commit - 9230203d371721cf5bde99f526fe40740aea3b21
commit + 8993f42562696079866fc2dec7191877b6cc1f18
blob - 396f264aae799c02ce62d47c6762b7dc5bd05331
blob + be72763dedcb12e2baabb6fda64e06d2fcf5f598
--- lib/diff_internal.h
+++ lib/diff_internal.h
@@ -177,7 +177,8 @@ 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);
+					 const struct diff_chunk_context *cc,
+					 unsigned int ncontext);
 
 struct diff_output_info *diff_output_info_alloc(void);
 
blob - 3047221aa5a15e4fc2a5dea8267e97864da21264
blob + bc44a51ea36b8a3a7cd5581e6787fcdda23168c6
--- lib/diff_output.c
+++ lib/diff_output.c
@@ -270,17 +270,19 @@ 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)
+    const struct diff_chunk_context *cc, unsigned int ncontext)
 {
 	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;
+	int rc, i, ch, idx;
+
+	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[cc->left.start - 1];
+		start_atom = &data->atoms.head[idx];
 	} else
 		return DIFF_RC_OK;
 
blob - 18bff74063081228c1c525cce2b35ad26a527bf8
blob + 74b22eb8207ab2c5f154a22229051900a3d7b1ce
--- lib/diff_output_unidiff.c
+++ lib/diff_output_unidiff.c
@@ -225,7 +225,7 @@ 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)
+		     const struct diff_chunk_context *cc, unsigned int ncontext)
 {
 	int rc, left_start, left_len, right_start, right_len;
 	off_t outoff = 0, *offp;
@@ -292,7 +292,7 @@ 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);
+		    result, cc, ncontext);
 		if (rc)
 			return rc;
 	}
@@ -416,7 +416,7 @@ diff_output_unidiff_chunk(struct diff_output_info **ou
 	}
 
 	return output_unidiff_chunk(outinfo, dest, state, info,
-	    result, false, show_function_prototypes, cc);
+	    result, false, show_function_prototypes, cc, 0);
 }
 
 int
@@ -569,7 +569,7 @@ 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);
+		    true, show_function_prototypes, &cc, context_lines);
 		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);
@@ -577,7 +577,7 @@ 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);
+		    true, show_function_prototypes, &cc, context_lines);
 	diff_output_unidiff_state_free(state);
 	return DIFF_RC_OK;
 }