9.10 Shared Files

Section A.14 of the Ada Reference Manual allows implementations to provide a wide variety of behavior if an attempt is made to access the same external file with two or more internal files.

To provide a full range of functionality, while at the same time minimizing the problems of portability caused by this implementation dependence, GNAT handles file sharing as follows:

• In the absence of a ‘shared=xxx’ form parameter, an attempt to open two or more files with the same full name is considered an error and is not supported. The exception Use_Error will be raised. Note that a file that is not explicitly closed by the program remains open until the program terminates.
• If the form parameter ‘shared=no’ appears in the form string, the file can be opened or created with its own separate stream identifier, regardless of whether other files sharing the same external file are opened. The exact effect depends on how the C stream routines handle multiple accesses to the same external files using separate streams.
• If the form parameter ‘shared=yes’ appears in the form string for each of two or more files opened using the same full name, the same stream is shared between these files, and the semantics are as described in Ada Reference Manual, Section A.14.

When a program that opens multiple files with the same name is ported from another Ada compiler to GNAT, the effect will be that Use_Error is raised.

The documentation of the original compiler and the documentation of the program should then be examined to determine if file sharing was expected, and ‘shared=xxx’ parameters added to Open and Create calls as required.

When a program is ported from GNAT to some other Ada compiler, no special attention is required unless the ‘shared=xxx’ form parameter is used in the program. In this case, you must examine the documentation of the new compiler to see if it supports the required file sharing semantics, and form strings modified appropriately. Of course it may be the case that the program cannot be ported if the target compiler does not support the required functionality. The best approach in writing portable code is to avoid file sharing (and hence the use of the ‘shared=xxx’ parameter in the form string) completely.

One common use of file sharing in Ada 83 is the use of instantiations of Sequential_IO on the same file with different types, to achieve heterogeneous input-output. Although this approach will work in GNAT if ‘shared=yes’ is specified, it is preferable in Ada to use Stream_IO for this purpose (using the stream attributes)