Check for and work around buggy fflush(NULL).

Some really old platforms (eg SunOS4) segfault on fflush(NULL) so check
for and work around.  With klausz at haus-gisela.de.

configure.ac

@@ -2059,6 +2059,16 @@ AC_CHECK_FUNCS([realpath], [
 	)
 ])
 
+AC_MSG_CHECKING([for working fflush(NULL)])
+AC_RUN_IFELSE(
+	[AC_LANG_PROGRAM([[#include <stdio.h>]], [[fflush(NULL); exit(0);]])],
+	AC_MSG_RESULT([yes]),
+	[AC_MSG_RESULT([no])
+	 AC_DEFINE([FFLUSH_NULL_BUG], [1],
+	    [define if fflush(NULL) does not work])],
+	AC_MSG_WARN([cross compiling: assuming working])
+)
+
 dnl    Checks for time functions
 AC_CHECK_FUNCS([gettimeofday time])
 dnl    Checks for utmp functions

openbsd-compat/bsd-misc.c

@@ -308,3 +308,19 @@ getsid(pid_t pid)
 }
 #endif
 
+#ifdef FFLUSH_NULL_BUG
+#undef fflush
+int _ssh_compat_fflush(FILE *f)
+{
+	int r1, r2, r3;
+
+	if (f == NULL) {
+		r2 = fflush(stdout);
+		r3 = fflush(stderr);
+		if (r1 == -1 || r2 == -1 || r3 == -1)
+			return -1;
+		return 0;
+	}
+	return fflush(f);
+}
+#endif

openbsd-compat/bsd-misc.h

@@ -153,4 +153,8 @@ pid_t getsid(pid_t);
 int flock(int, int);
 #endif
 
+#ifdef FFLUSH_NULL_BUG
+# define fflush(x)	(_ssh_compat_fflush(x))
+#endif
+
 #endif /* _BSD_MISC_H */