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.
#include <sys/shm.h> void *shmat(int shmid, const void *shmaddr, int shmflg);
The shmat() function operates on XSI shared memory (see the Base Definitions volume of POSIX.1-2017, Section 3.346, Shared Memory Object). It is unspecified whether this function interoperates with the realtime interprocess communication facilities defined in Section 2.8, Realtime.
The shmat() function attaches the shared memory segment associated with the shared memory identifier specified by shmid to the address space of the calling process. The segment is attached at the address specified by one of the following criteria:
If shmaddr is a null pointer, the segment is attached at the first available address as selected by the system.
If shmaddr is not a null pointer and (shmflg &SHM_RND) is non-zero, the segment is attached at the address given by (shmaddr -((uintptr_t)shmaddr %SHMLBA)). The character '%' is the C-language remainder operator.
If shmaddr is not a null pointer and (shmflg &SHM_RND) is 0, the segment is attached at the address given by shmaddr.
The segment is attached for reading if (shmflg &SHM_RDONLY) is non-zero and the calling process has read permission; otherwise, if it is 0 and the calling process has read and write permission, the segment is attached for reading and writing.
Upon successful completion, shmat() shall increment the value of shm_nattch in the data structure associated with the shared memory ID of the attached shared memory segment and return the segment's start address. Also, the shm_atime timestamp shall be set to the current time, as described in Section 2.7.1, IPC General Description.
Otherwise, the shared memory segment shall not be attached, shmat() shall return (void *)-1, and errno shall be set to indicate the error.
The shmat() function shall fail if:
Operation permission is denied to the calling process; see Section 2.7, XSI Interprocess Communication.
The value of shmid is not a valid shared memory identifier, the shmaddr is not a null pointer, and the value of (shmaddr -((uintptr_t)shmaddr %SHMLBA)) is an illegal address for attaching shared memory; or the shmaddr is not a null pointer, (shmflg &SHM_RND) is 0, and the value of shmaddr is an illegal address for attaching shared memory.
The number of shared memory segments attached to the calling process would exceed the system-imposed limit.
The available data space is not large enough to accommodate the shared memory segment.
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, exec, exit(), fork(), shmctl(), shmdt(), shmget(), shm_open(), shm_unlink()
The Base Definitions volume of POSIX.1-2017, Section 3.346, Shared Memory Object, <sys_shm.h>
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 at http://www.opengroup.org/unix/online.html .
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exec(3p), ipcs(1p), mmap(3p), shmctl(3p), shmdt(3p), shmget(3p), shm_open(3p), shm_unlink(3p), sys_shm.h(0p).