Rename cantwrite() to prepwrite(). The latter is less confusing,

since the macro isn't really a predicate, and it has side-effects.

lib/libc/stdio/local.h

@@ -93,9 +93,10 @@ struct __sFILEX {
 };
 
 /*
- * Return true iff the given FILE cannot be written now.
+ * Prepare the given FILE for writing, and return 0 iff it
+ * can be written now.  Otherwise, return EOF and set errno.
  */
-#define	cantwrite(fp) \
+#define	prepwrite(fp) \
  	((((fp)->_flags & __SWR) == 0 || \
  	    ((fp)->_bf._base == NULL && ((fp)->_flags & __SSTR) == 0)) && \
 	 __swsetup(fp))

lib/libc/stdio/vfprintf.c

@@ -642,7 +642,7 @@ __vfprintf(FILE *fp, const char *fmt0, va_list ap)
 	decimal_point = localeconv()->decimal_point;
 #endif
 	/* sorry, fprintf(read_only_file, "") returns EOF, not 0 */
-	if (cantwrite(fp))
+	if (prepwrite(fp) != 0)
 		return (EOF);
 
 	/* optimise fprintf(stderr) (and other unbuffered Unix files) */

lib/libc/stdio/vfwprintf.c

@@ -648,7 +648,7 @@ __vfwprintf(FILE *fp, const wchar_t *fmt0, va_list ap)
 #endif
 	convbuf = NULL;
 	/* sorry, fwprintf(read_only_file, L"") returns WEOF, not 0 */
-	if (cantwrite(fp))
+	if (prepwrite(fp) != 0)
 		return (EOF);
 
 	/* optimise fprintf(stderr) (and other unbuffered Unix files) */

lib/libc/stdio/wbuf.c

@@ -65,7 +65,7 @@ __swbuf(c, fp)
 	 * calls might wrap _w from negative to positive.
 	 */
 	fp->_w = fp->_lbfsize;
-	if (cantwrite(fp))
+	if (prepwrite(fp) != 0)
 		return (EOF);
 	c = (unsigned char)c;