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Re: [libvirt] [PATCH 2/3] Routine to truncate virBuffer

On 11/06/2011 06:58 AM, Bharata B Rao wrote:
Routine to truncate virBuffer

From: Bharata B Rao<bharata linux vnet ibm com>

Add a helper to truncate virBuffer.

  * virBufferTruncate:

+++ b/src/util/buf.c
@@ -123,6 +123,31 @@ virBufferGrow(virBufferPtr buf, unsigned int len)

+ * virBufferTruncate:
+ * @buf:  the buffer
+ * @len:  number of bytes by which the buffer is truncated
+ *
+ * Truncate the buffer by @len bytes.
+ *
+ * Returns zero on success or -1 on error
+ */
+virBufferTruncate(virBufferPtr buf, unsigned int len)

What good is returning an error, given that callers already have to use virBufferError for detecting other errors, and given that you aren't marking the function as ATTRIBUTE_RETURN_CHECK to require callers to respect that error? One of the points of the virBuffer API is that most functions can return void (for example, see virBufferAdjustIndent), because the user doesn't care about error checking until the end.

+    if (buf->error)
+        return -1;
+    if ((signed)(buf->use - len)<  0) {

I tend to write the cast as (int), not (signed), if a cast is even needed. You don't catch all possible overflows, though - if buf->use is 2, and len is 4294967295 (aka (unsigned)-1), then you've proceeded to expand(!) buf->use to 3, skipping an uninitialized byte. A better filter would be:

if (len > buf->use)

+        virBufferSetError(buf, -1);
+        return -1;
+    }
+    buf->use -= len;
+    buf->content[buf->use] = '\0';

This looks correct, once you filter out invalid len first.

+++ b/tests/virbuftest.c
@@ -123,7 +123,8 @@ static int testBufAutoIndent(const void *data ATTRIBUTE_UNUSED)
      virBufferAddChar(buf, '\n');
      virBufferEscapeShell(buf, " 11");
      virBufferAddChar(buf, '\n');
+    virBufferAsprintf(buf, "%d", 12);
+    virBufferTruncate(buf, 4);

That gives no change in the expected output.  I'd almost rather see:

virBufferAsprintf(buf, "%d", 123);
virBufferTruncate(buf, 1);

as well as an update to the expected output to see output ending in "12".

You're on the right track, but I don't think it is worth applying this until after 0.9.7 is released.

Eric Blake   eblake redhat com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org

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