From fe4b3bdec449f12e0b395cab6e61824abc909de1 Mon Sep 17 00:00:00 2001
From: David Harris <74973295+davidharrishmc@users.noreply.github.com>
Date: Wed, 23 Sep 2026 02:35:17 -0700
Subject: [PATCH] Fix $sscanf/$fscanf returning -1 after a partial match
 (#8457) (#8459)

---
 include/verilated.cpp         |  4 ++--
 test_regress/t/t_sys_sscanf.v | 14 ++++++++++++++
 2 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/include/verilated.cpp b/include/verilated.cpp
index ed7dac08e..0758d1736 100644
--- a/include/verilated.cpp
+++ b/include/verilated.cpp
@@ -1832,8 +1832,8 @@ IData _vl_vsscanf(FILE* fp,  // If a fscanf
     return got;
 
 done:
-    // Scan stopped early, return parsed or EOF
-    if (_vl_vsss_eof(fp, floc)) return -1;
+    // Scan stopped early; EOF only if input ended before the first conversion
+    if (got == 0 && _vl_vsss_eof(fp, floc)) return -1;
     return got;
 }
 
diff --git a/test_regress/t/t_sys_sscanf.v b/test_regress/t/t_sys_sscanf.v
index 7edfa0228..3d32dd0ca 100644
--- a/test_regress/t/t_sys_sscanf.v
+++ b/test_regress/t/t_sys_sscanf.v
@@ -35,6 +35,20 @@ module t;
     `checks(token, "\0\0\0Hello");
     `checkh(idx, 1);
 
+    // Input ends after some conversions: return the number assigned, not EOF
+    code = $sscanf("Hi 5", "%s %d %d", token, idx, val);
+    `checkh(code, 2);
+    `checks(token, "\0\0\0\0\0\0Hi");
+    `checkh(idx, 5);
+
+    // Matching failure before the first conversion: 0
+    code = $sscanf("Hi", "%d", idx);
+    `checkh(code, 0);
+
+    // Input ends before the first conversion: EOF
+    code = $sscanf("", "%d", idx);
+    `checkh(code, -1);
+
     $finish;
   end