[PATCH v2 02/20] iov: Introduce iov_tail_drop() and iov_slice()

[Laurent Vivier <lvivier@redhat.com>]

iov_tail_drop() discards a header from the iov_tail,

iov_slice() makes a new iov referencing a subset of the data
in the original iov.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
---
 iov.c | 68 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 iov.h |  4 ++++
 2 files changed, 72 insertions(+)

diff --git a/iov.c b/iov.c
index 8c63b7ea6e31..d2aca59e42ed 100644
--- a/iov.c
+++ b/iov.c
@@ -156,6 +156,59 @@ size_t iov_size(const struct iovec *iov, size_t iov_cnt)
 	return len;
 }
 
+/**
+ * iov_slice -  Make a new iov referencing a subset of the data in the original iov
+ *
+ * @dst_iov:     Pointer to the destination array of struct iovec describing
+ *		 the scatter/gather I/O vector to copy to.
+ * @dst_iov_cnt: Maximum number of elements in the destination iov array.
+ * @iov:	 Pointer to the source array of struct iovec describing
+ *		 the scatter/gather I/O vector to copy from.
+ * @iov_cnt:     Maximum number of elements in the source iov array.
+ * @offset:      Offset within the source iov from where copying should start.
+ * @bytes:	 On input, total number of bytes to copy from @iov to @dst_iov,
+ * 		 if @bytes is NULL, copy all the content of @iov,
+ * 		 on output actual number of bytes copied.
+ *
+ * Returns:      The number of elements successfully copied to the destination
+ *		 iov array, a negative value if there is no enough room in the
+ *		 destination iov array
+ */
+/* cppcheck-suppress unusedFunction */
+int iov_slice(struct iovec *dst_iov, size_t dst_iov_cnt,
+	      const struct iovec *iov, size_t iov_cnt,
+	      size_t offset, size_t *bytes)
+{
+	unsigned int i, j;
+	size_t remaining = bytes ? *bytes : SIZE_MAX;
+
+	i = iov_skip_bytes(iov, iov_cnt, offset, &offset);
+
+	/* copying data */
+	for (j = 0; i < iov_cnt && j < dst_iov_cnt && remaining; i++) {
+		size_t len = MIN(remaining, iov[i].iov_len - offset);
+
+		dst_iov[j].iov_base = (char *)iov[i].iov_base + offset;
+		dst_iov[j].iov_len = len;
+		j++;
+		remaining -= len;
+		offset = 0;
+	}
+	if (j == dst_iov_cnt) {
+		if (bytes) {
+			if (remaining)
+				return -1;
+		} else if (i != iov_cnt) {
+			return -1;
+		}
+	}
+
+	if (bytes)
+		*bytes -= remaining;
+
+	return j;
+}
+
 /**
  * iov_tail_prune() - Remove any unneeded buffers from an IOV tail
  * @tail:	IO vector tail (modified)
@@ -191,6 +244,21 @@ size_t iov_tail_size(struct iov_tail *tail)
 	return iov_size(tail->iov, tail->cnt) - tail->off;
 }
 
+/**
+ * iov_tail_drop() - Discard a header from an IOV tail
+ * @tail:	IO vector tail
+ * @len:	length to move the head of the tail
+ *
+ * Returns:	true if the tail still contains any bytes, otherwise false
+ */
+/* cppcheck-suppress unusedFunction */
+bool iov_tail_drop(struct iov_tail *tail, size_t len)
+{
+	tail->off = tail->off + len;
+
+	return iov_tail_prune(tail);
+}
+
 /**
  * iov_peek_header_() - Get pointer to a header from an IOV tail
  * @tail:	IOV tail to get header from
diff --git a/iov.h b/iov.h
index 9855bf0c0c32..3f8259aa8d28 100644
--- a/iov.h
+++ b/iov.h
@@ -28,6 +28,9 @@ size_t iov_from_buf(const struct iovec *iov, size_t iov_cnt,
 size_t iov_to_buf(const struct iovec *iov, size_t iov_cnt,
                   size_t offset, void *buf, size_t bytes);
 size_t iov_size(const struct iovec *iov, size_t iov_cnt);
+int  iov_slice(struct iovec *dst_iov, size_t dst_iov_cnt,
+               const struct iovec *iov, size_t iov_cnt,
+               size_t offset, size_t *bytes);
 
 /*
  * DOC: Theory of Operation, struct iov_tail
@@ -72,6 +75,7 @@ struct iov_tail {
 
 bool iov_tail_prune(struct iov_tail *tail);
 size_t iov_tail_size(struct iov_tail *tail);
+bool iov_tail_drop(struct iov_tail *tail, size_t len);
 void *iov_peek_header_(struct iov_tail *tail, size_t len, size_t align);
 void *iov_remove_header_(struct iov_tail *tail, size_t len, size_t align);
 
-- 
@@ -28,6 +28,9 @@ size_t iov_from_buf(const struct iovec *iov, size_t iov_cnt,
 size_t iov_to_buf(const struct iovec *iov, size_t iov_cnt,
                   size_t offset, void *buf, size_t bytes);
 size_t iov_size(const struct iovec *iov, size_t iov_cnt);
+int  iov_slice(struct iovec *dst_iov, size_t dst_iov_cnt,
+               const struct iovec *iov, size_t iov_cnt,
+               size_t offset, size_t *bytes);
 
 /*
  * DOC: Theory of Operation, struct iov_tail
@@ -72,6 +75,7 @@ struct iov_tail {
 
 bool iov_tail_prune(struct iov_tail *tail);
 size_t iov_tail_size(struct iov_tail *tail);
+bool iov_tail_drop(struct iov_tail *tail, size_t len);
 void *iov_peek_header_(struct iov_tail *tail, size_t len, size_t align);
 void *iov_remove_header_(struct iov_tail *tail, size_t len, size_t align);
 
-- 
2.49.0