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