Content-type: text/html Man page of protect_headers_setup


Section: Maintenance Commands (8)
Index Return to Main Contents


NAME - Sets up header files for the -protect_headers feature in cc, cxx, and cpp  


/usr/lib/cmplrs/cc/ -l [-f] [dirname]

/usr/lib/cmplrs/cc/ -d [-f] [dirname]

/usr/lib/cmplrs/cc/ -i [dirname]

/usr/lib/cmplrs/cc/ -v [dirname]  


Walks the specified directory tree and creates symbolic links to __DECC_include_prologue.h and __DECC_include_epilogue.h files. This requires write permission for the directories in which the links will be created. The script does not overwrite existing files with the same name (except for prologue and epilogue files or if the -f flag is also specified). It does overwrite existing symbolic links with the same name. Walks the specified directory tree and deletes the symbolic links named __DECC_include_prologue.h and __DECC_include_epilogue.h. This requires write permission for the directories from which the links will be deleted. The script does not remove real files unless the -f flag is also specified. When combined with -l, causes the script to overwrite existing files and symbolic links. When combined with -d, causes the script to remove real files. The -f flag cannot be used without the -l or -d flags. Walks the specified directory tree and outputs an inventory of all real files and symbolic links named __DECC_include_prologue.h and __DECC_include_epilogue.h. The inventory distinguishes between real files and symbolic links. It also makes note of symbolic links that differ from those that would have been created by the -l option. Walks the specified directory tree and verifies that all subdirectories that contain header files also contain valid prologue and epilogue files. It outputs a list of unprotected directories, including information on what is wrong with the each directory.  


The protect_headers_setup script walks the directory tree rooted at /usr/include (or dirname if specified) and performs one of several actions determined by the flag argument. For -l, it creates symbolic links with the following names: __DECC_include_prologue.h __DECC_include_epilogue.h

The script creates the links in each directory that contains header files. The links resolve to files located in /usr/lib/cmplrs/cc. The files contain directives that can be helpful to C and C++ compiler users who use the -xtaso_short, -nomember_alignment, or -Zpn (where n!=8) options.

If the optional dirname argument is specified, the command walks that directory tree instead of /usr/include.

The tree walk includes walking symbolic links that refer to directories.

When passed other flags, the protect_headers_setup script will delete these links (-d), inventory them (-i), or verify that they exist (-v).  

Header File Problem

When the system libraries were built, the compiler assumed that pointers were 64 bits and that structure members were naturally aligned. These are the C and C++ compiler defaults. The interfaces to the system libraries (the header files in the /usr/include tree) do not explicitly encode these assumptions.

You can alter the compiler's assumptions about pointer size (with -xtaso_short) and structure member alignment (with -Zpn [where !=8] or -nomember_alignment). If you use any of these options and your application includes a header file from the /usr/include tree and then calls a library function or uses types declared in that header file, problems may occur. In particular, the data layouts computed by the compiler when it processes the system header file declarations may differ from the layouts compiled into the system libraries. This situation can cause unpredictable results. Consider the following example: #include <stdio.h> int main () {
    fprintf(stdout, "hello, world); } In the preceding example, stdio.h defines stdout as (&_iob[1]). Array _iob has the type FILE, where FILE is a structure containing a number of members that are pointer types. Thus, the size of a FILE structure depends on the size of a pointer. As a result, if the program is compiled with -xtaso_short, the calculation of the address &_iob[1] will differ from the calculation done in libc and the program will not work as intended.

The problem is insidious because the program will compile and link cleanly with -xtaso_short. However, it will not run correctly because the data layouts will differ.

Similar problems can occur if a program is compiled with -Zpn (where n!=8) or -nomember_alignment, a header file is included from the /usr/include tree, and the program makes use of a structure declared in that header file. The program's notion of the physical layout of that structure may differ from the actual layout compiled into the standard system library.  

Solution to the Header File Problem

The -protect_headers feature in cc, cxx, and cpp can solve the pointer size and data alignment problems. The solution has two parts: Run the protect_headers_setup script to ensure that all system header file directories are protected from the previously described problems. A protected directory contains files (symbolic links, actually) with the following names: __DECC_include_prologue.h __DECC_include_epilogue.h
The prologue file contains #pragma directives that set the compiler's pointer size and data layout environment to appropriate values for system header files. The prologue file also suppresses certain warning-level and informational-level diagnostic messages that are inappropriate for system header files. ,IP The epilogue file contains #pragma directives that restore the values from the previous environment. When the -protect_headers feature is in effect and the compiler encounters a #include directive, the compiler searches for files with these special names in the directory containing the header file. If the directory contains a prologue file, that file is included just before the file specified in the #include directive. If the directory contains an epilogue file, that file is included just after the file specified in the #include directive. This ``bracketing'' ensures that system header files are always included in the appropriate system environment.

The -protect_headers feature is enabled by default in the cc compiler when processing C source files and in the cxx compiler when processing C or C++ source files. It is disabled by default for all other inputs to these compilers. It is also disabled by default for all inputs to the cpp preprocessor.

You can enable or disable the -protect_headers feature by using the following command-line flags to the cc, cxx, or cpp commands: Enables the feature. Disables the feature. Establishes the default behavior appropriate to the command and the type of input; that is, enables the feature for cc with C input and for cxx with C or C++ input, and disables it otherwise.

If more than one such flag appears on the command line, only the last one is applied.


The protect headers feature is not available with cc -oldc or cpp -oldc.

To alert users to the potential danger of compiling system headers with nonstandard data layout in effect, cc, cxx, and cpp may issue the following informational message: A nondefault pointer size or member alignment is specified and the system header files do not appear to be protected. This may yield unpredictable results. The protect_headers_setup script can help. See protect_headers_setup(8) or cc(1) for details.

The message is issued if the -protect_headers feature is enabled, the /usr/include directory does not contain prologue and epilogue files, and the user specifies -xtaso_short, -nomember_alignment, or -Zpn (where n!=8).

The message may be misleading in some cases because the -protect_headers feature is not always needed. The feature is not needed under either of the following circumstances: When an application does not include any files from the /usr/include tree When inclusions of system header files have been explicitly protected by bracketing the #include statements with #pragma directives

To prevent the message from appearing in the preceding cases, use the -protect_headers none switch in your compilations. You can also eliminate it by running the protect_headers_setup script. The prologue and epilogue files established by the script have no effect on the results of a program.  

When to Run the Script

Ideally, the protect_headers_setup script should be run immediately after the C or C++ compiler is installed. This will protect the /user/include directories.

If you later add a subdirectory to the /usr/include tree, the new directory will not be protected unless it contains the appropriate prologue and epilogue files. If protection is needed, you have two options: If the header files define an interface to a library that was built using the compiler's default assumptions about pointer size (64 bits) and structure member alignment, you should rerun the script to protect that new directory. If the header files define an interface to a library that was compiled with -xtaso_short or with unnatural alignment, you should add empty prologue and epilogue files to that directory (if they are not already present). This will prevent the protect_headers_setup script from adding links to the real prologue and epilogue files when it is next run.

The second situation is rare, and the creation of such libraries is not recommended.  


Commands: cc(1), cpp(1), cxx(1)

Programmer's Guide

DEC C Language Reference Manual delim off



Header File Problem
Solution to the Header File Problem
When to Run the Script

This document was created by man2html, using the manual pages.
Time: 02:40:39 GMT, October 02, 2010