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SHMGET (2) | System calls | Unix Manual Pages | :man


shmget - obtain a shared memory identifier


Return Values


.Lb libc


.In machine/param.h
.In sys/types.h
.In sys/ipc.h
.In sys/shm.h int shmget "key_t key" "size_t size" "int flag"


Based on the values of key and flag, 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 flag, a new one will be created.
  • The ftok(3) 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 flag 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).


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.


The shmget system call will fail if:
Size specified is greater than the size of the previously existing segment. Size specified is less than the system imposed minimum, or greater than the system imposed maximum.
No shared memory segment was found matching key, and IPC_CREAT was not specified.
The kernel was unable to allocate enough memory to satisfy the request.
IPC_CREAT and IPC_EXCL were specified, and a shared memory segment corresponding to key already exists.


shmat(2), shmctl(2), shmdt(2), ftok(3)

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