From 8eceacc0d3ac8714b9f4d986bc4bb17ff19d05bd Mon Sep 17 00:00:00 2001
From: "Sergio R. Caprile" <scaprile@cesanta.com>
Date: Tue, 27 Oct 2026 17:30:26 -0300
Subject: [PATCH] survive to width.precision screw ups in %s

---
 mongoose.c       | 7 ++++++-
 src/fmt.c        | 7 ++++++-
 test/unit_test.c | 5 +++++
 3 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/mongoose.c b/mongoose.c
index e06b827f..4f9d77e6 100644
--- a/mongoose.c
+++ b/mongoose.c
@@ -2968,7 +2968,12 @@ size_t mg_vxprintf(void (*out)(char, void *), void *param, const char *fmt,
         n++;
       } else if (c == 's') {
         char *p = va_arg(*ap, char *);
-        if (pr == ~0U) pr = p == NULL ? 0 : strlen(p);
+        if (p != NULL && pr != ~0U) {
+          for (j = 0; j < pr && p[j] != '\0'; j++) ;  // portable strnlen()
+          pr = j;
+        } else {
+          pr = p == NULL ? 0 : strlen(p);
+        }
         for (j = 0; !minus && pr < w && j + pr < w; j++)
           n += scpy(out, param, &pad, 1);
         n += scpy(out, param, p, pr);
diff --git a/src/fmt.c b/src/fmt.c
index 0f19e691..dffbd20f 100644
--- a/src/fmt.c
+++ b/src/fmt.c
@@ -214,7 +214,12 @@ size_t mg_vxprintf(void (*out)(char, void *), void *param, const char *fmt,
         n++;
       } else if (c == 's') {
         char *p = va_arg(*ap, char *);
-        if (pr == ~0U) pr = p == NULL ? 0 : strlen(p);
+        if (p != NULL && pr != ~0U) {
+          for (j = 0; j < pr && p[j] != '\0'; j++) ;  // portable strnlen()
+          pr = j;
+        } else {
+          pr = p == NULL ? 0 : strlen(p);
+        }
         for (j = 0; !minus && pr < w && j + pr < w; j++)
           n += scpy(out, param, &pad, 1);
         n += scpy(out, param, p, pr);
diff --git a/test/unit_test.c b/test/unit_test.c
index 22434439..20b6a82f 100644
--- a/test/unit_test.c
+++ b/test/unit_test.c
@@ -2399,10 +2399,15 @@ static void test_str(void) {
   ASSERT(sn("%.*s", 1, "ab"));
   ASSERT(sn("%.1s", "ab"));
   ASSERT(sn("%.99s", "a"));
+  ASSERT(sn("%10.8s", "abc"));
+  ASSERT(sn("%-6.4s", "ab"));
+  ASSERT(sn("%5.1s", ""));
   ASSERT(sn("%11s", "a"));
   ASSERT(sn("%s", "a\0b"));
   ASSERT(sn("%2s", "a"));
   ASSERT(sn("%.*s", 3, "a\0b"));
+  ASSERT(mg_snprintf(NULL, 0, "%.8s", (char *) NULL) == 0);
+  ASSERT(mg_snprintf(NULL, 0, "%8.4s", (char *) NULL) == 8);
   ASSERT(sn("%d", 7));
   ASSERT(sn("%d", 123));
 #if MG_ARCH == MG_ARCH_UNIX