMPORT_C int shmctl(int shmid, int cmd, struct shmid_ds *buf);

Description

 IPC_STAT Fetch the segment's struct shmid_ds ,
 storing it in the memory pointed to by buf.
 IPC_SET Changes the shm_perm.uid, shm_perm.gid, and shm_perm.mode members of the segment's struct shmid_ds to match those of the struct pointed to by buf.
 IPC_RMID Removes the segment from the system.
 The removal will not take
 effect until all processes having attached the segment have exited;
 however, once the IPC_RMID operation has taken place, no further
 processes will be allowed to attach the segment.

Performs the action specified by cmd on the shared memory segment identified by shmid:

IPC_STAT Fetch the segment's struct shmid_dsshmid_ds , storing it in the memory pointed to by buf. IPC_SET Changes the shm_perm.uid, shm_perm.gid, and shm_perm.mode members of the segment's struct shmid_dsshmid_ds to match those of the struct pointed to by buf. IPC_RMID Removes the segment from the system.

The removal will not take effect until all processes having attached the segment have exited; however, once the IPC_RMID operation has taken place, no further processes will be allowed to attach the segment.

The shmid_dsshmid_ds structure is defined as follows:

struct shmid_ds {
    struct ipc_perm shm_perm;   /* operation permission structure */
    int             shm_segsz;  /* size of segment in bytes */
    pid_t           shm_lpid;   /* process ID of last shared memory op */
    pid_t           shm_cpid;   /* process ID of creator */
    short           shm_nattch; /* number of current attaches */
    time_t          shm_atime;  /* time of last shmat() */
    time_t          shm_dtime;  /* time of last shmdt() */
    time_t          shm_ctime;  /* time of last change by shmctl() */
    void           *shm_internal; /* sysv stupidity */
};

Examples:

# 217 "d:/EPOC/release/9.4/common/generic/openenv/core/include/sys/shm.dosc" 2
# 218 "d:/EPOC/release/9.4/common/generic/openenv/core/include/sys/shm.dosc" 2
# 219 "d:/EPOC/release/9.4/common/generic/openenv/core/include/sys/shm.dosc" 2
# 220 "d:/EPOC/release/9.4/common/generic/openenv/core/include/sys/shm.dosc" 2
# 221 "d:/EPOC/release/9.4/common/generic/openenv/core/include/sys/shm.dosc" 2

#define SHM_SEG_SIZE

int main(void)
{
    int shm_id;
    char *shm_addr;
    /*
 Create a shared memory segment
     */
    if ((shm_id = shmget( (key_t)0 ,  1024 ,
                          001000  |  002000  | 0666))
         == -1) {
        printf("Shared memory create failed with errno %d",  (*__errno()) );
        return -1;
    }
    /*
 Attach the shared memory segment to the
 process address space
     */
    if((shm_addr = (char *)shmat(shm_id,  0 , 0)) == (void *)-1) {
       printf("Shared memory attach failed with errno %d",  (*__errno()) );
       return -1;
    }
    /*
 Copy data to shared memory segment
     */
    strcpy(shm_addr, "some_random_data");
    /*
 Detach the shared memory segment
     */
    if(shmdt(shm_addr) == -1) {
       printf("Shared memory detach failed with errno %d",  (*__errno()) );
    }
    /*
 Remove the shared memory segment
     */
    if(shmctl(shm_id,  0 ,  0 ) == -1) {
       printf("Shared memory destroy failed with errno %d",  (*__errno()) );
    }
    return 0;
}

Parameters

int shmid int cmd struct shmid_dsshmid_ds *buf

Return value

int The shmctl function returns the value 0 if successful; otherwise the value -1 is returned and errno is set to indicate the error.