---
name: "shm_open"
section: "2"
platform: "macos"
package: "xnu"
version: "12377.121.6"
release: "macOS 26.5"
platforms: ["freebsd", "macos"]
url: "https://new.carta.tech/shm_open"
---

# shm\_open(2)

open a shared memory object

## NAME

**shm\_open** – open a shared memory object

## SYNOPSIS

**#include \<sys/mman.h\>**

**#include \<fcntl.h\>**

*int*\

**shm\_open**(*const char \*name*, *int oflag*, *...*);

The parameter "mode\_t mode" is optional.

## DESCRIPTION

The shared memory object referenced by *name* is opened for reading and/or writing as specified by the argument *oflag* and the file descriptor returned to the calling process. The returned file descriptor will be the lowest non-open file descriptor for the calling process, and is not shared with any other processes, as it is a new file descriptor. The new file descriptor will have the FD\_CLOEXEC flag set. Repeated calls to **shm\_open** with the same string value for **name**() will return a file descriptor referring to the same shared memory object, provided that the object has not been unlinked by a call to **shm\_unlink**(). The *oflag* argument may indicate the file is to be created if it does not exist (by specifying the O\_CREAT flag), in which case the file is created with mode *mode* as described in [chmod(2)](/chmod/2.md) and modified by the process' umask value (see [umask(2)](/umask/2.md)).

The value of *oflag* is formed by *or*'ing the following values:

```
O_RDONLY	open for reading only
O_RDWR		open for reading and writing
O_CREAT		create object if it does not exist
O_EXCL		error if create and object exists
O_TRUNC		truncate size to 0
```

Exactly one of O\_RDONLY or O\_RDWR must be specified.

If O\_TRUNC is specified and the file exists, the file is truncated to zero length. If O\_EXCL is set with O\_CREAT and the file already exists, **shm\_open**() returns an error.  This may be used to implement a simple exclusive access locking mechanism.

If successful, **shm\_open**() returns a non-negative integer, termed a file descriptor. It returns -1 and sets *errno* on failure. The file pointer used to mark the current position within the memory object is set to the beginning of the object.

When a new shared memory object is created it is given the owner and group corresponding to the effective user and group of the calling process. There is no visible entry in the file system for the created object in this implementation.

When a shared memory object is created, it persists until it it unlinked and all other references are gone. Objects do not persist across a system reboot.

The system imposes a limit on the number of file descriptors open simultaneously by one process. [getdtablesize(2)](/getdtablesize/2.md) returns the current system limit.

## ERRORS

The named object is opened unless:

- \[EACCES\]

  The required permissions (for reading and/or writing) are denied for the given flags.

- \[EACCES\]

  O\_CREAT is specified, the object does not exist, and permission to create the object is denied.

- \[EEXIST\]

  O\_CREAT and O\_EXCL were specified and the object exists.

- \[EINTR\]

  The **shm\_open**() operation was interrupted by a signal.

- \[EINVAL\]

  The **shm\_open**() operation is not supported.

- \[EMFILE\]

  The process has already reached its limit for open file descriptors.

- \[ENAMETOOLONG\]

  *name* exceeded the name size limit. This is currently PSHMNAMLEN characters (defined in ⟨sys/posix\_shm.h⟩), but this may change in the future.

- \[ENFILE\]

  The system file table is full.

- \[ENOENT\]

  O\_CREAT is not set and the named object does not exist.

- \[ENOSPC\]

  O\_CREAT is specified, the file does not exist, and there is insufficient space available to create the object.

## SEE ALSO

[chmod(2)](/chmod/2.md), [close(2)](/close/2.md), [getdtablesize(2)](/getdtablesize/2.md), [mmap(2)](/mmap/2.md), [shm\_unlink(2)](/shm_unlink/2.md), [umask(2)](/umask/2.md)

## HISTORY

**shm\_open**() is specified in the POSIX Realtime Extension (1003.1b-1993/1003.1i-1995).