[PATCH wwwdocs] gcc-14: Some very common historic Autoconf probes that no longer work

[Florian Weimer]

---
 htdocs/gcc-14/porting_to.html | 43 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 43 insertions(+)

diff --git a/htdocs/gcc-14/porting_to.html b/htdocs/gcc-14/porting_to.html
index 123b5e9f..ab65c5e7 100644
--- a/htdocs/gcc-14/porting_to.html
+++ b/htdocs/gcc-14/porting_to.html
@@ -437,6 +437,49 @@ issues addressed in more recent versions.)  Versions 
before 2.69 may
 have generic probes (for example for standard C support) that rely on
 C features that were removed in C99 and thus fail with GCC 14.
 
+<p>
+Older Autoconf versions (for example, Autoconf 2.13) generate core
+probes that are incompatible with C99.  These include the basic
+compiler functionality check:
+
+<pre>
+#include "confdefs.h"
+main(){return(0);}
+</pre>
+
+And a check for standard C compatibility:
+
+<pre>
+#define XOR(e, f) (((e) && !(f)) || (!(e) && (f)))
+int main () { int i; for (i = 0; i &lt; 256; i++)
+if (XOR (islower (i), ISLOWER (i)) || toupper (i) != TOUPPER (i)) exit(2);
+exit (0); }
+</pre>
+
+(Several variants with different line breaks and whitespace were in
+use.)  If it is not possible to port the configure script to current
+Autoconf, these issues can be patched directly:
+
+<pre>
+#include "confdefs.h"
+<ins>int</ins> main(){return(0);}
+</pre>
+
+And for the second probe:
+
+<pre>
+#define XOR(e, f) (((e) && !(f)) || (!(e) && (f)))
+int main () { int i; for (i = 0; i &lt; 256; i++)
+if (XOR (islower (i), ISLOWER (i)) || toupper (i) != TOUPPER (i)) <ins>return 
2</ins>;
+<ins>return 0</ins>; }
+</pre>
+
+There is a long tail of less frequent issues, involving keyword
+checking (for <code>inline</code>), and checks for <code>dlopen</code>
+and <code>mmap</code>.  A <code>setvbuf</code> probe was previously
+expected to fail at run time because it triggered undefined behavior,
+now it fails because of a compilation error.
+
 <h4 id="errors-as-warnings">Turning errors back into warnings</h4>
 
 <p>

base-commit: 1fcd61437d6a3d7bf24b993b09d525486dc9a2e5