[PATCH v6 2/3] iov: Introduce IOV_PUSH_HEADER() macro

[Laurent Vivier <lvivier@redhat.com>]

Add iov_push_header_() and its typed wrapper IOV_PUSH_HEADER() to write
a header into an iov_tail at the current offset and advance past it.

This is the write counterpart to IOV_PEEK_HEADER() / IOV_REMOVE_HEADER(),
using iov_from_buf() to copy the header data across iovec boundaries.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
---
 iov.c | 23 +++++++++++++++++++++++
 iov.h | 11 +++++++++++
 2 files changed, 34 insertions(+)

diff --git a/iov.c b/iov.c
index 2289b425529e..6357c477bea6 100644
--- a/iov.c
+++ b/iov.c
@@ -360,6 +360,29 @@ void *iov_peek_header_(struct iov_tail *tail, void *v, size_t len, size_t align)
 	return v;
 }
 
+/**
+ * iov_push_header_() - Write a new header to an IOV tail
+ * @tail:	IOV tail to write header to
+ * @v:		Pointer to header data to write
+ * @len:	Length of header to write, in bytes
+ *
+ * Return: number of bytes written
+ */
+/* cppcheck-suppress unusedFunction */
+size_t iov_push_header_(struct iov_tail *tail, const void *v, size_t len)
+{
+	size_t l;
+
+	if (!iov_tail_prune(tail))
+		return 0; /* No space */
+
+	l = iov_from_buf(tail->iov, tail->cnt, tail->off, v, len);
+
+	tail->off = tail->off + l;
+
+	return l;
+}
+
 /**
  * iov_remove_header_() - Remove a header from an IOV tail
  * @tail:	IOV tail to remove header from (modified)
diff --git a/iov.h b/iov.h
index 074266e127ef..8e4d5f54b9e4 100644
--- a/iov.h
+++ b/iov.h
@@ -93,6 +93,7 @@ bool iov_tail_prune(struct iov_tail *tail);
 size_t iov_tail_size(struct iov_tail *tail);
 bool iov_drop_header(struct iov_tail *tail, size_t len);
 void *iov_peek_header_(struct iov_tail *tail, void *v, size_t len, size_t align);
+size_t iov_push_header_(struct iov_tail *tail, const void *v, size_t len);
 void *iov_remove_header_(struct iov_tail *tail, void *v, size_t len, size_t align);
 ssize_t iov_tail_clone(struct iovec *dst_iov, size_t dst_iov_cnt,
 		       struct iov_tail *tail);
@@ -115,6 +116,16 @@ ssize_t iov_tail_clone(struct iovec *dst_iov, size_t dst_iov_cnt,
 					       sizeof(var_),		\
 					       __alignof__(var_))))
 
+/**
+ * IOV_PUSH_HEADER() - Write a new header to an IOV tail
+ * @tail_:	IOV tail to write header to
+ * @var_:	A variable containing the header data to write
+ *
+ * Return: number of bytes written
+ */
+#define IOV_PUSH_HEADER(tail_, var_)					\
+	(iov_push_header_((tail_), &(var_), sizeof(var_)))
+
 /**
  * IOV_REMOVE_HEADER() - Remove and return typed header from an IOV tail
  * @tail_:	IOV tail to remove header from (modified)
-- 
2.53.0