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Files Reference

shm.h File


Describes the structures that are used by the subroutines that perform shared memory operations.


#include <sys/shm.h>


The shm.h header file defines the following symbolic constants, types, and structures:


typedef unsigned short   shmatt_t;

Symbolic Constants:

SHMLBA          segment low boundary address multiple
SHMLBA_EXTSHM   SHMLBA value when environment variable EXTSHM=ON
SHM_RDONLY      attach read-only (else read-write)
SHM_RND         round attach address to SHMLBA
SHM_MAP         map a file instead of share a segment
SHM_FMAP        fast file map
SHM_COPY        deferred update
SHM_R           read permission
SHM_W           write permission
SHM_DEST        destroy segment when # attached = 0
ZERO_MEM        for disclaim
SHMHISEG        highest shared memory segment allowed
SHMLOSEG        lowest shared memory segment allowed
NSHMSEGS        number of shared memory segments allowed

There is a shared mem id data structure for each shared memory and mapped file segment in the system.


The structure shmid_ds contains the following members:

struct ipc_perm    shm_perm       operation permission struct 
size_t             shm_segsz      size of segment in bytes
pid_t              shm_lpid       process ID of last shared memory operation 
pid_t              shm_cpid       pid of creator
shmatt_t           shm_nattch     number of current attaches
time_t             shm_atime      last shmat time
time_t             shm_dtime      time of last shmdt
time_t             shm_ctime      time of last change by shmctl

The structure shminfo contains the following members:

unsigned int        shmmax           max shared memory segment size
int                 shmmin           min shared memory segment size
int                 shmmni           # of shared memory identifiers

The types pid_t, time_t, key_t, and size_t are defined as described in <sys/types.h>. The following are declared as functions:

void  *shmat(int, const void *, int);
int    shmctl(int, int, struct shmid_ds *);
int    shmdt(const void *);
int    shmget(key_t, size_t, int);

In addition, all of the symbols from <sys/ipc.h> will be defined when this header is included.

Related Information

The types.h file.

The shmat, shmctl, shmdt, and shmget subroutines.

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