pr_opensharedmemory.md

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# 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`.