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118 lines
3.7 KiB
ReStructuredText
118 lines
3.7 KiB
ReStructuredText
Anonymous Shared Memory
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=======================
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This chapter describes the NSPR API for anonymous shared memory.
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- `Anonymous Memory Protocol <#Anonymous_Memory_Protocol>`__
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- `Anonymous Shared Memory
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Functions <#Anonymous_Shared_Memory_Functions>`__
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.. _Anonymous_Memory_Protocol:
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Anonymous Memory Protocol
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-------------------------
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NSPR provides an anonymous shared memory based on NSPR's :ref:`PRFileMap`
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type. The anonymous file-mapped shared memory provides an inheritable
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shared memory, as in: the child process inherits the shared memory.
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Compare the file-mapped anonymous shared memory to to a named shared
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memory described in prshm.h. The intent is to provide a shared memory
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that is accessible only by parent and child processes. ... It's a
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security thing.
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Depending on the underlying platform, the file-mapped shared memory may
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be backed by a file. ... surprise! ... On some platforms, no real file
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backs the shared memory. On platforms where the shared memory is backed
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by a file, the file's name in the filesystem is visible to other
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processes for only the duration of the creation of the file, hopefully a
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very short time. This restricts processes that do not inherit the shared
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memory from opening the file and reading or writing its contents.
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Further, when all processes using an anonymous shared memory terminate,
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the backing file is deleted. ... If you are not paranoid, you're not
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paying attention.
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The file-mapped shared memory requires a protocol for the parent process
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and child process to share the memory. NSPR provides two protocols. Use
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one or the other; don't mix and match.
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In the first protocol, the job of passing the inheritable shared memory
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is done via helper-functions with PR_CreateProcess. In the second
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protocol, the parent process is responsible for creating the child
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process; the parent and child are mutually responsible for passing a
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``FileMap`` string. NSPR provides helper functions for extracting data
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from the :ref:`PRFileMap` object. ... See the examples below.
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Both sides should adhere strictly to the protocol for proper operation.
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The pseudo-code below shows the use of a file-mapped shared memory by a
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parent and child processes. In the examples, the server creates the
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file-mapped shared memory, the client attaches to it.
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.. _First_protocol:
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First protocol
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~~~~~~~~~~~~~~
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**Server:**
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.. code::
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fm = PR_OpenAnonFileMap(dirName, size, FilemapProt);
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addr = PR_MemMap(fm);
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attr = PR_NewProcessAttr();
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PR_ProcessAttrSetInheritableFileMap( attr, fm, shmname );
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PR_CreateProcess(Client);
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PR_DestroyProcessAttr(attr);
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... yadda ...
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PR_MemUnmap( addr );
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PR_CloseFileMap(fm);
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**Client:**
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.. code::
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... started by server via PR_CreateProcess()
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fm = PR_GetInheritedFileMap( shmname );
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addr = PR_MemMap(fm);
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... yadda ...
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PR_MemUnmap(addr);
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PR_CloseFileMap(fm);
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.. _Second_protocol:
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Second protocol
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~~~~~~~~~~~~~~~
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**Server:**
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.. code::
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fm = PR_OpenAnonFileMap(dirName, size, FilemapProt);
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fmstring = PR_ExportFileMapAsString( fm );
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addr = PR_MemMap(fm);
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... application specific technique to pass fmstring to child
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... yadda ... Server uses his own magic to create child
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PR_MemUnmap( addr );
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PR_CloseFileMap(fm);
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**Client:**
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.. code::
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... started by server via his own magic
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... application specific technique to find fmstring from parent
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fm = PR_ImportFileMapFromString( fmstring )
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addr = PR_MemMap(fm);
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... yadda ...
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PR_MemUnmap(addr);
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PR_CloseFileMap(fm);
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.. _Anonymous_Shared_Memory_Functions:
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Anonymous Shared Memory Functions
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---------------------------------
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- :ref:`PR_OpenAnonFileMap`
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- :ref:`PR_ProcessAttrSetInheritableFileMap`
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- :ref:`PR_GetInheritedFileMap`
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- :ref:`PR_ExportFileMapAsString`
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- :ref:`PR_ImportFileMapFromString`
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