NAME

shmop: shmat, shmdt − shared memory operations

SYNOPSIS

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

void ∗shmat(int shmid, void ∗shmaddr, int shmflg);

int shmdt (void ∗shmaddr);

DESCRIPTION

shmat attaches the shared memory segment associated with the shared memory identifier specified by shmid to the data segment of the calling process.  The segment is attached at the address specified by one of the following criteria:

If shmaddr is equal to (void ∗) 0, the segment is attached at the first available address as selected by the system. 

If shmaddr is not equal to (void ∗) 0 and (shmflg&SHM_RND) is true, the segment is attached at the address given by (shmaddr - (shmaddr modulus SHMLBA)). 

If shmaddr is not equal to (void ∗) 0 and (shmflg&SHM_RND) is false, the segment is attached at the address given by shmaddr.

shmdt detaches from the calling process’s data segment the shared memory segment located at the address specified by shmaddr.

The segment is attached for reading if (shmflg&SHM_RDONLY) is true ­{READ}, otherwise it is attached for reading and writing ­{READ/WRITE}. 

shmat fails and does not attach the shared memory segment if one or more of the following are true:

EINVAL shmid is not a valid shared memory identifier. 

EACCES Operation permission is denied to the calling process [see intro(2)]. 

ENOMEM The available data space is not large enough to accommodate the shared memory segment. 

EINVAL shmaddr is not equal to zero, and the value of (shmaddr - (shmaddr modulus SHMLBA)) is an illegal address. 

EINVAL shmaddr is not equal to zero, (shmflg&SHM_RND) is false, and the value of shmaddr is an illegal address. 

EMFILE The number of shared memory segments attached to the calling process would exceed the system-imposed limit. 

EINVAL shmdt fails and does not detach the shared memory segment if shmaddr is not the data segment start address of a shared memory segment. 

SEE ALSO

intro(2), exec(2), exit(2), fork(2), shmctl(2), shmget(2). 

DIAGNOSTICS

Upon successful completion, the return value is as follows:

shmat returns the data segment start address of the attached shared memory segment. 

shmdt returns a value of 0. 

Otherwise, a value of −1 is returned and errno is set to indicate the error. 

NOTES

The user must explicitly remove shared memory segments after the last reference to them has been removed. 

If a parity error occurs in a shared memory segment, it may be persistent, since the shared memory segment is persistent.  If the page that contains the parity error is clean (i.e. has an up-to-date backing copy on disk), then the system will recover that page from the disk copy.  If the page is dirty (i.e. there is no up-to-date backing copy anywhere), then the parity error will remain in the shared memory until the segment is explicitly removed.  In either case, a SIGBUS signal is sent to any process that accesses a memory location with a parity error.  This may affect new processes that attach an existing shared memory segment that has a persistent parity error.