shm.h File Reference

SHM_RDONLY

Attach read-only (else read-write)

SHM_RND

Round attach address to SHMLBA

SHMLBA

Segment low boundary address multiple

SHM_R

SHM_W

SHM_LOCK

SHM_UNLOCK

SHM_STAT

SHM_INFO

Typedef pid_t

typedef __pid_t

pid_t

_PID_T_DECLARED

Typedef time_t

typedef __time_t

time_t

_TIME_T_DECLARED

Typedef size_t

typedef __size_t

size_t

_SIZE_T_DECLARED

shmget ( key_t, int, int )

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

 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;
}

Return Value: 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.

shmat ( int, const void *, int )

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

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
shmflg	Note: This description also covers the following functions - shmdt()

Return Value: Upon success, shmat returns the address where the segment is attached; otherwise, -1 is returned and errno is set to indicate the error. The shmdt function returns the value 0 if successful; otherwise the value -1 is returned and errno is set to indicate the error.

shmdt ( const void * )

IMPORT_C int

shmdt

(

const void *

shmaddr

)

Parameters
shmaddr	Refer to shmat() for the documentation

shmctl ( int, int, struct shmid_ds * )

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

 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_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.

The shmid_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:

#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;
}

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