[Qemu-devel] [PATCH v3 09/12] iov: add iov_get_ptr() to reference vector data

[Stefan Hajnoczi <stefanha@redhat.com>]

The iov_get_ptr() data returns a pointer to contiguous data within a
vector.  This allows the caller to manipulate data inside the vector
without copying in/out using iov_from_buf()/iov_to_buf() when we know
that data is contiguous within an iovec element.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 iov.c | 25 +++++++++++++++++++++++++
 iov.h | 11 +++++++++++
 2 files changed, 36 insertions(+)

diff --git a/iov.c b/iov.c
index 6eed089..bc78c34 100644
--- a/iov.c
+++ b/iov.c
@@ -395,3 +395,28 @@ size_t iov_discard(struct iovec **iov, unsigned int *iov_cnt, ssize_t bytes)
     }
     return total;
 }
+
+void *iov_get_ptr(struct iovec *iov, unsigned int iov_cnt,
+                  ssize_t offset, size_t bytes)
+{
+    if (offset < 0) {
+        offset += iov_size(iov, iov_cnt);
+        if (offset < 0) {
+            return NULL; /* offset before beginning of vector */
+        }
+    }
+
+    while (iov_cnt > 0) {
+        if (offset < iov->iov_len) {
+            if (bytes > iov->iov_len - offset) {
+                return NULL; /* would span iovec elements */
+            }
+            return iov->iov_base + offset;
+        }
+
+        offset -= iov->iov_len;
+        iov_cnt--;
+        iov++;
+    }
+    return NULL; /* offset beyond end of vector */
+}
diff --git a/iov.h b/iov.h
index d6d1fa6..674dd51 100644
--- a/iov.h
+++ b/iov.h
@@ -108,3 +108,14 @@ unsigned iov_copy(struct iovec *dst_iov, unsigned int dst_iov_cnt,
  * smaller than requested if the vector is too small.
  */
 size_t iov_discard(struct iovec **iov, unsigned int *iov_cnt, ssize_t bytes);
+
+/*
+ * Get a pointer into a vector at offset if the given number of bytes is
+ * contiguous and not split across iovec elements.  NULL is returned if
+ * memory would span iovec elements or exceed the length of the vector.
+ *
+ * The offset is ssize_t so that an offset from the end of the vector can
+ * be specified with a negative number.
+ */
+void *iov_get_ptr(struct iovec *iov, unsigned int iov_cnt, ssize_t offset,
+                  size_t bytes);
-- 
1.8.0