fix AC_C_CONST to work with CFLAGS -O2 -Wall -Werror

[Didier Barvaux]

Hi all,

I seems that GCC fails to compile the code generated by the AC_C_CONST
macro if the -O2 -Wall -Werror CFLAGS are specified.

I tried with both autoconf 2.61 and 2.62.17-684d2-dirty (taken from git
repo). GCC version is 4.1.2.

Below is the part of the config.log file describing what appends:

configure:25803: checking for an ANSI C-conforming const
configure:25878: gcc -c -g -O2 -Wno-unused-function -Wall -Werror conftest.c >&5
cc1: warnings being treated as errors
conftest.c: In function 'main':
conftest.c:76: warning: 't' is used uninitialized in this function
conftest.c:92: warning: 'b' is used uninitialized in this function
conftest.c:98: warning: 'cs[0]' is used uninitialized in this function
configure:25885: $? = 1
configure: failed program was:
[...]
| int
| main ()
| {
| /* FIXME: Include the comments suggested by Paul. */
| #ifndef __cplusplus
|   /* Ultrix mips cc rejects this.  */
|   typedef int charset[2];
|   const charset cs;
|   /* SunOS 4.1.1 cc rejects this.  */
|   char const *const *pcpcc;
|   char **ppc;
|   /* NEC SVR4.0.2 mips cc rejects this.  */
|   struct point {int x, y;};
|   static struct point const zero = {0,0};
|   /* AIX XL C 1.02.0.0 rejects this.
|      It does not let you subtract one const X* pointer from another in
|      an arm of an if-expression whose if-part is not a constant
|      expression */
|   const char *g = "string";
|   pcpcc = &g + (g ? g-g : 0);
|   /* HPUX 7.0 cc rejects these. */
|   ++pcpcc;
|   ppc = (char**) pcpcc;
|   pcpcc = (char const *const *) ppc;
|   { /* SCO 3.2v4 cc rejects this.  */
|     char *t;
|     char const *s = 0 ? (char *) 0 : (char const *) 0;
| 
|     *t++ = 0;
|     if (s) return 0;
|   }
|   { /* Someone thinks the Sun supposedly-ANSI compiler will reject this.  */
|     int x[] = {25, 17};
|     const int *foo = &x[0];
|     ++foo;
|   }
|   { /* Sun SC1.0 ANSI compiler rejects this -- but not the above. */
|     typedef const int *iptr;
|     iptr p = 0;
|     ++p;
|   }
|   { /* AIX XL C 1.02.0.0 rejects this saying
|        "k.c", line 2.27: 1506-025 (S) Operand must be a modifiable lvalue. */
|     struct s { int j; const int *ap[3]; };
|     struct s *b; b->j = 5;
|   }
|   { /* ULTRIX-32 V3.1 (Rev 9) vcc rejects this */
|     const int foo = 10;
|     if (!foo) return 0;
|   }
|   return !cs[0] && !zero.x;
| #endif
| 
|   ;
|   return 0;
| }
configure:25900: result: no


Below is a patch that corrects the problem with AC_C_CONST. This patch
works great on my system. It may perhaps break the macro on other
systems, I don't know.

diff --git a/lib/autoconf/c.m4 b/lib/autoconf/c.m4
index ffaa06e..75b0745 100644
--- a/lib/autoconf/c.m4
+++ b/lib/autoconf/c.m4
@@ -1587,7 +1587,7 @@ AC_DEFUN([AC_C_CONST],
 #ifndef __cplusplus
   /* Ultrix mips cc rejects this.  */
   typedef int charset[2];
-  const charset cs;
+  const charset cs = { 0, 0 };
   /* SunOS 4.1.1 cc rejects this.  */
   char const *const *pcpcc;
   char **ppc;
@@ -1605,10 +1605,7 @@ AC_DEFUN([AC_C_CONST],
   ppc = (char**) pcpcc;
   pcpcc = (char const *const *) ppc;
   { /* SCO 3.2v4 cc rejects this.  */
-    char *t;
     char const *s = 0 ? (char *) 0 : (char const *) 0;
-
-    *t++ = 0;
     if (s) return 0;
   }
   { /* Someone thinks the Sun supposedly-ANSI compiler will reject this.  */
@@ -1624,7 +1621,9 @@ AC_DEFUN([AC_C_CONST],
   { /* AIX XL C 1.02.0.0 rejects this saying
        "k.c", line 2.27: 1506-025 (S) Operand must be a modifiable lvalue. */
     struct s { int j; const int *ap[3]; };
-    struct s *b; b->j = 5;
+    struct s a;
+    struct s *b = &a;
+    b->j = 5;
   }
   { /* ULTRIX-32 V3.1 (Rev 9) vcc rejects this */
     const int foo = 10;


Any comments are welcome.

Didier Barvaux

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