NAME

shmat, shmdt − shared memory operations

SYNOPSIS

#include <sys/shm.h>

char *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. 

Series 700/800 Systems
If the shared memory segment is not already attached, shmaddr must be specified as zero and the segment is attached at a location selected by the operating system.  That location is identical in all processes accessing that shared memory object. 

If the shared memory segment is already attached, a non-zero value of shmaddr is accepted, provided the specified address is identical to the current attach address of the segment. 

Series 300/400 Systems
shmaddr can be specified as a non-zero value as a machine-dependent extension (see DEPENDENCIES below).  However, those systems do not necessarily guarantee that a given shared memory object appears at the same address in all processes that access it, unless the user specifies an address. 

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

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

RETURN VALUE

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. 

ERRORS

shmat() fails and does not attach the shared memory segment if any of the following conditions are encountered (see DEPENDENCIES):

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

[EACCES] Operation permission is denied to the calling process. 

[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 exceed the system-imposed limit. 

shmdt() fails and returns −1 if the following condition is encountered:

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

EXAMPLES

The following call to shmat attaches the shared memory segment to the process.  This example assumes the process has a valid shmid, which can be obtained by calling shmget(2).

char *shmptr, *shmat();
shmptr = shmat(myshmid, (char *)0, 0);

The following call to shmdt() then detaches the shared memory segment. 

shmdt (shmptr);

DEPENDENCIES

Series 300/400

shmaddr can be non-zero, and 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 varies 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 % SHMLBA)). The character % is the C language modulus operator.

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

This form of shmat() fails and does not attach the shared memory segment if any of the following conditions are encountered:

[EACCES] shmid is the ID of a shared memory segment currently being used by the system to implement other features (see graphics(7) and iomap(7)).

[EINVAL] shmaddr is not equal to zero, and the value of (shmaddr − (shmaddr % 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. 

[ENOMEM] The calling process is locked (see plock(2)) and there is not sufficient lockable memory to support the process-related data structure overhead.

Series 700/800

shmat() fails and returns −1 if the following is encountered:

[EINVAL] The calling process is already attached to shmid.

SEE ALSO

ipcs(1), exec(2), exit(2), fork(2), shmctl(2), shmget(2), stdipc(3C). 

STANDARDS CONFORMANCE

shmat(): SVID2 [Series 300/400 only], XPG2, XPG3, XPG4

shmdt(): SVID2, XPG2, XPG3, XPG4

Hewlett-Packard Company  —  HP-UX Release 9.0: August 1992