June 17, 2002 SHMAT 2

### NAME

shmat, shmdt - map/unmap shared memory

### LIBRARY

Standard C Library (libc, -lc)

### SYNOPSIS

```void``*` shmat(```int shmid``, const void *shmaddr``, int shmflg`)
```int` shmdt(```const void *shmaddr`)

### DESCRIPTION

shmat(``) maps the shared memory segment associated with the shared memory identifier ```shmid` into the address space of the calling process. The address at which the segment is mapped is determined by the ```shmaddr` parameter. If it is equal to 0, the system will pick an address itself. Otherwise, an attempt is made to map the shared memory segment at the address ```shmaddr` specifies. If ```SHM_RND` is set in ```shmflg`, the system will round the address down to a multiple of ```SHMLBA` bytes (```SHMLBA` is defined in <```sys/shm.h`>).

A shared memory segment can be mapped read-only by specifying the ```SHM_RDONLY` flag in ```shmflg`.

shmdt(``) unmaps the shared memory segment that is currently mapped at ```shmaddr` from the calling process' address space. ```shmaddr` must be a value returned by a prior shmat(``) call. A shared memory segment will remain in existence until it is removed by a call to shmctl(2) with the ```IPC_RMID` command.

### RETURN VALUES

shmat(``) returns the address at which the shared memory segment has been mapped into the calling process' address space when successful, shmdt(``) returns 0 on successful completion. Otherwise, a value of -1 is returned, and the global variable errno is set to indicate the error.

### ERRORS

shmat(``) will fail if:

[```EACCES`]
  The calling process has no permission to access this shared memory segment.

[```ENOMEM`]
  There is not enough available data space for the calling process to map the shared memory segment.

[```EINVAL`]
  ```shmid` is not a valid shared memory identifier.

  ```shmaddr` specifies an illegal address.

[```EMFILE`]
  The number of shared memory segments has reached the system-wide limit.

shmdt(``) will fail if:

[```EINVAL`]
  ```shmaddr` is not the start address of a mapped shared memory segment.

### SEE ALSO

ipcrm(1), ipcs(1), mmap(2), shmctl(2), shmget(2)

### STANDARDS

The shmat and shmdt system calls conform to X/Open System Interfaces and Headers Issue 5 (``XSH5'') .

### HISTORY

Shared memory segments appeared in the first release of `AT&T` System V `UNIX`.