Movatterモバイル変換

shmget(3p) — Linux manual page

SHMGET(3P)              POSIX Programmer's ManualSHMGET(3P)

PROLOG        top

       This manual page is part of the POSIX Programmer's Manual.  The       Linux implementation of this interface may differ (consult the       corresponding Linux manual page for details of Linux behavior), or       the interface may not be implemented on Linux.

NAME        top

       shmget — get an XSI shared memory segment

SYNOPSIS        top

       #include <sys/shm.h>       int shmget(key_tkey, size_tsize, intshmflg);

DESCRIPTION        top

       Theshmget() function operates on XSI shared memory (see the Base       Definitions volume of POSIX.1‐2017,Section 3.346,Shared MemoryObject).  It is unspecified whether this function interoperates       with the realtime interprocess communication facilities defined inSection 2.8,Realtime.       Theshmget() function shall return the shared memory identifier       associated withkey.       A shared memory identifier, associated data structure, and shared       memory segment of at leastsize bytes (see<sys/shm.h>) are       created forkey if one of the following is true:        *  The argumentkey is equal to IPC_PRIVATE.        *  The argumentkey does not already have a shared memory           identifier associated with it and (shmflg &IPC_CREAT) is non-           zero.       Upon creation, the data structure associated with the new shared       memory identifier shall be initialized as follows:        *  The values ofshm_perm.cuid,shm_perm.uid,shm_perm.cgid, andshm_perm.gid are set to the effective user ID and effective           group ID, respectively, of the calling process.        *  The low-order nine bits ofshm_perm.mode are set to the low-           order nine bits ofshmflg.        *  The value ofshm_segsz is set to the value ofsize.        *  The values ofshm_lpid,shm_nattch,shm_atime, andshm_dtime           are set to 0.        *  The value ofshm_ctime is set to the current time, as           described inSection 2.7.1,IPC General Description.       When the shared memory segment is created, it shall be initialized       with all zero values.

RETURN VALUE        top

       Upon successful completion,shmget() shall return a non-negative       integer, namely a shared memory identifier; otherwise, it shall       return -1 and seterrno to indicate the error.

ERRORS        top

       Theshmget() function shall fail if:EACCESA shared memory identifier exists forkey but operation              permission as specified by the low-order nine bits ofshmflg would not be granted; seeSection 2.7,XSIInterprocess Communication.EEXISTA shared memory identifier exists for the argumentkey but              (shmflg &IPC_CREAT) &&(shmflg &IPC_EXCL) is non-zero.EINVALA 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.EINVALNo shared memory segment is to be created and a shared              memory segment exists forkey but the size of the segment              associated with it is less thansize.ENOENTA shared memory identifier does not exist for the argumentkey and (shmflg &IPC_CREAT) is 0.ENOMEMA shared memory identifier and associated shared memory              segment are to be created, but the amount of available              physical memory is not sufficient to fill the request.ENOSPCA 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.The following sections are informative.

EXAMPLES        top

       None.

APPLICATION USAGE        top

       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 inSection 2.7,XSI Interprocess Communication       can be easily modified to use the alternative interfaces.

RATIONALE        top

       None.

FUTURE DIRECTIONS        top

       None.

SEE ALSO        top

Section 2.7,XSI Interprocess Communication,Section 2.8,Realtime,ftok(3p),shmat(3p),shmctl(3p),shmdt(3p),shm_open(3p),shm_unlink(3p)       The Base Definitions volume of POSIX.1‐2017,Section 3.346,SharedMemory Object,sys_shm.h(0p)

COPYRIGHT        top

       Portions of this text are reprinted and reproduced in electronic       form from IEEE Std 1003.1-2017, Standard for Information       Technology -- Portable Operating System Interface (POSIX), The       Open Group Base Specifications Issue 7, 2018 Edition, Copyright       (C) 2018 by the Institute of Electrical and Electronics Engineers,       Inc and The Open Group.  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 athttp://www.opengroup.org/unix/online.html .       Any typographical or formatting errors that appear in this page       are most likely to have been introduced during the conversion of       the source files to man page format. To report such errors, seehttps://www.kernel.org/doc/man-pages/reporting_bugs.html .IEEE/The Open Group                2017SHMGET(3P)

Pages that refer to this page:sys_shm.h(0p), ipcs(1p), _Exit(3p), ftok(3p), shmat(3p), shmctl(3p), shmdt(3p)

HTML rendering created 2025-09-06 byMichael Kerrisk, author ofThe Linux Programming Interface. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH.