NAME

shmget − get shared memory segment

SYNTAX

#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>

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

DESCRIPTION

The system call returns the shared memory identifier associated with key.  A shared memory identifier and associated data structure and shared memory segment of size size bytes are created for key if one of the following are true:

The key is equal to IPC_PRIVATE.  For further information, see intro(2). 

The key does not already have a shared memory identifier associated with it, and (shmflg & IPC_CREAT ) is "true". 

Upon creation, the data structure associated with the new shared memory identifier is initialized as follows:

The shm_perm.cuid,shm_perm.uid,shm_perm.cgid, and shm_perm.gid are set equal to the effective user ID and effective group ID, respectively, of the calling process. 

The low-order 9 bits of shm_perm.mode are set equal to the low-order 9 bits of shmflg.  The shm_segsz is set equal to the value of size. 

The shm_lpid,shm_nattch,shm_atime, and shm_dtime are set equal to 0.  The shm_ctime is set equal to the current time. 

RETURN VALUE

Upon successful completion, a non-negative integer, namely a shared memory identifier is returned.  Otherwise, a value of −1 is returned and errno is set to indicated the error. 

DIAGNOSTICS

The system call will fail if one or more of the following are true:

[EINVAL] The size is less than the system imposed minimum or greater than the system imposed maximum. 

[EACCES] A shared memory identifier exists for key but operations permission as specified by the low-order 9 bits of shmflg would not be granted.  For further information, see

[EINVAL] A shared memory identifier exists for key but the size of the segment associated with it is less than size and size is not equal to zero. 

[ENOENT] A shared memory identifier does not exist for key and (shmflg & IPC_CREAT ) is "false". 

[ENOSPC] A shared memory identifier is to be created but the system imposed limit on the maximum number of allowed shared memory identifiers system wide would be exceeded. 

[ENOMEM] A shared memory identifier and associate shared memory segment are to be created by the amount of available physical memory is not sufficient to fill the request. 

[EEXIST] A shared memory identifier exists for key but ((shmflg & IPC_CREAT ) & (shmflg & IPC_EXCL )) is "true". 

SEE ALSO

shmctl(2), shmop(2), ftok(3)

BUGS

The System V shared memory functions are implemented at user level on top of the mmap(2) interface.  Not all the System V functionality is implemented. 

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