fix failed write reporting by fwrite in line-buffered mode

when a write error occurred while flushing output due to a newline,
fwrite falsely reported all bytes up to and including the newline as
successfully written. in general, due to buffering such "spurious
success" returns are acceptable for stdio; however for line-buffered
mode it was subtly wrong. errors were still visible via ferror() or as
a short-write return if there was more data past the newline that
should have been written, but since the contract for line-buffered
mode is that everything up through the newline be written out
immediately, a discrepency was observable in the actual file contents.

diff --git a/src/stdio/fwrite.c b/src/stdio/fwrite.c
index 81ec271e1bcb7ba477f92c8af74e4cc159deb032..fa30c0d384d3128dcc964d15d2bff1a0c5e5e30f 100644 (file)
--- a/src/stdio/fwrite.c
+++ b/src/stdio/fwrite.c
@@ -13,8 +13,8 @@ size_t __fwritex(const unsigned char *restrict s, size_t l, FILE *restrict f)
                /* Match /^(.*\n|)/ */
                for (i=l; i && s[i-1] != '\n'; i--);
                if (i) {
-                       if (f->write(f, s, i) < i)
-                               return i;
+                       size_t n = f->write(f, s, i);
+                       if (n < i) return n;
                        s += i;
                        l -= i;
                }