attr: expand mnl_attr_get_uint() documentation

This function is modelled after rta_getattr_uint() in libnetlink to fetch the
netlink attribute payload of NLA_UINT, although it was extended to make it
universal for 8-bit, 16-bit, 32-bit or 64-bit integers.

Acked-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

1 files changed, 18 insertions, 1 deletions

diff --git a/src/attr.c b/src/attr.c
index 399318e..20e99b1 100644
--- a/src/attr.c
+++ b/src/attr.c
@@ -393,7 +393,24 @@ EXPORT_SYMBOL uint64_t mnl_attr_get_u64(const struct nlattr *attr)
  * mnl_attr_get_uint - returns 64-bit unsigned integer attribute.
  * \param attr pointer to netlink attribute
  *
- * This function returns the 64-bit value of the attribute payload.
+ * This helper function reads the variable-length netlink attribute NLA_UINT
+ * that provides a 32-bit or 64-bit integer payload. Its use is recommended only
+ * in these cases.
+ *
+ * Recommended validation for NLA_UINT is:
+ *
+ * \verbatim
+	if (!mnl_attr_validate(attr, NLA_U32) &&
+	    !mnl_attr_validate(attr, NLA_U64)) {
+		perror("mnl_attr_validate");
+		return MNL_CB_ERROR;
+	}
+\endverbatim
+ *
+ * \returns the 64-bit value of the attribute payload. On error, it returns
+ * UINT64_MAX if the length of the netlink attribute is not an 8-bit, 16-bit,
+ * 32-bit and 64-bit integer. Therefore, there is no way to distinguish between
+ * UINT64_MAX and an error. Also, errno is never set.
  */
 EXPORT_SYMBOL uint64_t mnl_attr_get_uint(const struct nlattr *attr)
 {