When precompiling a header file, the compiler preprocesses the file too. In other words, a precompiled file is preprocessed in the context of its precompilation, not in the context of its later compilation.
The preprocessor also tracks macros used to guard #include files to reduce parsing time. If a file's contents are surrounded with
#ifndef MYHEADER_H
#define MYHEADER_H
/* file contents */
#endif
the compiler will not load the file twice, saving some time in the process.
Pragma settings inside a precompiled file affect only the source code within that file. The pragma settings for an item declared in a precompiled file (such as data or a function) are saved, then restored when the precompiled header file is included.
For example, the source code in the following listing specifies that the variable xxx is a far variable.
/* my_pch.pch */ /* Generate a precompiled header named pch.mch. */ #pragma precompile_target "my_pch.mch" #pragma far_data on extern int xxx;
The source code in Listing: Pragma Settings in an Included Precompiled File includes the precompiled version of Listing: Pragma Settings in a Precompiled Header.
/* test.c */ /* Far data is disabled. */ #pragma far_data off /* This precompiled file sets far_data on. */ #include "my_pch.mch" /* far_data is still off but xxx is still a far variable. */
The pragma setting in the precompiled file is active within the precompiled file, even though the source file including the precompiled file has a different setting.