Semaphore.OpenExisting Method

Definition

Namespace:

System.Threading

Assembly:

System.Threading.dll

Opens a specified named semaphore, if it already exists.

Overloads

OpenExisting(String)

Opens the specified named semaphore, if it already exists.

OpenExisting(String)

Source:

Semaphore.cs

Opens the specified named semaphore, if it already exists.

public:
 static System::Threading::Semaphore ^ OpenExisting(System::String ^ name);

[System.Runtime.Versioning.SupportedOSPlatform("windows")]
public static System.Threading.Semaphore OpenExisting (string name);

[<System.Runtime.Versioning.SupportedOSPlatform("windows")>]
static member OpenExisting : string -> System.Threading.Semaphore

Public Shared Function OpenExisting (name As String) As Semaphore

Parameters

name

String

The name of the synchronization object to be shared with other processes. The name is case-sensitive. The backslash character (\) is reserved and may only be used to specify a namespace. For more information on namespaces, see the remarks section. There may be further restrictions on the name depending on the operating system. For example, on Unix-based operating systems, the name after excluding the namespace must be a valid file name.

Returns

Semaphore

An object that represents the named system semaphore.

Attributes

SupportedOSPlatformAttribute

Exceptions

ArgumentException

name is an empty string.

-or-

.NET Framework only: name is longer than MAX_PATH (260 characters).

ArgumentNullException

name is null.

WaitHandleCannotBeOpenedException

A synchronization object with the provided name cannot be created. A synchronization object of a different type might have the same name. In some cases, this exception may be thrown for invalid names.

IOException

name is invalid. This can be for various reasons, including some restrictions that may be placed by the operating system, such as an unknown prefix or invalid characters. Note that the name and common prefixes "Global\" and "Local\" are case-sensitive.

-or-

There was some other error. The HResult property may provide more information.

PathTooLongException

The name is too long. Length restrictions may depend on the operating system or configuration.

UnauthorizedAccessException

The named semaphore exists, but the user does not have the security access required to use it.

Examples

The following code example demonstrates the cross-process behavior of a named semaphore with access control security. The example uses the OpenExisting(String) method overload to test for the existence of a named semaphore.

If the semaphore does not exist, it is created with a maximum count of two and with access control security that denies the current user the right to use the semaphore, but which grants the right to read and change permissions on the semaphore.

If you run the compiled example from two command windows, the second copy will throw an access violation exception on the call to the OpenExisting(String) method overload. The exception is caught, and the example uses the OpenExisting(String, SemaphoreRights) method overload to open the semaphore with the rights needed to read and change the permissions.

After the permissions are changed, the semaphore is opened with the rights required to enter and release it. If you run the compiled example from a third command window, it runs using the new permissions.

#using <System.dll>
using namespace System;
using namespace System::Threading;
using namespace System::Security::AccessControl;
using namespace System::Security::Permissions;

public ref class Example
{
public:
   [SecurityPermissionAttribute(SecurityAction::Demand, Flags = SecurityPermissionFlag::UnmanagedCode)]
   static void main()
   {
      String^ semaphoreName = L"SemaphoreExample5";

      Semaphore^ sem = nullptr;
      bool doesNotExist = false;
      bool unauthorized = false;
      
      // Attempt to open the named semaphore.
      try
      {
         // Open the semaphore with (SemaphoreRights.Synchronize
         // | SemaphoreRights.Modify), to enter and release the
         // named semaphore.
         //
         sem = Semaphore::OpenExisting( semaphoreName );
      }
      catch ( WaitHandleCannotBeOpenedException^ ex ) 
      {
         Console::WriteLine( L"Semaphore does not exist." );
         doesNotExist = true;
      }
      catch ( UnauthorizedAccessException^ ex ) 
      {
         Console::WriteLine( L"Unauthorized access: {0}", ex->Message );
         unauthorized = true;
      }

