Sun Studio 12: C User's Guide

B.2.124 -xpch=v

This compiler option activates the precompiled-header feature. v can be auto, autofirst, collect:pch_filename, or use:pch_filename. You can take advantage of this feature through the -xpch (detailed in B.2.124 -xpch=v) and -xpchstop (detailed in B.2.125 -xpchstop=[file|<include>]) options in combination with the #pragma hdrstop directive (detailed under 2.8.8 hdrstop).

Use the -xpch option to create a precompiled-header file and improve your compilation time. The precompiled-header file is designed to reduce compile time for applications whose source files share a common set of include files containing a large amount of source code. A precompiled header works by collecting information about a sequence of header files from one source file, and then using that information when recompiling that source file, and when compiling other source files that have the same sequence of headers. The information that the compiler collects is stored in a precompiled-header file.

See Also:

B.2.124.1 Creating a Precompiled-Header File Automatically

You can let the compiler generate the precompiled- header file for you automatically. Choose between one of the following two ways to do this. One way is for the compiler to create the precompiled-header file from the first include file it finds in the source file. The other way is for the compiler to select from the set of include files found in the source file starting with the first include file and extending through a well- defined point that determines which include file is the last one. Use one of the following two flags to determine which method the compiler uses to automatically generate a precompiled header:

Table B–34 The -xpch Flags




The contents of the precompiled-header file is based on the longest viable prefix (see the following section for an explanation of how a viable prefix is identified) that the compiler finds in the source file. This flag produces a precompiled header file that consists of the largest possible number of header files. 


This flag produces a precompiled-header file that contains only the first header found in the source file. 

B.2.124.2 Creating a Precompiled-Header File Manually

If you decide to create your precompiled-header file manually, you must start by first using -xpch, and specifying the collect mode. The compilation command that specifies -xpch=collect must only specify one source file. In the following example, the -xpch option creates a precompiled-header file called myheader.cpch based on the source file a.c:

cc -xpch=collect:myheader a.c

A valid precompiled-header filename always has the suffix .cpch. When you specify pch_filename, you can add the suffix or let the compiler add it for you. For example, if you specify cc -xpch=collect:foo a.c, the precompiled-header file is called foo.cpch.

B.2.124.3 How the Compiler Handles an Existing Precompiled-Header File

If the compiler cannot use the precompiled-header file, under -xpch=auto and -xpch=autofirst, it generates a new precompiled-header file. If the compiler cannot use the precompiled-header file under -xpch=use, a warning is issued and the compilation is done using the real headers.

B.2.124.4 Directing the Compiler to Use a Specific Precompiled-Header File

You can also direct the compiler to use a specific precompiled header. Specify -xpch=use:pch_filename to do this. You can specify any number of source files with the same sequence of include files as the source file that was used to create the precompiled-header file. For example, your command in use mode could look like this: cc -xpch=use:foo.cpch foo.c bar.c foobar.c.

You should only use an existing precompiled-header file if the following are true. If any of the following is not true, you should recreate the precompiled-header file:

B.2.124.5 The Viable Prefix

In order to share a precompiled-header file across multiple source files, those source files must share a common set of include files as their initial sequence of tokens. A token is a keyword, name or punctuation mark. Comments and code that is excluded by #if directives are not recognized by the compiler as tokens. This initial sequence of tokens is known as the viable prefix. In other words, the viable prefix is the top portion of the source file that is common to all source files. The compiler uses this viable prefix as the basis for creating a precompiled-header file and thereby determining which header files from the source are pre-compiled.

The viable prefix that the compiler finds during the current compilation must match the viable prefix that it used to create the precompiled-header file. In other words, viable prefix must be interpreted consistently across all the source files that use the same precompiled-header file.

The viable prefix of a source file can only be comprised of comments and any of the following pre-processor directives:

#ident (if identical, passed through as is)
#pragma (if identical)

Any of these may reference macros. The #else, #elif, and #endif directives must match within the viable prefix. Comments are ignored.

The compiler determines the end point of the viable prefix automatically when you specify -xpch=auto or -xpch=autofirst and is defined as follows. For -xpch=collect or -xpch=use, the viable prefix ends with a #pragma hdrstop.

Within the viable prefix of each file that shares a precompiled-header file, each corresponding #define and #undef directive must reference the same symbol (in the case of #define, each one must reference the same value). Their order of appearance within each viable prefix must be the same as well. Each corresponding pragma must also be the same and appear in the same order across all the files sharing a precompiled header.

B.2.124.6 Screening a Header File for Problems

What makes a header file precompilable? A header file is precompilable when it is interpreted consistently across different source files. Specifically, when it contains only complete declarations. That is, a declaration in any one file must stand alone as a valid declaration. Incomplete type declarations, such as struct S;, are valid declarations. The complete type declaration can appear in some other file. Consider these example header files:

file a.h
struct S {
#include "x.h" /* not allowed */

file b.h
struct T; // ok, complete declaration
struct S {
   int i;
[end of file, continued in another file] /* not allowed*/

A header file that is incorporated into a precompiled-header file must not violate the following. The results of compiling a program that violates any of these constraints is undefined.

B.2.124.7 The Precompiled-Header File Cache

When the compiler creates a precompiled-header file automatically, the compiler writes it to the SunWS_cache directory. This directory always resides in the location where the object file is created. Updates to the file are preformed under a lock so that it works properly under dmake.

If you need to force the compiler to rebuild automatically-generated precompiled-header files, you can clear the precompiled-header file cache-directory with the CCadmin tool. See the CCadmin(1) man page for more information.

B.2.124.8 Warnings

B.2.124.9 Precompiled-Header File Dependencies and make Files

The compiler generates dependency information for precompiled-header files when you specify -xpch=collect. You need to create the appropriate rules in your make files to take advantage of these dependencies. Consider this sample make file:

%.o : %.c shared.cpch
         $(CC) -xpch=use:shared -xpchstop=foo.h -c $<
default : a.out

foo.o + shared.cpch : foo.c
         $(CC) -xpch=collect:shared -xpchstop=foo.h foo.c -c

a.out : foo.o bar.o foobar.o
         $(CC) foo.o bar.o foobar.o

clean :
         rm -f *.o shared.cpch .make.state a.out

These make rules, along with the dependencies generated by the compiler, force a manually created precompiled- header file to be recreated if any source file you used with -xpch=collect, or any of the headers that are part of the precompiled-header file, have changed. This prevents the use of an out of date precompiled-header file.

You do not have to create any additional make rules in your makefiles for -xpch=auto or -xpch=autofirst.