.\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it) .\" .\" Permission is granted to make and distribute verbatim copies of this .\" manual provided the copyright notice and this permission notice are .\" preserved on all copies. .\" .\" Permission is granted to copy and distribute modified versions of this .\" manual under the conditions for verbatim copying, provided that the .\" entire resulting derived work is distributed under the terms of a .\" permission notice identical to this one .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date. The author(s) assume no .\" responsibility for errors or omissions, or for damages resulting from .\" the use of the information contained herein. The author(s) may not .\" have taken the same level of care in the production of this manual, .\" which is licensed free of charge, as they might when working .\" professionally. .\" .\" Formatted or processed versions of this manual, if unaccompanied by .\" the source, must acknowledge the copyright and authors of this work. .\" .\" Modified Sun Nov 28 17:06:19 1993, Rik Faith (faith@cs.unc.edu) .\" with material from Luigi P. Bai (lpb@softint.com) .\" Portions Copyright 1993 Luigi P. Bai .\" .TH SHMOP 2 "November 28, 1993" "Linux 0.99.13" "Linux Programmer's Manual" .SH NAME shmop \- shared memory operations .SH SYNOPSIS .nf .B # include .B # include .B # include .fi .sp .BI "char *shmat ( int " shmid , .BI "char *" shmaddr , .BI "int " shmflg " )" .sp .BI "int shmdt ( char *" shmaddr ")" .SH DESCRIPTION The function .B shmat attaches the shared memory segment identified by .B shmid to the data segment of the calling process. The attaching address is specified by .I shmaddr with one of the following criteria: .IP If .I shmaddr is .BR 0 , the system tries to find an unmapped region in the range 1 \- 1.5G starting from the upper value and coming down from there. .IP If .I shmaddr isn't .B 0 and .B SHM_RND is asserted in .IR shmflg , the attach occurs at address equal to the rounding down of .I shmaddr to a multiple of .BR SHMLBA . Otherwise .I shmaddr must be a page aligned address at which the attach occurs. .PP If .B SHM_RDONLY is asserted in .IR shmflg , the segment is attached for reading and the process must have read access permissions to the segment. Otherwise the segment is attached for read and write and the process must have read and write access permissions to the segment. There is no notion of write-only shared memory segment. .PP The .B brk value of the calling process is not altered by the attach. The segment will automatically detached at process exit. The same segment may be attached as a read and as a read-write one, and more than once, in the process's address space. .PP On a successful .B shmat call the system updates the members of the structure .B shmid_ds associated to the shared memory segment as follows: .IP .B shm_atime is set to the current time. .IP .B shm_lpid is set to the process-ID of the calling process. .IP .B shm_nattch is incremented by one. .PP Note that the attach succeeds also if the shared memory segment is marked as to be deleted. .PP The function .B shmdt detaches from the calling process's data segment the shared memory segment located at the address specified by .IR shmaddr . The detaching shared memory segment must be one among the currently attached ones (to the process's address space) with .I shmaddr equal to the value returned by the its attaching .B shat call. .PP On a successful .B shmdt call the system updates the members of the structure .B shmid_ds associated to the shared memory segment as follows: .IP .B shm_dtime is set to the current time. .IP .B shm_lpid is set to the process-ID of the calling process. .IP .B shm_nattch is decremented by one. If it becomes 0 and the segment is marked for deletion, the segment is deleted. .PP The occupied region in the user space of the calling process is unmapped. .PP .SH "SYSTEM CALLS" .TP .B fork() After a .B fork() the child inherits the attached shared memory segments. .TP .B exec() After an .B exec() all attached shared memory segments are detached (not destroyed). .TP .B exit() Upon .B exit() all attached shared memory segments are detached (not destroyed). .PP .SH "RETURN VALUE" On a failure both functions return .B \-1 with .B errno indicating the error, otherwise .B shmat returns the address of the attached shared memory segment, and .B shmdt returns .BR 0 . .SH ERRORS When .B shmat fails, at return .B errno will be set to one among the following values: .TP 11 .B EACCES The calling process has no access permissions for the requested attach type. .TP .B EINVAL Invalid .I shmid value, unaligned (i.e., not page-aligned and \fBSHM_RND\fP was not specified) or invalid .I shmaddr value, or failing attach at .BR brk . .TP .B ENOMEM Could not allocate memory for the descriptor or for the page tables. .PP The function .B shmdt can fails only if there is no shared memory segment attached at .IR shmaddr , in such a case at return .B errno will be set to .BR EINVAL . .SH NOTES On executing a .BR fork (2) system call, the child inherits all the attached shared memory segments. .PP The shared memory segments attached to a process executing an .BR exec (2) system call will not be attached to the resulting process. .PP The following is a system parameter affecting a .B shmat system call: .TP 11 .B SHMLBA Segment low boundary address multiple. Must be page aligned. For the current implementation the .B SHMBLA value is .BR PAGE_SIZE . .PP The implementation has no intrinsic limit to the per process maximum number of shared memory segments .RB ( SHMSEG ) .SH "SEE ALSO" .BR ipc (5), .BR shmctl (2), .BR shmget (2).