      // There are three cases: (1) The semaphore does not exist.
      // (2) The semaphore exists, but the current user doesn't
      // have access. (3) The semaphore exists and the user has
      // access.
      //
      if ( doesNotExist )
      {
         // The semaphore does not exist, so create it.
         //
         // The value of this variable is set by the semaphore
         // constructor. It is true if the named system semaphore was
         // created, and false if the named semaphore already existed.
         //
         bool semaphoreWasCreated;
         
         // Create an access control list (ACL) that denies the
         // current user the right to enter or release the
         // semaphore, but allows the right to read and change
         // security information for the semaphore.
         //
         String^ user = String::Concat( Environment::UserDomainName,
            L"\\", Environment::UserName );
         SemaphoreSecurity^ semSec = gcnew SemaphoreSecurity;

         SemaphoreAccessRule^ rule = gcnew SemaphoreAccessRule( user,
            static_cast<SemaphoreRights>(
               SemaphoreRights::Synchronize |
               SemaphoreRights::Modify ),
            AccessControlType::Deny );
         semSec->AddAccessRule( rule );

         rule = gcnew SemaphoreAccessRule( user,
            static_cast<SemaphoreRights>(
               SemaphoreRights::ReadPermissions |
               SemaphoreRights::ChangePermissions ),
            AccessControlType::Allow );
         semSec->AddAccessRule( rule );
         
         // Create a Semaphore object that represents the system
         // semaphore named by the constant 'semaphoreName', with
         // maximum count three, initial count three, and the
         // specified security access. The Boolean value that
         // indicates creation of the underlying system object is
         // placed in semaphoreWasCreated.
         //
         sem = gcnew Semaphore( 3,3,semaphoreName,semaphoreWasCreated,semSec );
         
         // If the named system semaphore was created, it can be
         // used by the current instance of this program, even
         // though the current user is denied access. The current
         // program enters the semaphore. Otherwise, exit the
         // program.
         //
         if ( semaphoreWasCreated )
         {
            Console::WriteLine( L"Created the semaphore." );
         }
         else
         {
            Console::WriteLine( L"Unable to create the semaphore." );
            return;
         }

      }
      else if ( unauthorized )
      {
         // Open the semaphore to read and change the access
         // control security. The access control security defined
         // above allows the current user to do this.
         //
         try
         {
            sem = Semaphore::OpenExisting( semaphoreName,
               static_cast<SemaphoreRights>(
                  SemaphoreRights::ReadPermissions |
                  SemaphoreRights::ChangePermissions ));
            
            // Get the current ACL. This requires
            // SemaphoreRights.ReadPermissions.
            SemaphoreSecurity^ semSec = sem->GetAccessControl();

            String^ user = String::Concat( Environment::UserDomainName,
               L"\\", Environment::UserName );
            
            // First, the rule that denied the current user
            // the right to enter and release the semaphore must
            // be removed.
            SemaphoreAccessRule^ rule = gcnew SemaphoreAccessRule( user,
               static_cast<SemaphoreRights>(
                  SemaphoreRights::Synchronize |
                  SemaphoreRights::Modify ),
               AccessControlType::Deny );
            semSec->RemoveAccessRule( rule );
            
            // Now grant the user the correct rights.
            //
            rule = gcnew SemaphoreAccessRule( user,
               static_cast<SemaphoreRights>(
                  SemaphoreRights::Synchronize |
                  SemaphoreRights::Modify ),
               AccessControlType::Allow );
            semSec->AddAccessRule( rule );
            
            // Update the ACL. This requires
            // SemaphoreRights.ChangePermissions.
            sem->SetAccessControl( semSec );

            Console::WriteLine( L"Updated semaphore security." );
            
            // Open the semaphore with (SemaphoreRights.Synchronize
            // | SemaphoreRights.Modify), the rights required to
            // enter and release the semaphore.
            //
            sem = Semaphore::OpenExisting( semaphoreName );

         }
         catch ( UnauthorizedAccessException^ ex ) 
         {
            Console::WriteLine( L"Unable to change permissions: {0}", ex->Message );
            return;
         }
      }
      
