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shmget(2) System Calls Manual shmget(2) NAME shmget - Returns (and possibly creates) the ID for a shared memory re- gion SYNOPSIS #include <sys/shm.h> int shmget( key_t key, size_t size, int flags); If programs are being developed for multiple platforms, application de- velopers may want to specify #include statements for <sys/types.h> and <sys/ipc.h> before the one for <sys/shm.h>. The additional #include statements are not required on DIGITAL UNIX systems or by ISO or X/Open standards, but may be required on other vendors' systems that conform to these standards. STANDARDS Interfaces documented on this reference page conform to industry stan- dards as follows: shmget(): XPG4, XPG4-UNIX Refer to the standards(5) reference page for more information about in- dustry standards and associated tags. PARAMETERS Specifies the key that identifies the shared memory region. The value for the key parameter can be IPC_PRIVATE or a random number other than zero (0). If the value of key is IPC_PRIVATE, it can be used to assure the return of a new, unused shared memory region. Specifies the mini- mum number of bytes to allocate for the region. Specifies the creation flags. Possible values are: If the key specified does not exist, the shmget() function creates an ID using the specified key. If the specified key does exist, and IPC_EXCL is not set, the ID for the specified key is returned. If the specified key does exist and IPC_EXCL is set, the shmget() function fails and returns an error. If the key al- ready exists, the shmget() function fails and returns an error notification. DESCRIPTION The shmget() function returns (and possibly creates) the ID for the shared memory region identified by the key parameter. If IPC_PRIVATE is used for the key parameter, the shmget() function returns the ID for a private (that is, newly created) shared memory region. The flags pa- rameter supplies creation options for the shmget() function. If the key parameter does not already exist, the IPC_CREAT flag instructs the shmget() function to create a new shared memory region for the key and return the kernel-assigned ID for the region. After creating a new shared memory region ID, the shmget() function initializes the shmid_ds structure associated with the ID as follows: The shm_perm.cuid and shm_perm.uid fields are set equal to the effec- tive user ID of the calling process. The shm_perm.cgid and shm_perm.gid fields are set equal to the effective group ID of the calling process. The low-order nine bits of the shm_perm.mode field are set equal to the low-order nine bits of flags. The shm_segsz field is set equal to size. The shm_lpid, shm_nattch, shm_atime, and shm_dtime fields are all set equal to 0 (zero). The shm_ctime field is set equal to the current time. [DIGITAL] The shm_cpid field is set to the process ID of the calling process. [DIGITAL] To reduce the overhead associated with managing large shared memory regions among many processes, the DIGITAL UNIX kernel makes use of shared page tables, also referred to as segmented shared memory (SSM). When the segment size requested by shmget is greater than or equal to the value of ssm-threshold (an attribute in /etc/syscon- figtab), the memory region is managed using shared page tables. When attaching to such a segment, the segment must always be aligned and sized correctly. The alignment and size factor is given by the system- defined SSM_SIZE value (see machine/pmap.h). When the attachment oc- curs, the segment's memory region is aligned on an SSM_SIZE boundary and its size is rounded up to the next SSM_SIZE boundary - thus, the segment size specified by shmget does not need to be a multiple of SSM_SIZE. (Use of segmented shared memory can be disabled by setting the ssm-threshold attribute to zero.) RETURN VALUES Upon successful completion, a shared memory identifier is returned. If the shmget() function fails, a value of -1 is returned and errno is set to indicate the error. ERRORS The shmget() function sets errno to the specified values for the fol- lowing conditions: A shared memory region ID already exists for the key parameter, but operation permission as specified by the low-order nine bits of the flags parameter was not granted. A shared memory region ID already exists for the key parameter, but IPC_CREAT and IPC_EXCL were used for the flags parameter. The value of the size parameter is less than the system-defined minimum or greater than the system-defined max- imum. Or, a shared memory region ID already exists for the key parame- ter, but the number of bytes allocated for the region is less than size and size is not equal to 0 (zero). A shared memory region ID does not exist for the key parameter, and IPC_CREAT was not used for the flags parameter. An attempt was made to create a shared memory region ID and its associated shmid_ds structure, but there was not enough physical memory available. An attempt to create a new shared memory region ID exceeded the system-wide limit on the maximum number of IDs allowed. RELATED INFORMATION Functions: shmat(2), shmctl(2), shmdt(2), sysconfig(8), table(2), ftok(3) Files: shmid_ds(4) Standards: standards(5) delim off shmget(2)