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# 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.