      // Enter the semaphore, and hold it until the program
      // exits.
      //
      try
      {
         sem->WaitOne();
         Console::WriteLine( L"Entered the semaphore." );
         Console::WriteLine( L"Press the Enter key to exit." );
         Console::ReadLine();
         sem->Release();
      }
      catch ( UnauthorizedAccessException^ ex ) 
      {
         Console::WriteLine( L"Unauthorized access: {0}", ex->Message );
      }
   }
};

using System;
using System.Threading;
using System.Security.AccessControl;

internal class Example
{
    internal static void Main()
    {
        const string semaphoreName = "SemaphoreExample5";

        Semaphore sem = null;
        bool doesNotExist = false;
        bool unauthorized = false;

        // Attempt to open the named semaphore.
        try
        {
            // Open the semaphore with (SemaphoreRights.Synchronize
            // | SemaphoreRights.Modify), to enter and release the
            // named semaphore.
            //
            sem = Semaphore.OpenExisting(semaphoreName);
        }
        catch(WaitHandleCannotBeOpenedException)
        {
            Console.WriteLine("Semaphore does not exist.");
            doesNotExist = true;
        }
        catch(UnauthorizedAccessException ex)
        {
            Console.WriteLine("Unauthorized access: {0}", ex.Message);
            unauthorized = true;
        }

        // There are three cases: (1) The semaphore does not exist.
        // (2) The semaphore exists, but the current user doesn't 
        // have access. (3) The semaphore exists and the user has
        // access.
        //
        if (doesNotExist)
        {
            // The semaphore does not exist, so create it.
            //
            // The value of this variable is set by the semaphore
            // constructor. It is true if the named system semaphore was
            // created, and false if the named semaphore already existed.
            //
            bool semaphoreWasCreated;

            // Create an access control list (ACL) that denies the
            // current user the right to enter or release the 
            // semaphore, but allows the right to read and change
            // security information for the semaphore.
            //
            string user = Environment.UserDomainName + "\\" 
                + Environment.UserName;
            SemaphoreSecurity semSec = new SemaphoreSecurity();

            SemaphoreAccessRule rule = new SemaphoreAccessRule(
                user, 
                SemaphoreRights.Synchronize | SemaphoreRights.Modify, 
                AccessControlType.Deny);
            semSec.AddAccessRule(rule);

            rule = new SemaphoreAccessRule(
                user, 
                SemaphoreRights.ReadPermissions | SemaphoreRights.ChangePermissions,
                AccessControlType.Allow);
            semSec.AddAccessRule(rule);

            // Create a Semaphore object that represents the system
            // semaphore named by the constant 'semaphoreName', with
            // maximum count three, initial count three, and the
            // specified security access. The Boolean value that 
            // indicates creation of the underlying system object is
            // placed in semaphoreWasCreated.
            //
            sem = new Semaphore(3, 3, semaphoreName, 
                out semaphoreWasCreated, semSec);

            // If the named system semaphore was created, it can be
            // used by the current instance of this program, even 
            // though the current user is denied access. The current
            // program enters the semaphore. Otherwise, exit the
            // program.
            // 
            if (semaphoreWasCreated)
            {
                Console.WriteLine("Created the semaphore.");
            }
            else
            {
                Console.WriteLine("Unable to create the semaphore.");
                return;
            }
        }
        else if (unauthorized)
        {
            // Open the semaphore to read and change the access
            // control security. The access control security defined
            // above allows the current user to do this.
            //
            try
            {
                sem = Semaphore.OpenExisting(
                    semaphoreName, 
                    SemaphoreRights.ReadPermissions 
                        | SemaphoreRights.ChangePermissions);

                // Get the current ACL. This requires 
                // SemaphoreRights.ReadPermissions.
                SemaphoreSecurity semSec = sem.GetAccessControl();
                
                string user = Environment.UserDomainName + "\\" 
                    + Environment.UserName;

                // First, the rule that denied the current user 
                // the right to enter and release the semaphore must
                // be removed.
                SemaphoreAccessRule rule = new SemaphoreAccessRule(
                    user, 
                    SemaphoreRights.Synchronize | SemaphoreRights.Modify, 
                    AccessControlType.Deny);
                semSec.RemoveAccessRule(rule);

