From 9200553b229bbccbe24389998d06aa34065bc249 Mon Sep 17 00:00:00 2001
From: Victor Zverovich <victor.zverovich@gmail.com>
Date: Wed, 17 Jun 2026 08:36:43 +0200
Subject: [PATCH] Fix hang/assertion when printing to a pipe with closed read
 end (#4797)

---
 include/fmt/format-inl.h | 10 ++++++++--
 test/format-test.cc      | 19 +++++++++++++++++++
 2 files changed, 27 insertions(+), 2 deletions(-)

diff --git a/include/fmt/format-inl.h b/include/fmt/format-inl.h
index 665f7308..ecd3ffec 100644
--- a/include/fmt/format-inl.h
+++ b/include/fmt/format-inl.h
@@ -1526,7 +1526,10 @@ template <typename F> class file_base {
       FMT_THROW(system_error(errno, FMT_STRING("ungetc failed")));
   }
 
-  void flush() { fflush(this->file_); }
+  void flush() {
+    if (fflush(this->file_) != 0)
+      FMT_THROW(system_error(errno, FMT_STRING("fflush failed")));
+  }
 };
 
 // A FILE wrapper for glibc.
@@ -1572,7 +1575,10 @@ template <typename F> class glibc_file : public file_base<F> {
     return memchr(end, '\n', static_cast<size_t>(size));
   }
 
-  void flush() { fflush_unlocked(this->file_); }
+  void flush() {
+    if (fflush_unlocked(this->file_) != 0)
+      FMT_THROW(system_error(errno, FMT_STRING("fflush failed")));
+  }
 };
 
 // A FILE wrapper for Apple's libc.
diff --git a/test/format-test.cc b/test/format-test.cc
index 61a53c72..9a2258d5 100644
--- a/test/format-test.cc
+++ b/test/format-test.cc
@@ -18,6 +18,7 @@
 #include <cfenv>        // fegetexceptflag and FE_ALL_EXCEPT
 #include <climits>      // INT_MAX
 #include <cmath>        // std::signbit
+#include <csignal>      // std::signal, SIGPIPE
 #include <cstring>      // std::strlen
 #include <iterator>     // std::back_inserter
 #include <list>         // std::list
@@ -2592,6 +2593,24 @@ TEST(format_test, invalid_glibc_buffer) {
 
   fmt::print(file, "------\n");
 }
+
+TEST(format_test, print_to_broken_pipe) {
+  // Ignore SIGPIPE so that a failing write reports EPIPE instead of
+  // terminating the test process. It must stay ignored until the file is
+  // closed below because closing also flushes the remaining buffered data.
+  auto old_handler = std::signal(SIGPIPE, SIG_IGN);
+  {
+    auto pipe = fmt::pipe();
+    pipe.read_end.close();
+    auto write_end = pipe.write_end.fdopen("w");
+
+    // The data must exceed the file's buffer to force a flush during
+    // formatting, whose underlying write() then fails with EPIPE.
+    auto data = std::string(1024 * 1024, 'x');
+    EXPECT_THROW(fmt::print(write_end.get(), "{}", data), std::system_error);
+  }
+  std::signal(SIGPIPE, old_handler);
+}
 #endif  // FMT_USE_FCNTL
 
 // Only defined after the test case.