# PR_OpenSharedMemory

Opens an existing shared memory segment or, if one with the specified
name doesn't exist, creates a new one.

## Syntax

```{code}
#include <prshm.h>

NSPR_API( PRSharedMemory * )
  PR_OpenSharedMemory(
     const char             *name,
     PRSize             size,
     PRIntn             flags,
     PRIntn             mode
);

/* Define values for PR_OpenShareMemory(...,create) */
#define PR_SHM_CREATE 0x1  /* create if not exist */
#define PR_SHM_EXCL   0x2  /* fail if already exists */
```

### Parameters

The function has the following parameters:

name

: The name of the shared memory segment.

size

: The size of the shared memory segment.

flags

: Options for creating the shared memory.

mode

: Same as passed to {ref}`PR_Open`.

### Returns

Pointer to opaque structure `PRSharedMemory`, or `NULL` if an error
occurs. Retrieve the reason for the failure by calling {ref}`PR_GetError`
and {ref}`PR_GetOSError`.

## Description

{ref}`PR_OpenSharedMemory` creates a new shared memory segment or
associates a previously created memory segment with the specified name.
When parameter `create` is (`PR_SHM_EXCL` | `PR_SHM_CREATE`) and
the shared memory already exists, the function returns `NULL` with the
error set to `PR_FILE_EXISTS_ERROR`.

When parameter `create` is `PR_SHM_CREATE` and the shared memory
already exists, a handle to that memory segment is returned. If the
segment does not exist, it is created and a pointer to the related
`PRSharedMemory` structure is returned.

When parameter `create` is 0, and the shared memory exists, a pointer
to a `PRSharedMemory` structure is returned. If the shared memory does
not exist, `NULL` is returned with the error set to
`PR_FILE_NOT_FOUND_ERROR`.