                // Now grant the user the correct rights.
                // 
                rule = new SemaphoreAccessRule(user, 
                     SemaphoreRights.Synchronize | SemaphoreRights.Modify, 
                     AccessControlType.Allow);
                semSec.AddAccessRule(rule);

                // Update the ACL. This requires
                // SemaphoreRights.ChangePermissions.
                sem.SetAccessControl(semSec);

                Console.WriteLine("Updated semaphore security.");

                // Open the semaphore with (SemaphoreRights.Synchronize 
                // | SemaphoreRights.Modify), the rights required to
                // enter and release the semaphore.
                //
                sem = Semaphore.OpenExisting(semaphoreName);
            }
            catch(UnauthorizedAccessException ex)
            {
                Console.WriteLine("Unable to change permissions: {0}", ex.Message);
                return;
            }
        }

        // Enter the semaphore, and hold it until the program
        // exits.
        //
        try
        {
            sem.WaitOne();
            Console.WriteLine("Entered the semaphore.");
            Console.WriteLine("Press the Enter key to exit.");
            Console.ReadLine();
            sem.Release();
        }
        catch(UnauthorizedAccessException ex)
        {
            Console.WriteLine("Unauthorized access: {0}", ex.Message);
        }
    }
}

Imports System.Threading
Imports System.Security.AccessControl

Friend Class Example

    <MTAThread> _
    Friend Shared Sub Main()
        Const semaphoreName As String = "SemaphoreExample5"

        Dim sem As Semaphore = Nothing
        Dim doesNotExist as Boolean = False
        Dim unauthorized As Boolean = False

        ' Attempt to open the named semaphore.
        Try
            ' Open the semaphore with (SemaphoreRights.Synchronize
            ' Or SemaphoreRights.Modify), to enter and release the
            ' named semaphore.
            '
            sem = Semaphore.OpenExisting(semaphoreName)
        Catch ex As WaitHandleCannotBeOpenedException
            Console.WriteLine("Semaphore does not exist.")
            doesNotExist = True
        Catch ex As UnauthorizedAccessException
            Console.WriteLine("Unauthorized access: {0}", ex.Message)
            unauthorized = True
        End Try

        ' There are three cases: (1) The semaphore does not exist.
        ' (2) The semaphore exists, but the current user doesn't 
        ' have access. (3) The semaphore exists and the user has
        ' access.
        '
        If doesNotExist Then
            ' The semaphore does not exist, so create it.
            '
            ' The value of this variable is set by the semaphore
            ' constructor. It is True if the named system semaphore was
            ' created, and False if the named semaphore already existed.
            '
            Dim semaphoreWasCreated As Boolean

            ' Create an access control list (ACL) that denies the
            ' current user the right to enter or release the 
            ' semaphore, but allows the right to read and change
            ' security information for the semaphore.
            '
            Dim user As String = Environment.UserDomainName _ 
                & "\" & Environment.UserName
            Dim semSec As New SemaphoreSecurity()

            Dim rule As New SemaphoreAccessRule(user, _
                SemaphoreRights.Synchronize Or SemaphoreRights.Modify, _
                AccessControlType.Deny)
            semSec.AddAccessRule(rule)

            rule = New SemaphoreAccessRule(user, _
                SemaphoreRights.ReadPermissions Or _
                SemaphoreRights.ChangePermissions, _
                AccessControlType.Allow)
            semSec.AddAccessRule(rule)

            ' Create a Semaphore object that represents the system
            ' semaphore named by the constant 'semaphoreName', with
            ' maximum count three, initial count three, and the
            ' specified security access. The Boolean value that 
            ' indicates creation of the underlying system object is
            ' placed in semaphoreWasCreated.
            '
            sem = New Semaphore(3, 3, semaphoreName, _
                semaphoreWasCreated, semSec)

