Distinguish printf-like functions that support %m from those that don't.

The elog/ereport family of functions certainly support the %m format spec,
because they implement it "by hand".  But elsewhere we have printf wrappers
that might or might not allow it depending on whether the platform's printf
does.  (Most non-glibc versions don't, and notably, src/port/snprintf.c
doesn't.)  Hence, rather than using the gnu_printf format archetype
interchangeably for all these functions, use it only for elog/ereport.
This will allow us to get compiler warnings for mistakes like the ones
fixed in commit a13b47a59, at least on platforms where printf doesn't
take %m and gcc is correctly configured to know it.  (Unfortunately,
that won't happen on Linux, nor on macOS according to my testing.
It remains to be seen what the buildfarm's gcc-on-Windows animals will
think of this, but we may well have to rely on less-popular platforms
to warn us about unportable code of this kind.)

Discussion: https://postgr.es/m/2975.1526862605@sss.pgh.pa.us

1 files changed, 5 insertions, 1 deletions

diff --git a/src/include/c.h b/src/include/c.h
index 1e50103095b..0a4757e29fd 100644
--- a/src/include/c.h
+++ b/src/include/c.h
@@ -126,10 +126,14 @@
 /* GCC and XLC support format attributes */
 #if defined(__GNUC__) || defined(__IBMC__)
 #define pg_attribute_format_arg(a) __attribute__((format_arg(a)))
-#define pg_attribute_printf(f,a) __attribute__((format(PG_PRINTF_ATTRIBUTE, f, a)))
+/* Use for functions wrapping stdio's printf, which often doesn't take %m: */
+#define pg_attribute_printf(f,a) __attribute__((format(printf, f, a)))
+/* Use for elog/ereport, which implement %m for themselves: */
+#define pg_attribute_printf_m(f,a) __attribute__((format(PG_PRINTF_ATTRIBUTE_M, f, a)))
 #else
 #define pg_attribute_format_arg(a)
 #define pg_attribute_printf(f,a)
+#define pg_attribute_printf_m(f,a)
 #endif
 
 /* GCC, Sunpro and XLC support aligned, packed and noreturn */