protect_headers_setup(8)protect_headers_setup(8)NAME
protect_headers_setup.sh - Sets up header files for the -protect_head‐
ers feature in cc, cxx, and cpp
SYNOPSIS
/usr/lib/cmplrs/cc/protect_headers_setup.sh -l [-f] [dirname]
/usr/lib/cmplrs/cc/protect_headers_setup.sh -d [-f] [dirname]
/usr/lib/cmplrs/cc/protect_headers_setup.sh -i [dirname]
/usr/lib/cmplrs/cc/protect_headers_setup.sh -v [dirname]
OPTIONS
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 option
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_epi‐
logue.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 option is also specified. When combined with -l, causes
the script to overwrite existing files and symbolic links. When com‐
bined with -d, causes the script to remove real files. The -f option
cannot be used without the -l or -d options. Walks the specified
directory tree and outputs an inventory of all real files and symbolic
links named <__DECC_include_prologue.h> and <__DECC_include_epi‐
logue.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 direc‐
tory 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.
DESCRIPTION
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 option 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 directo‐
ries.
When passed other options, 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 point‐
ers 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 n!=8] or
-nomember_alignment). If you use any of these options and your applica‐
tion 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 com‐
piler's pointer size and data layout environment to appropriate
values for system header files. The prologue file also sup‐
presses certain warning-level and informational-level diagnostic
messages that are inappropriate for system header files. The
epilogue file contains #pragma directives that restore the val‐
ues 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 “brack‐
eting” 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 options 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 dis‐
ables it otherwise.
If more than one such option appears on the command line, only the last
one is applied.
To alert users to the potential danger of compiling system headers with
nonstandard data layout in effect, cc, cxx, and cpp may issue the fol‐
lowing 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_head‐
ers_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_head‐
ers 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 elimi‐
nate 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 pro‐
logue 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 align‐
ment, you should add empty prologue and epilogue files to that direc‐
tory (if they are not already present). This will prevent the pro‐
tect_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.
SEE ALSO
Commands: cc(1), cpp(1), cxx(1)
Programmer's Guide
Compaq C Language Reference Manual
protect_headers_setup(8)
