This topic provides information about the operating system facilities provided for sharing libraries and memory allocation.

The operating system provides facilities for the creation and use of dynamically bound shared libraries. Dynamic binding allows external symbols referenced in user code and defined in a shared library to be resolved by the loader at run time.

The shared library code is not present in the executable image on disk. Shared code is loaded into memory once in the shared library segment and shared by all processes that reference it. The advantages of shared libraries are:

• Less disk space is used because the shared library code is not included in the executable programs.
• Less memory is used because the shared library code is only loaded once.
• Load time may be reduced because the shared library code may already be in memory.
• Performance may be improved because fewer page faults will be generated when the shared library code is already in memory. However, there is a performance cost in calls to shared library routines of one to eight instructions.

The symbols defined in the shared library code that are to be made available to referencing modules must be explicitly exported using an exports file, unless the -bexpall options is used. The first line of the file optionally contains the path name of the shared library. Subsequent lines contain the symbols to be exported.

• Shared objects and run time linking
  

  By default, programs are linked so that a reference to a symbol imported from a shared object is bound to that definition at load time.

• Shared libraries and lazy loading
  

  By default, when a module is loaded, the system loader automatically loads all of the module's dependents at the same time. Loading of dependents occurs because when a module is linked, a list of the module's dependent modules is saved in the loader section of the module.

• Named shared library areas
  

  By default, AIX® shares libraries among processes using a global set of segments, referred to as the global shared library area.

• Creating a shared library
  

  This section describes how to create a shared library.

• Program address space overview
  

  The Base Operating System provides a number of services for programming application program memory use.

• Understanding memory mapping
  

  The speed at which application instructions are processed on a system is proportionate to the number of access operations required to obtain data outside of program-addressable memory.

• Interprocess communication limits
  

  This topic describes system limits for Interprocess communication (IPC) mechanisms.

• Creating a mapped data file with the shmat subroutine
  

  This section describes how to create a mapped data file using shmat subroutine.

• Creating a copy-on-write mapped data file with the shmat subroutine
  

  This section describes how to create a copy-on-write mapped data file with the shmat subroutine.

• Creating a shared memory segment with the shmat subroutine
  

  This section describes how to create a shared memory segment with the shmat subroutine

• Paging space programming requirements
  

  The amount of paging space required by an application depends on the type of activities performed on the system. If paging space runs low, processes may be lost.

• List of memory manipulation services
  

  The memory functions operate on arrays of characters in memory called memory areas.

• List of memory mapping services
  

  The memory mapping subroutines operate on memory regions that have been mapped with the mmapsubroutine.