[Libguestfs] [PATCH nbdkit 1/2] include: Fix NBDKIT_HANDLE_NOT_NEEDED for C90 compilers.

[Richard W.M. Jones]

When an ANSI/C90 plugin compiled with ‘-pedantic’ uses
NBDKIT_HANDLE_NOT_NEEDED it gets the error:

  ISO C forbids conversion of function pointer to object pointer type

This is because the existing macro worked by returning a function
pointer but in C90 function pointers cannot be cast to data pointers
since on some ancient architectures code and data pointers were
incompatible.

We only need a convenient global data pointer here, and the address of
‘errno’ should be fine.
---
 include/nbdkit-common.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/include/nbdkit-common.h b/include/nbdkit-common.h
index 36fce20..cb9954e 100644
--- a/include/nbdkit-common.h
+++ b/include/nbdkit-common.h
@@ -40,6 +40,7 @@
 
 #include <stdarg.h>
 #include <stdint.h>
+#include <errno.h>
 
 #ifdef __cplusplus
 extern "C" {
@@ -78,7 +79,7 @@ extern char *nbdkit_realpath (const char *path);
 /* A static non-NULL pointer which can be used when you don't need a
  * per-connection handle.
  */
-#define NBDKIT_HANDLE_NOT_NEEDED ((void *) &nbdkit_error)
+#define NBDKIT_HANDLE_NOT_NEEDED (&errno)
 
 #ifdef __cplusplus
 }
-- 
2.20.1