SHMAT

Section: System Calls (2)
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BSD mandoc
NetBSD

NAME

shmat shmdt - map/unmap shared memory

SYNOPSIS

Fd #include <sys/types.h> Fd #include <sys/ipc.h> Fd #include <sys/shm.h> Ft void * Fn shmat int shmid void *shmaddr int shmflg Ft int Fn shmdt void *shmaddr

DESCRIPTION

Fn shmat maps the shared memory segment associated with the shared memory identifier Fa shmid into the address space of the calling process. The address at which the segment is mapped is determined by the Fa 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 Fa shmaddr specifies. If SHM_RND is set in Fa shmflg , the system will round the address down to a multiple of SHMLBA bytes (SHMLBA is defined in Aq Pa sys/shm.h ).

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

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

RETURN VALUES

Fn shmat returns the address at which the shared memory segment has been mapped into the calling process' address space when successful, Fn 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

Fn shmat will fail if:

Bq Er EACCESS: The calling process has no permission to access this shared memory segment.
Bq Er ENOMEM: There is not enough available data space for the calling process to map the shared memory segment.
Bq Er EINVAL: Fa shmid is not a valid shared memory identifier.
Fa shmaddr specifies an illegal address.
Bq Er EMFILE: The number of shared memory segments has reached the system-wide limit.

Fn shmdt will fail if:

Bq Er EINVAL: Fa shmaddr is not the start address of a mapped shared memory segment.

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Time: 16:28:59 GMT, April 18, 2022