shmget(2) [freebsd man page]
SHMGET(2) BSD System Calls Manual SHMGET(2) NAME
shmget -- obtain a shared memory identifier LIBRARY
Standard C Library (libc, -lc) SYNOPSIS
#include <sys/types.h> #include <sys/ipc.h> #include <sys/shm.h> int shmget(key_t key, size_t size, int flag); DESCRIPTION
Based on the values of key and flag, shmget() returns the identifier of a newly created or previously existing shared memory segment. The key is analogous to a filename: it provides a handle that names an IPC object. There are three ways to specify a key: o IPC_PRIVATE may be specified, in which case a new IPC object will be created. o An integer constant may be specified. If no IPC object corresponding to key is specified and the IPC_CREAT bit is set in flag, a new one will be created. o The ftok(3) may be used to generate a key from a pathname. The mode of a newly created IPC object is determined by OR'ing the following constants into the flag argument: S_IRUSR Read access for owner. S_IWUSR Write access for owner. S_IRGRP Read access for group. S_IWGRP Write access for group. S_IROTH Read access for other. S_IWOTH Write access for other. When creating a new shared memory segment, size indicates the desired size of the new segment in bytes. The size of the segment may be rounded up to a multiple convenient to the kernel (i.e., the page size). RETURN VALUES
Upon successful completion, shmget() returns the positive integer identifier of a shared memory segment. Otherwise, -1 is returned and errno set to indicate the error. ERRORS
The shmget() system call will fail if: [EINVAL] Size specified is greater than the size of the previously existing segment. Size specified is less than the system imposed minimum, or greater than the system imposed maximum. [ENOENT] No shared memory segment was found matching key, and IPC_CREAT was not specified. [ENOSPC] The kernel was unable to allocate enough memory to satisfy the request. [EEXIST] IPC_CREAT and IPC_EXCL were specified, and a shared memory segment corresponding to key already exists. SEE ALSO
shmat(2), shmctl(2), shmdt(2), stat(2), ftok(3) BSD
December 17, 2010 BSD
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shmget(2) System Calls shmget(2) NAME
shmget - get shared memory segment identifier SYNOPSIS
#include <sys/types.h> #include <sys/ipc.h> #include <sys/shm.h> int shmget(key_t key, size_t size, int shmflg); DESCRIPTION
The shmget() function returns the shared memory identifier associated with key. A shared memory identifier and associated data structure and shared memory segment of at least size bytes (see Intro(2)) are created for key if one of the following are true: o The key argument is equal to IPC_PRIVATE. o The key argument does not already have a shared memory identifier associated with it, and (shmflg&IPC_CREAT) is true. Upon creation, the data structure associated with the new shared memory identifier is initialized as follows: o The values of shm_perm.cuid, shm_perm.uid, shm_perm.cgid, and shm_perm.gid are set equal to the effective user ID and effective group ID, respectively, of the calling process. o The access permission bits of shm_perm.mode are set equal to the access permission bits of shmflg. shm_segsz is set equal to the value of size. o The values of shm_lpid, shm_nattch shm_atime, and shm_dtime are set equal to 0. o The shm_ctime is set equal to the current time. Shared memory segments must be explicitly removed after the last reference to them has been removed. RETURN VALUES
Upon successful completion, a non-negative integer representing a shared memory identifier is returned. Otherwise, -1 is returned and errno is set to indicate the error. ERRORS
The shmget() function will fail if: EACCES A shared memory identifier exists for key but operation permission (see Intro(2)) as specified by the low-order 9 bits of shmflg would not be granted. EEXIST A shared memory identifier exists for key but both (shmflg&IPC_CREAT) and (shmflg&IPC_EXCL) are true. EINVAL The size argument is less than the system-imposed minimum or greater than the system-imposed maximum. See NOTES. A shared memory identifier exists for key but the size of the segment associated with it is less than size and size is not equal to 0. ENOENT A shared memory identifier does not exist for key and (shmflg&IPC_CREAT) is false. ENOMEM A shared memory identifier and associated shared memory segment are to be created but the amount of available memory is not suf- ficient to fill the request. ENOSPC A shared memory identifier is to be created but the system-imposed limit on the maximum number of allowed shared memory identi- fiers system-wide would be exceeded. See NOTES. ATTRIBUTES
See attributes(5) for descriptions of the following attributes: +-----------------------------+-----------------------------+ | ATTRIBUTE TYPE | ATTRIBUTE VALUE | +-----------------------------+-----------------------------+ |Interface Stability |Standard | +-----------------------------+-----------------------------+ SEE ALSO
rctladm(1M), Intro(2), setrctl(2), shmctl(2), shmop(2), ftok(3C), getpagesize(3C), attributes(5), standards(5) NOTES
The project.max-shm-memory resource control restricts the total amount of shared memory a project can allocate. The zone.max-shm-memory resource control restricts the total amount of shared memory that can be allocated by a zone. The system-imposed maximum on the size of a shared memory segment is therefore a function of the sizes of any other shared memory segments the calling project might have allocated that are still in use, as well as any other shared memory segments allocated and still in use by processes in the zone. For accounting purposes, segment sizes are rounded up to the nearest multiple of the system page size. See getpagesize(3C). The system-imposed limit on the number of shared memory identifiers is maintained on a per-project basis using the project.max-shm-ids resource control. The zone.max-shm-ids resource control restricts the total number of shared memory identifiers that can be allocated by a zone. See rctladm(1M) and setrctl(2) for information about using resource controls. SunOS 5.11 14 Aug 2006 shmget(2)