SHMOP(2)

NAME

shmop − shared memory operations

SYNOPSIS

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

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

int shmdt (shmaddr)
char ∗shmaddr

HP-UX COMPATABILITY

Level: HP-UX/STANDARD

Origin: System V Release 2

DESCRIPTION

Shmat attaches the shared memory segment associated with the shared memory identifier specified by shmid to the data segment of the calling process. If the shared memory segment has not already been attached shmaddr must be specified as zero, and the segment will be attached at a location selected by the operating system. That location will be the same in all processes accessing that shared memory object. If the shared memory segment has already been attached a non-zero value of shmaddr will be accepted as long as the specified address is the same as the current attach address of the segment. Some implementations may permit the specification of a non-zero value as a machine dependent extension, as discussed in HARDWARE DEPENDENCIES below. Systems which do this do not necessarily guarantee that a given shared memory object will appear at the same address in all processes which access it, unless the user specifies an address.

The segment is attached for reading if (shmflg & SHM_RDONLY) is “true” {READ}, otherwise it is attached for reading and writing {READ/WRITE}. It is not possible to attach a segment for write only.

Shmat will fail and 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 zero and the machine does not permit non-zero values or shmaddr is not equal to the current attach location for the shared memory segment.

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

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

Shmdt fails if the following is true.

[EINVAL] Shmdt will fail and not detach the shared memory segment if shmaddr is not the data segment start address of a shared memory segment.

HARDWARE DEPENDENCIES

Series 500:

Shmaddr must be zero in all cases for shmat. Otherwise, an error is generated. In addition, SHM_RDONLY is not supported, and if it is set in shmflg, an error is generated.

[EINVAL] Shmflg has SHM_RDONLY set.

Series 200:

Shmaddr may be non-zero. If it is, the segment is attached at the address specified by one of the following criteria:

If shmaddr is equal to zero, the segment is attached at the first available address as selected by the system. The selected value will vary for each process accessing that shared memory object.

If shmaddr is not equal to zero 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 zero and (shmflg & SHM_RND) is “false”, the segment is attached at the address given by shmaddr.

Shmat will fail and not attach the shared memory segment if one or more of the following are true:

[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.

RETURN VALUES

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.

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