            ' If the named system semaphore was created, it can be
            ' used by the current instance of this program, even 
            ' though the current user is denied access. The current
            ' program enters the semaphore. Otherwise, exit the
            ' program.
            ' 
            If semaphoreWasCreated Then
                Console.WriteLine("Created the semaphore.")
            Else
                Console.WriteLine("Unable to create the semaphore.")
                Return
            End If

        ElseIf unauthorized Then

            ' Open the semaphore to read and change the access
            ' control security. The access control security defined
            ' above allows the current user to do this.
            '
            Try
                sem = Semaphore.OpenExisting(semaphoreName, _
                    SemaphoreRights.ReadPermissions Or _
                    SemaphoreRights.ChangePermissions)

                ' Get the current ACL. This requires 
                ' SemaphoreRights.ReadPermissions.
                Dim semSec As SemaphoreSecurity = sem.GetAccessControl()
                
                Dim user As String = Environment.UserDomainName _ 
                    & "\" & Environment.UserName

                ' First, the rule that denied the current user 
                ' the right to enter and release the semaphore must
                ' be removed.
                Dim rule As New SemaphoreAccessRule(user, _
                    SemaphoreRights.Synchronize Or SemaphoreRights.Modify, _
                    AccessControlType.Deny)
                semSec.RemoveAccessRule(rule)

                ' Now grant the user the correct rights.
                ' 
                rule = New SemaphoreAccessRule(user, _
                    SemaphoreRights.Synchronize Or SemaphoreRights.Modify, _
                    AccessControlType.Allow)
                semSec.AddAccessRule(rule)

                ' Update the ACL. This requires
                ' SemaphoreRights.ChangePermissions.
                sem.SetAccessControl(semSec)

                Console.WriteLine("Updated semaphore security.")

                ' Open the semaphore with (SemaphoreRights.Synchronize 
                ' Or SemaphoreRights.Modify), the rights required to
                ' enter and release the semaphore.
                '
                sem = Semaphore.OpenExisting(semaphoreName)

            Catch ex As UnauthorizedAccessException
                Console.WriteLine("Unable to change permissions: {0}", _
                    ex.Message)
                Return
            End Try

        End If

        ' Enter the semaphore, and hold it until the program
        ' exits.
        '
        Try
            sem.WaitOne()
            Console.WriteLine("Entered the semaphore.")
            Console.WriteLine("Press the Enter key to exit.")
            Console.ReadLine()
            sem.Release()
        Catch ex As UnauthorizedAccessException
            Console.WriteLine("Unauthorized access: {0}", _
                ex.Message)
        End Try
    End Sub 
End Class

Remarks

The name may be prefixed with Global\ or Local\ to specify a namespace. When the Global namespace is specified, the synchronization object may be shared with any processes on the system. When the Local namespace is specified, which is also the default when no namespace is specified, the synchronization object may be shared with processes in the same session. On Windows, a session is a login session, and services typically run in a different non-interactive session. On Unix-like operating systems, each shell has its own session. Session-local synchronization objects may be appropriate for synchronizing between processes with a parent/child relationship where they all run in the same session. For more information about synchronization object names on Windows, see Object Names.

If a synchronization object of the requested type exists in the namespace, the existing synchronization object is opened. If a synchronization object does not exist in the namespace, or a synchronization object of a different type exists in the namespace, a WaitHandleCannotBeOpenedException is thrown.

The OpenExisting method tries to open the specified named semaphore. To create the system semaphore when it does not already exist, use one of the Semaphore constructors that has a name parameter.

Multiple calls to this method that use the same value for name do not necessarily return the same Semaphore object, even though the objects that are returned represent the same named system semaphore.

This method overload is equivalent to calling the OpenExisting method overload and specifying SemaphoreRights.Synchronize and SemaphoreRights.Modify rights, combined by using the bitwise OR operation.

Specifying the SemaphoreRights.Synchronize flag allows a thread to enter the semaphore, and specifying the SemaphoreRights.Modify flag allows a thread to call the Release method.

See also

• Managed Threading
• Semaphore