/[autoconf]/autoconf/doc/autoconf.texi
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revision 1.695 by akim, Wed Oct 23 07:42:13 2002 UTC revision 1.696 by akim, Fri Oct 25 08:44:23 2002 UTC
# Line 4493  commands @var{action-if-found}, otherwis Line 4493  commands @var{action-if-found}, otherwis
4493  header file is available, consider using @code{AC_CHECK_HEADERS}  header file is available, consider using @code{AC_CHECK_HEADERS}
4494  instead.  instead.
4495    
4496  Previous versions of Autoconf merely checked whether the header was  For compatibility issues with older versions of Autoconf, please read
4497  accepted by the preprocessor.  This was changed because the old test  below.
 was inappropriate for typical uses.  Headers are typically used to  
 compile, not merely to preprocess, and the old behavior sometimes  
 accepted headers that clashed at compile-time.  If you need to check  
 whether a header is preprocessable, you can use @code{AC_TRY_CPP} or  
 one of its variants (@pxref{Examining Declarations}).  
4498  @end defmac  @end defmac
4499    
4500  @defmac AC_CHECK_HEADERS (@var{header-file}@dots{}, @ovar{action-if-found}, @ovar{action-if-not-found}, @dvar{includes, default-includes})  @defmac AC_CHECK_HEADERS (@var{header-file}@dots{}, @ovar{action-if-found}, @ovar{action-if-not-found}, @dvar{includes, default-includes})
# Line 4512  is given, it is additional shell code to Line 4507  is given, it is additional shell code to
4507  files is found.  You can give it a value of @samp{break} to break out of  files is found.  You can give it a value of @samp{break} to break out of
4508  the loop on the first match.  If @var{action-if-not-found} is given, it  the loop on the first match.  If @var{action-if-not-found} is given, it
4509  is executed when one of the header files is not found.  is executed when one of the header files is not found.
4510    
4511    For compatibility issues with older versions of Autoconf, please read
4512    below.
4513  @end defmac  @end defmac
4514    
4515    Previous versions of Autoconf merely checked whether the header was
4516    accepted by the preprocessor.  This was changed because the old test was
4517    inappropriate for typical uses.  Headers are typically used to compile,
4518    not merely to preprocess, and the old behavior sometimes accepted
4519    headers that clashed at compile-time.  If you need to check whether a
4520    header is preprocessable, you can use @code{AC_TRY_CPP} or one of its
4521    variants (@pxref{Examining Declarations}).
4522    
4523    This scheme, which improves the robustness of the test, also requires
4524    that you make sure that headers that must be included before the
4525    @var{header-file} be part of the @var{includes}, (@pxref{Default
4526    Includes}).  If looking for @file{bar.h}, which requires that
4527    @file{foo.h} be included before if it exists, we suggest the following
4528    scheme:
4529    
4530    @verbatim
4531    AC_CHECK_HEADERS([foo.h])
4532    AC_CHECK_HEADERS([bar.h], [], [],
4533    [#if HAVE_FOO_H
4534    # include <foo.h>
4535    # endif
4536    ])
4537    @end verbatim
4538    
4539  @node Declarations  @node Declarations
4540  @section Declarations  @section Declarations
4541  @cindex Declaration, checking  @cindex Declaration, checking

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