Since diff.git commit 8993f42562696079866fc2dec7191877b6cc1f18 which
intentionally changed the line index where we start searching upwards
for a function prototype, there is a difference in behaviour compared
to traditional usr.bin/diff -p. Unfortunately this difference can result
in confusing output, such as the following reported by tb@ and sthen@.

Prototype line appears after changed lines, instead of before:
[[[
(this blob exists in the OpenBSD src.git repo on Github)
blob - 25333f98510f8805602824e43e841f1df4f95769
file + btrace.c
--- btrace.c
+++ btrace.c
@@ -117,6 +117,7 @@ uint64_t             bt_filtered;   /* # of events filtered out */
 struct dtioc_arg_info  **dt_args;      /* array of probe arguments */

 struct dt_evt           bt_devt;       /* fake event for BEGIN/END */
+#define FOO    0
 uint64_t                bt_filtered;   /* # of events filtered out */

 struct syms            *kelf, *uelf;
]]]

Prototype line appears within changed lines, instead of before:
[[[
(this blob exists in the OpenBSD ports.git repo on Github)
blob - 62353bfe8ea8bc8fab8208fe41bf34edfdac30d2
file + Makefile
--- Makefile
+++ Makefile
@@ -2,7 +2,7 @@ DISTNAME =		nim-1.6.12
 
 COMMENT =		statically typed systems programming language
 
-DISTNAME =		nim-1.6.12
+DISTNAME =		nim-1.6.14
 EXTRACT_SUFX =		.tar.xz
 
 CATEGORIES =		lang
]]]

My patch below reverts commits 8993f42562696079866fc2dec7191877b6cc1f18,
such that our diff output matches the output of usr/bin/diff -p again:
[[[
--- btrace.c
+++ btrace.c
@@ -117,6 +117,7 @@ size_t			 dt_ndtpi;	/* # of elements in the array */
 struct dtioc_arg_info  **dt_args;	/* array of probe arguments */
 
 struct dt_evt		 bt_devt;	/* fake event for BEGIN/END */
+#define FOO	0
 uint64_t		 bt_filtered;	/* # of events filtered out */
 
 struct syms		*kelf, *uelf;
]]]
[[[
--- Makefile
+++ Makefile
@@ -2,7 +2,7 @@ ONLY_FOR_ARCHS =        amd64 arm64 i386

 COMMENT =              statically typed systems programming language

-DISTNAME =             nim-1.6.12
+DISTNAME =             nim-1.6.14
 EXTRACT_SUFX =         .tar.xz

 CATEGORIES =           lang
]]]

Of course this re-introduces the original problem which Tom intended
to fix. I don't know if there is a way to prevent the above problematic
cases while still showing the prototype of a newly added function.
Is this possible?

There is one diff test which needs adjustment to expected results
after fixing our deviation from usr.bin/diff behaviour.

All Got regress tests keep passing as-is.

ok?

diff /home/stsp/src/diff
commit - f46fa9b5e78156c0360f508336dbe3dfa04b503f
path + /home/stsp/src/diff
blob - 6855889d9612f734106a574e7b7ff947b56823d5
file + lib/diff_internal.h
--- lib/diff_internal.h
+++ lib/diff_internal.h
@@ -148,8 +148,7 @@ int diff_output_trailing_newline_msg(struct diff_outpu
 int diff_output_match_function_prototype(char *prototype, size_t prototype_size,
 					 int *last_prototype_idx,
 					 const struct diff_result *result,
-					 const struct diff_chunk_context *cc,
-					 unsigned int ncontext);
+					 const struct diff_chunk_context *cc);
 
 struct diff_output_info *diff_output_info_alloc(void);
 
blob - f44e799638f771a0588c11e3d5b6b1200b5c64f1
file + lib/diff_output.c
--- lib/diff_output.c
+++ lib/diff_output.c
@@ -263,19 +263,17 @@ is_function_prototype(unsigned char ch)
 int
 diff_output_match_function_prototype(char *prototype, size_t prototype_size,
     int *last_prototype_idx, const struct diff_result *result,
-    const struct diff_chunk_context *cc, unsigned int ncontext)
+    const struct diff_chunk_context *cc)
 {
 	struct diff_atom *start_atom, *atom;
 	const struct diff_data *data;
 	unsigned char buf[DIFF_FUNCTION_CONTEXT_SIZE];
 	const char *state = NULL;
-	int rc, i, ch, idx;
+	int rc, i, ch;
 
-	idx = MIN(cc->left.start + (ncontext ? ncontext : 0), cc->left.end - 1);
-
 	if (result->left->atoms.len > 0 && cc->left.start > 0) {
 		data = result->left;
-		start_atom = &data->atoms.head[idx];
+		start_atom = &data->atoms.head[cc->left.start - 1];
 	} else
 		return DIFF_RC_OK;
 
blob - 4757099644a9c1eec21e1af9102ba13ba417d609
file + lib/diff_output_unidiff.c
--- lib/diff_output_unidiff.c
+++ lib/diff_output_unidiff.c
@@ -237,7 +237,7 @@ output_unidiff_chunk(struct diff_output_info *outinfo,
 		     const struct diff_input_info *info,
 		     const struct diff_result *result,
 		     bool print_header, bool show_function_prototypes,
-		     const struct diff_chunk_context *cc, unsigned int ncontext)
+		     const struct diff_chunk_context *cc)
 {
 	int rc, left_start, left_len, right_start, right_len;
 	off_t outoff = 0, *offp;
@@ -304,7 +304,7 @@ output_unidiff_chunk(struct diff_output_info *outinfo,
 	if (show_function_prototypes) {
 		rc = diff_output_match_function_prototype(state->prototype,
 		    sizeof(state->prototype), &state->last_prototype_idx,
-		    result, cc, ncontext);
+		    result, cc);
 		if (rc)
 			return rc;
 	}
@@ -433,7 +433,7 @@ diff_output_unidiff_chunk(struct diff_output_info **ou
 	}
 
 	return output_unidiff_chunk(outinfo, dest, state, info,
-	    result, false, show_function_prototypes, cc, 0);
+	    result, false, show_function_prototypes, cc);
 }
 
 int
@@ -586,7 +586,7 @@ diff_output_unidiff(struct diff_output_info **output_i
 		      " print left %d-%d right %d-%d\n",
 		      cc.left.start, cc.left.end, cc.right.start, cc.right.end);
 		output_unidiff_chunk(outinfo, dest, state, info, result,
-		    true, show_function_prototypes, &cc, context_lines);
+		    true, show_function_prototypes, &cc);
 		cc = next;
 		debug("new unprinted chunk is left %d-%d right %d-%d\n",
 		      cc.left.start, cc.left.end, cc.right.start, cc.right.end);
@@ -594,7 +594,7 @@ diff_output_unidiff(struct diff_output_info **output_i
 
 	if (!diff_chunk_context_empty(&cc))
 		output_unidiff_chunk(outinfo, dest, state, info, result,
-		    true, show_function_prototypes, &cc, context_lines);
+		    true, show_function_prototypes, &cc);
 	diff_output_unidiff_state_free(state);
 	return DIFF_RC_OK;
 }
blob - 2ae051a0a1e04a0e219eb0a362d5d74f2039a76f
file + test/expect124.diff
--- test/expect124.diff
+++ test/expect124.diff
@@ -1,6 +1,6 @@
 --- test124.left-p.txt
 +++ test124.right-p.txt
-@@ -11,5 +11,5 @@ return_test(int test) {
+@@ -11,5 +11,5 @@ doSomethingThenPrintHello(int test)
  
  struct testfile *
  return_test(int test) {