From e816e97762ae661684e3c7d01cb3e68985c2db41 Mon Sep 17 00:00:00 2001
From: Gabriel Ravier <gabravier@gmail.com>
Date: Thu, 23 Mar 2023 21:58:14 +0100
Subject: [PATCH] Fix printf-family function not treating negative precisions
 as if they had been omitted

The C standard states that, within the context of a printf-family
function, when specifying the precision of a conversion specification:

> A negative precision argument is taken as if the precision were
> omitted.
- C Standard, 7.23.6.1. The fprintf function

Cosmopolitan currently fails to do so and treats a negative precision
as if it had a value of 0, which is non-conforming.

This patch fixes this.
---
 libc/fmt/fmt.c           |  4 +++-
 test/libc/fmt/fmt_test.c | 19 +++++++++++++++++++
 2 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/libc/fmt/fmt.c b/libc/fmt/fmt.c
index 99b8e8afd..b6f90a588 100644
--- a/libc/fmt/fmt.c
+++ b/libc/fmt/fmt.c
@@ -16,10 +16,11 @@
 │ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/fmt/fmt.h"
+
 #include "libc/assert.h"
 #include "libc/errno.h"
 #include "libc/fmt/conv.h"
-#include "libc/fmt/fmt.h"
 #include "libc/fmt/fmt.internal.h"
 #include "libc/fmt/internal.h"
 #include "libc/fmt/itoa.h"
@@ -255,6 +256,7 @@ _Hide int __fmt(void *fn, void *arg, const char *format, va_list va) {
     }
     if (prec < 0) {
       prec = 0;
+      flags &= ~FLAGS_PRECISION;
     }
 
     // evaluate length field
diff --git a/test/libc/fmt/fmt_test.c b/test/libc/fmt/fmt_test.c
index bccd22e9a..4f481d44f 100644
--- a/test/libc/fmt/fmt_test.c
+++ b/test/libc/fmt/fmt_test.c
@@ -17,6 +17,7 @@
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/fmt/fmt.h"
+
 #include "libc/limits.h"
 #include "libc/log/log.h"
 #include "libc/math.h"
@@ -84,6 +85,24 @@ TEST(fmt, b) {
 }
 
 TEST(fmt, s) {
+  EXPECT_STREQ("123456", _gc(xasprintf("%4.*s", -5, "123456")));
+  EXPECT_STREQ("123456789", _gc(xasprintf("%4.*s", -5, "123456789")));
+  EXPECT_STREQ("12345678901234567890",
+               _gc(xasprintf("%4.*s", -5, "12345678901234567890")));
+  EXPECT_STREQ("123456789012345678901234567890",
+               _gc(xasprintf("%4.*s", -5, "123456789012345678901234567890")));
+  EXPECT_STREQ(
+      "1234567890123456789012345678901234567890",
+      _gc(xasprintf("%4.*s", -5, "1234567890123456789012345678901234567890")));
+  EXPECT_STREQ(
+      "12345678901234567890123456789012345678901234567890",
+      _gc(xasprintf("%4.*s", -5,
+                    "12345678901234567890123456789012345678901234567890")));
+  EXPECT_STREQ(
+      "123456789012345678901234567890123456789012345678901234567890",
+      _gc(xasprintf(
+          "%4.*s", -5,
+          "123456789012345678901234567890123456789012345678901234567890")));
   EXPECT_STREQ("Wide character output test",
                _gc(xasprintf("%S", L"Wide character output test")));
 }