2.127 Pragma Polling

Syntax:

pragma Polling (ON | OFF);

This pragma controls the generation of polling code. This is normally off. If pragma Polling (ON) is used then periodic calls are generated to the routine Ada.Exceptions.Poll. This routine is a separate unit in the runtime library, and can be found in file a-excpol.adb.

Pragma Polling can appear as a configuration pragma (for example it can be placed in the gnat.adc file) to enable polling globally, or it can be used in the statement or declaration sequence to control polling more locally.

A call to the polling routine is generated at the start of every loop and at the start of every subprogram call. This guarantees that the Poll routine is called frequently, and places an upper bound (determined by the complexity of the code) on the period between two Poll calls.

The primary purpose of the polling interface is to enable asynchronous aborts on targets that cannot otherwise support it (for example Windows NT), but it may be used for any other purpose requiring periodic polling. The standard version is null, and can be replaced by a user program. This will require re-compilation of the Ada.Exceptions package that can be found in files a-except.ads and a-except.adb.

A standard alternative unit (in file 4wexcpol.adb in the standard GNAT distribution) is used to enable the asynchronous abort capability on targets that do not normally support the capability. The version of Poll in this file makes a call to the appropriate runtime routine to test for an abort condition.

Note that polling can also be enabled by use of the `-gnatP' switch. See the section on switches for gcc in the GNAT User’s Guide.