From 49414826afee624113ed37bd84cc5e8f5c477145 Mon Sep 17 00:00:00 2001
From: goldsimon <goldsimon@gmx.de>
Date: Mon, 6 Mar 2017 21:53:48 +0100
Subject: [PATCH] pbuf: added new function pbuf_free_header() to gradually hide
 bytes and free pbufs from the front of a pbuf chain

---
 src/core/pbuf.c         | 30 ++++++++++++++++++++++++++++++
 src/include/lwip/pbuf.h |  1 +
 2 files changed, 31 insertions(+)

diff --git a/src/core/pbuf.c b/src/core/pbuf.c
index 4dd1a26a..635f2f35 100644
--- a/src/core/pbuf.c
+++ b/src/core/pbuf.c
@@ -683,6 +683,36 @@ pbuf_header_force(struct pbuf *p, s16_t header_size_increment)
    return pbuf_header_impl(p, header_size_increment, 1);
 }
 
+/** Similar to pbuf_header(-size) but de-refs header pbufs for (size >= p->len)
+ *
+ * @param q pbufs to operate on
+ * @param size The number of bytes to remove from the beginning of the pbuf list.
+ *             While size >= p->len, pbufs are freed.
+ *        ATTENTION: this is the opposite direction as @ref pbuf_header, but
+ *                   takes an u16_t not s16_t!
+ * @return the new head pbuf
+ */
+struct pbuf*
+pbuf_free_header(struct pbuf *q, u16_t size)
+{
+  struct pbuf *p = q;
+  u16_t free_left = size;
+  while (free_left && p) {
+    s16_t free_len = (free_left > INT16_MAX ? INT16_MAX : (s16_t)free_left);
+    if (free_len >= p->len) {
+      struct pbuf *f = p;
+      free_left -= p->len;
+      p = p->next;
+      f->next = 0;
+      pbuf_free(f);
+    } else {
+      pbuf_header(p, -free_len);
+      free_left -= free_len;
+    }
+  }
+  return p;
+}
+
 /**
  * @ingroup pbuf
  * Dereference a pbuf chain or queue and deallocate any no-longer-used
diff --git a/src/include/lwip/pbuf.h b/src/include/lwip/pbuf.h
index c2238028..667c2383 100644
--- a/src/include/lwip/pbuf.h
+++ b/src/include/lwip/pbuf.h
@@ -248,6 +248,7 @@ struct pbuf *pbuf_alloced_custom(pbuf_layer l, u16_t length, pbuf_type type,
 void pbuf_realloc(struct pbuf *p, u16_t size);
 u8_t pbuf_header(struct pbuf *p, s16_t header_size);
 u8_t pbuf_header_force(struct pbuf *p, s16_t header_size);
+struct pbuf *pbuf_free_header(struct pbuf *q, u16_t size);
 void pbuf_ref(struct pbuf *p);
 u8_t pbuf_free(struct pbuf *p);
 u16_t pbuf_clen(const struct pbuf *p);