[Qemu-devel] [PATCH v4 07/11] iov: add iov_discard() to remove data

[Stefan Hajnoczi <stefanha@redhat.com>]

The iov_discard() function removes data from the front or back of the
vector.  This is useful when peeling off header/footer structs.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 iov.c | 41 +++++++++++++++++++++++++++++++++++++++++
 iov.h | 13 +++++++++++++
 2 files changed, 54 insertions(+)

diff --git a/iov.c b/iov.c
index a81eedc..6eed089 100644
--- a/iov.c
+++ b/iov.c
@@ -354,3 +354,44 @@ size_t qemu_iovec_memset(QEMUIOVector *qiov, size_t offset,
 {
     return iov_memset(qiov->iov, qiov->niov, offset, fillc, bytes);
 }
+
+size_t iov_discard(struct iovec **iov, unsigned int *iov_cnt, ssize_t bytes)
+{
+    size_t total = 0;
+    struct iovec *cur;
+    int direction;
+
+    if (*iov_cnt == 0) {
+        return 0;
+    }
+
+    if (bytes < 0) {
+        bytes = -bytes;
+        direction = -1;
+        cur = *iov + (*iov_cnt - 1);
+    } else {
+        direction = 1;
+        cur = *iov;
+    }
+
+    while (*iov_cnt > 0) {
+        if (cur->iov_len > bytes) {
+            if (direction > 0) {
+                cur->iov_base += bytes;
+            }
+            cur->iov_len -= bytes;
+            total += bytes;
+            break;
+        }
+
+        bytes -= cur->iov_len;
+        total += cur->iov_len;
+        cur += direction;
+        *iov_cnt -= 1;
+    }
+
+    if (direction > 0) {
+        *iov = cur;
+    }
+    return total;
+}
diff --git a/iov.h b/iov.h
index 34c8ec9..d6d1fa6 100644
--- a/iov.h
+++ b/iov.h
@@ -95,3 +95,16 @@ void iov_hexdump(const struct iovec *iov, const unsigned int iov_cnt,
 unsigned iov_copy(struct iovec *dst_iov, unsigned int dst_iov_cnt,
                  const struct iovec *iov, unsigned int iov_cnt,
                  size_t offset, size_t bytes);
+
+/*
+ * Remove a given number of bytes from the front or back of a vector.
+ * This may update iov and/or iov_cnt to exclude iovec elements that are
+ * no longer required.
+ *
+ * Data is discarded from the front of the vector if bytes is positive and
+ * from the back of the vector if bytes is negative.
+ *
+ * The number of bytes actually discarded is returned.  This number may be
+ * smaller than requested if the vector is too small.
+ */
+size_t iov_discard(struct iovec **iov, unsigned int *iov_cnt, ssize_t bytes);
-- 
1.8.0