---
name: "sys_shm.h"
section: "0p"
platform: "linux"
package: "man-pages"
version: "6.19-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/sys_shm.h"
---

# sys\_shm.h(0p)

XSI shared memory facility

## PROLOG

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

sys/shm.h — XSI shared memory facility

## SYNOPSIS

```
#include <sys/shm.h>
```

## DESCRIPTION

The *\<sys/shm.h\>* header shall define the following symbolic constants:

- SHM\_RDONLY

  Attach read-only (else read-write).

- SHM\_RND

  Round attach address to SHMLBA.

- SHMLBA

  Segment low boundary address multiple.

The *\<sys/shm.h\>* header shall define the following data types through **typedef**:

- **shmatt\_t**

  Unsigned integer used for the number of current attaches that must be able to store values at least as large as a type **unsigned short**.

The *\<sys/shm.h\>* header shall define the **shmid\_ds** structure, which shall include the following members:

```
struct ipc_perm shm_perm   Operation permission structure.
size_t          shm_segsz  Size of segment in bytes.
pid_t           shm_lpid   Process ID of last shared memory operation.
pid_t           shm_cpid   Process ID of creator.
shmatt_t        shm_nattch Number of current attaches.
time_t          shm_atime  Time of last shmat().
time_t          shm_dtime  Time of last shmdt().
time_t          shm_ctime  Time of last change by shmctl().
```

The *\<sys/shm.h\>* header shall define the **pid\_t**, **size\_t**, and **time\_t** types as described in *\<sys/types.h\>*.

The following shall be declared as functions and may also be defined as macros. Function prototypes shall be provided.

```
void *shmat(int, const void *, int);
int   shmctl(int, int, struct shmid_ds *);
int   shmdt(const void *);
int   shmget(key_t, size_t, int);
```

In addition, the *\<sys/shm.h\>* header shall include the *\<sys/ipc.h\>* header.

*The following sections are informative.*

## APPLICATION USAGE

None.

## RATIONALE

None.

## FUTURE DIRECTIONS

None.

## SEE ALSO

***\<sys\_ipc.h\>***, ***\<sys\_types.h\>***

The System Interfaces volume of POSIX.1‐2017, *shmat()*, *shmctl()*, *shmdt()*, *shmget()*

## COPYRIGHT

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 .

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, see https://www.kernel.org/doc/man-pages/reporting\_bugs.html .