# PR_OpenAnonFileMap

Creates or opens a named semaphore with the specified name

## Syntax

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

NSPR_API( PRFileMap *)
PR_OpenAnonFileMap(
  const char *dirName,
  PRSize size,
  PRFileMapProtect prot
);
```

### Parameters

The function has the following parameters:

`dirName`

: A pointer to a directory name that will contain the anonymous file.

`size`

: The size of the shared memory.

`prot`

: How the shared memory is mapped.

### Returns

Pointer to {ref}`PRFileMap` or `NULL` on error.

## Description

If the shared memory already exists, a handle is returned to that shared
memory object.

On Unix platforms, {ref}`PR_OpenAnonFileMap` uses `dirName` as a
directory name, without the trailing '/', to contain the anonymous file.
A filename is generated for the name.

On Windows platforms, `dirName` is ignored.