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Message-ID: <56B4C0AA.3020705@gmx.at>
Date: Fri, 5 Feb 2016 16:32:58 +0100
From: hombre <hombre67@....at>
To: musl@...ts.openwall.com
Subject: bug in fwrite/__towrite
Hello,
I think there is a bug in fwrite/__towrite.
This is my unittest that fails:
static void test_write_read2(const char *fname)
{
char wbuf[3];
char c;
FILE *file = fopen(fname, "wb");
assert(file != NULL);
wbuf[0] = 'a';
wbuf[1] = 'b';
wbuf[2] = 'c';
size_t written = fwrite(wbuf, 1, 3, file);
assert(written == 3);
fclose(file);
file = fopen(fname, "rb+");
size_t nread = fread(&c, 1, 1, file);
assert(nread == 1);
assert(c == 'a');
c = 'B';
written = fwrite(&c, 1, 1, file);
assert(written == 1);
nread = fread(&c, 1, 1, file);
assert(nread == 1); /* <================== nread is 0 here ! */
assert(c == 'c');
fclose(file);
}
Please note that I have not tested this with the original musl-libc in
linux. I found this bug while I was trying to port parts of musl to a
small embedded os. But I think the bug is not in my port. Here is what I
think is wrong:
- fwrite calls __towrite when the write buffer is not active
if (!f->wend && __towrite(f)) return 0;
- __towrite clears the read buffer, but the underlying filepointer is
not adjusted. I think that the filepointer should be adjusted, when the
read buffer is not empty.
int __towrite(FILE *f)
{
...
/* Clear read buffer (easier than summoning nasal demons) */
f->rpos = f->rend = 0;
...
Here is my fix:
int __towrite(FILE *f)
{
...
/* Clear read buffer (easier than summoning nasal demons) */
if (f->rpos) {
/* Adjust underlying filepointer for unread data in buffer. */
if (f->seek(f, -(f->rend - f->rpos), SEEK_CUR) < 0)
return -1;
f->rpos = f->rend = 0;
}
...
Regards,
Erwin
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