TASKING VX-toolset for ARM User Guide

C++ LanguageWhen the host system does not support memory mapping, so that everything to be saved in theprecompiled header file is assigned to preallocated memory (MS-Windows), two additional restrictionsapply:• The total memory needed at the header stop point cannot exceed the size of the block of preallocatedmemory.• No single program entity saved can exceed 16384, the preallocation unit.When a precompiled header file is produced, it contains, in addition to the snapshot of the compiler state,some information that can be checked to determine under what circumstances it can be reused. Thisincludes:• The compiler version, including the date and time the compiler was built.• The current directory (i.e., the directory in which the compilation is occurring).• The command line options.• The initial sequence of preprocessing directives from the primary source file, including #includedirectives.• The date and time of the header files specified in #include directives.This information comprises the PCH prefix. The prefix information of a given source file can be comparedto the prefix information of a PCH file to determine whether the latter is applicable to the current compilation.As an illustration, consider two source files:// a.cc#include "xxx.h"... // Start of code// b.cc#include "xxx.h"... // Start of codeWhen a.cc is compiled with --pch, a precompiled header file named a.pch is created.Then, when b.ccis compiled (or when a.cc is recompiled), the prefix section of a.pch is read in for comparison with thecurrent source file. If the command line options are identical, if xxx.h has not been modified, and soforth, then, instead of opening xxx.h and processing it line by line, the C++ compiler reads in the rest ofa.pch and thereby establishes the state for the rest of the compilation.It may be that more than one PCH file is applicable to a given compilation. If so, the largest (i.e., the onerepresenting the most preprocessing directives from the primary source file) is used. For instance, considera primary source file that begins with#include "xxx.h"#include "yyy.h"#include "zzz.h"If there is one PCH file for xxx.h and a second for xxx.h and yyy.h, the latter will be selected (assumingboth are applicable to the current compilation). Moreover, after the PCH file for the first two headers isread in and the third is compiled, a new PCH file for all three headers may be created.79