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shmget — get an XSI shared memory segment
int shmget(key_t key, size_t size, int shmflg);
The shmget() function operates on XSI shared memory (see the Base Definitions volume of POSIX.1‐2008, Section 3.342, Shared Memory Object). It is unspecified whether this function interoperates with the realtime interprocess communication facilities defined in Section 2.8, Realtime.
The shmget() function shall return the shared memory identifier associated with key.
A shared memory identifier, associated data structure, and shared memory segment of at least size bytes (see <sys/shm.h>) are created for key if one of the following is true:
- The argument key is equal to IPC_PRIVATE.
Upon creation, the data structure associated with the new shared memory identifier shall be initialized as follows:
- The argument key does not already have a shared memory identifier associated with it and ( shmflg &IPC_CREAT) is non-zero.
- The values of shm_perm.cuid, shm_perm.uid, shm_perm.cgid, and shm_perm.gid are set to the effective user ID and effective group ID, respectively, of the calling process.
- The low-order nine bits of shm_perm.mode are set to the low-order nine bits of shmflg.
- The value of shm_segsz is set to the value of size.
- The values of shm_lpid, shm_nattch, shm_atime, and shm_dtime are set to 0.
When the shared memory segment is created, it shall be initialized with all zero values.
Upon successful completion, shmget() shall return a non-negative integer, namely a shared memory identifier; otherwise, it shall return −1 and set errno to indicate the error.
The shmget() function shall fail if:
- The value of shm_ctime is set to the current time, as described in Section 2.7.1, IPC General Description.
- A shared memory identifier exists for key but operation permission as specified by the low-order nine bits of shmflg would not be granted; see Section 2.7, XSI Interprocess Communication.
- A shared memory identifier exists for the argument key but ( shmflg &IPC_CREAT) &&(shmflg &IPC_EXCL) is non-zero.
- A shared memory segment is to be created and the value of size is less than the system-imposed minimum or greater than the system-imposed maximum.
- No shared memory segment is to be created and a shared memory segment exists for key but the size of the segment associated with it is less than size.
- A shared memory identifier does not exist for the argument key and ( shmflg &IPC_CREAT) is 0.
- A shared memory identifier and associated shared memory segment shall be created, but the amount of available physical memory is not sufficient to fill the request.
The following sections are informative.
The POSIX Realtime Extension defines alternative interfaces for interprocess communication. Application developers who need to use IPC should design their applications so that modules using the IPC routines described in Section 2.7, XSI Interprocess Communication can be easily modified to use the alternative interfaces.
Section 2.7, XSI Interprocess Communication, Section 2.8, Realtime, ftok(), shmat(), shmctl(), shmdt(), shm_open(), shm_unlink()
The Base Definitions volume of POSIX.1‐2008, Section 3.342, Shared Memory Object, <sys_shm.h>
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, Copyright (C) 2013 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.unix.org/online.html .
- A shared memory identifier is to be created, but the system-imposed limit on the maximum number of allowed shared memory identifiers system-wide would be exceeded.
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