Preprocessor Scope in Precompiled Files

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.

Listing: Pragma Settings in a Precompiled Header

/* 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 the following listing includes the precompiled version of the listing displayed above.

Listing: Pragma Settings in an Included Precompiled File

/* 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.